davidbgk
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larlet-fr-david-cache

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<h1>
  <span><a id="jumper" href="#jumpto" title="Un peu perdu ?">?</a></span>
  24/192 Music Downloads (archive)
  <time>Pour la pérennité des contenus liés. Non-indexé, retrait sur simple email.</time>
</h1>
<section>
  <article>
    <h3><a href="https://www.xiph.org/~xiphmont/demo/neil-young.html">Source originale du contenu</a></h3>
    <div id="toc_intro">

  <img src="https://www.xiph.org/~xiphmont/demo/players2-small.jpg"/>

  <p class="aside">
Also see Xiph.Org's new
    video, <a href="https://video.xiph.org/vid2.shtml">Digital Show
      &amp; Tell</a>, for detailed demonstrations of digital sampling
    in action on real equipment!
</p>

  <p>Articles last month revealed that musician Neil
    Young and Apple's Steve Jobs discussed offering
    digital music downloads of 'uncompromised studio quality'.
    Much of the press and user commentary was particularly
    enthusiastic about the prospect of uncompressed 24 bit 192kHz
    downloads.  24/192 featured prominently in my own
    conversations with Mr. Young's group several months ago.</p>

  <p>Unfortunately, there is no point to distributing music in
    24-bit/192kHz format. Its playback fidelity is slightly
    inferior to 16/44.1 or 16/48, and it takes up 6 times the
    space.</p>

  <p>There are a few real problems with the audio quality and
    'experience' of digitally distributed music today. 24/192
    solves none of them.  While everyone fixates on 24/192 as a
    magic bullet, we're not going to see any actual
    improvement.</p>
</div>

<div id="toc_ftbn">
  <h2>First, the bad news</h2>

  <p>In the past few weeks, I've had conversations with
    intelligent, scientifically minded individuals who believe
    in 24/192 downloads and want to know how anyone could
    possibly disagree.  They asked good questions that deserve
    detailed answers.</p>

  <p>I was also interested in what motivated high-rate digital
    audio advocacy.  Responses indicate that few people
    understand basic signal theory or the
    <a href="http://en.wikipedia.org/wiki/Sampling_theorem">sampling
      theorem</a>, which is hardly surprising.  Misunderstandings
    of the mathematics, technology, and physiology arose in most
    of the conversations, often asserted by professionals who
    otherwise possessed significant audio expertise.  Some even
    argued that the sampling theorem doesn't really explain how
    digital audio actually works [<a href="#foot1">1</a>].</p>

  <p>Misinformation and superstition only serve charlatans.  So,
    let's cover some of the basics of why 24/192 distribution
    makes no sense before suggesting some improvements that
    actually do.</p>
</div>

<div id="toc_gmye">
  <h3>Gentlemen, meet your ears</h3>

  <p>The ear hears via hair cells that sit on the resonant
    basilar membrane in the cochlea.  Each hair cell is
    effectively tuned to a narrow frequency band determined by
    its position on the membrane. Sensitivity peaks in the
    middle of the band and falls off to either side in a
    lopsided cone shape overlapping the bands of other nearby
    hair cells.  A sound is inaudible if there are no hair cells
    tuned to hear it.</p>

  <img src="https://www.xiph.org/~xiphmont/demo/cochlea-and-responses.png"/>
<div class="caption">
<p>Above left: anatomical cutaway drawing of a human cochlea with the
basilar membrane colored in beige.  The membrane is
tuned to resonate at different frequencies along its length,
with higher frequencies near the base and lower frequencies
at the apex.  Approximate locations of several frequencies
are marked.</p>

<p>Above right: schematic diagram representing hair cell response
along the basilar membrane as a bank of overlapping filters.</p>
</div>

  <p>This is similar to an analog radio that picks up the
    frequency of a strong station near where the tuner is
    actually set.  The farther off the station's frequency is,
    the weaker and more distorted it gets until it disappears
    completely, no matter how strong. There is an upper (and
    lower) audible frequency limit, past which the sensitivity
    of the last hair cells drops to zero, and hearing ends.</p>
</div>

<div id="toc_sratas">
  <h3>Sampling rate and the audible spectrum</h3>

  <p>I'm sure you've heard this many, many times: The human
    hearing range spans 20Hz to 20kHz.  It's important to know
    how researchers arrive at those specific numbers.</p>

  <p>First, we measure the 'absolute threshold of hearing'
    across the entire audio range for a group of listeners.
    This gives us a curve representing the very quietest sound
    the human ear can perceive for any given frequency as
    measured in ideal circumstances on healthy ears.  Anechoic
    surroundings, precision calibrated playback equipment, and
    rigorous statistical analysis are the easy part.  Ears and
    auditory concentration both fatigue quickly, so testing must
    be done when a listener is fresh.  That means lots of breaks
    and pauses.  Testing takes anywhere from many hours to many
    days depending on the methodology.</p>

  <p>Then we collect data for the opposite extreme, the
    'threshold of pain'.  This is the point where the audio
    amplitude is so high that the ear's physical and neural
    hardware is not only completely overwhelmed by the input,
    but experiences physical pain.  Collecting this data is
    trickier.  You don't want to permanently damage anyone's
    hearing in the process.</p>

<img src="https://www.xiph.org/~xiphmont/demo/ath-top.png"/>
<div class="caption">
<p>Above: Approximate equal loudness curves derived from
Fletcher and Munson (1933) plus modern sources for
frequencies &gt; 16kHz. The absolute threshold of hearing and
threshold of pain curves are marked in red.  Subsequent
researchers refined these readings, culminating in the Phon
scale and the ISO 226 standard equal loudness curves. Modern
data indicates that the ear is significantly less sensitive
to low frequencies than Fletcher and Munson's results. </p>
</div>

  <p>The upper limit of the human audio range is defined to be
    where the absolute threshold of hearing curve crosses the
    threshold of pain.  To even faintly perceive the audio at
    that point (or beyond), it must simultaneously be unbearably
    loud.</p>

<p>At low frequencies, the cochlea works like a bass reflex
cabinet.  The <em>helicotrema</em> is an opening at the apex
of the basilar membrane that acts as a port tuned to
somewhere between 40Hz to 65Hz depending on the
individual. Response rolls off steeply below this
frequency.</p>

  <p>Thus, 20Hz - 20kHz is a generous range.  It thoroughly
    covers the audible spectrum, an assertion backed by nearly a
    century of experimental data.</p>
</div>

<div id="toc_ggage">
  <h3>Genetic gifts and golden ears</h3>

  <p>Based on my correspondences, many people believe in
    individuals with extraordinary gifts of hearing.  Do such
    'golden ears' really exist?</p>

  <p>It depends on what you call a golden ear.</p>

  <p>Young, healthy ears hear better than old or damaged ears.
    Some people are exceptionally well trained to hear nuances
    in sound and music most people don't even know exist.  There
    was a time in the 1990s when I could identify every major
    mp3 encoder by sound (back when they were all pretty bad),
    and could demonstrate this reliably in double-blind testing
    [<a href="#foot2">2</a>].</p>

  <p>When healthy ears combine with highly trained
    discrimination abilities, I would call that person a golden
    ear.  Even so, below-average hearing can also be trained to
    notice details that escape untrained listeners.  Golden ears
    are more about training than hearing beyond the physical
    ability of average mortals.</p>

  <p>Auditory researchers would love to find, test, and document
    individuals with truly exceptional hearing, such as a
    greatly extended hearing range. Normal people are nice and
    all, but everyone wants to find a genetic freak for a really
    juicy paper. We haven't found any such people in the
    past 100 years of testing, so they probably don't exist.
    Sorry.  We'll keep looking.</p>
</div>

<div id="toc_s">
  <h3>Spectrophiles</h3>

  <p>Perhaps you're skeptical about everything I've just
    written; it certainly goes against most marketing material.
    Instead, let's consider a hypothetical Wide Spectrum Video
    craze that doesn't carry preexisting audiophile baggage.</p>

<img src="https://www.xiph.org/~xiphmont/demo/visspec.png"/>
<div class="caption">
<p>Above: The approximate log scale response of the human
eye's rods and cones, superimposed on the visible
spectrum.  These sensory organs respond to light in
overlapping spectral bands, just as the ear's hair cells
are tuned to respond to overlapping bands of sound
frequencies.</p>
</div>

  <p>The human eye sees a limited range of frequencies of
    light, aka, the visible spectrum.  This is directly
    analogous to the audible spectrum of sound waves.  Like the
    ear, the eye has sensory cells (rods and cones) that detect
    light in different but overlapping frequency bands.</p>

  <p>The visible spectrum extends from about 400THz (deep red)
    to 850THz (deep violet) [<a href="#foot3">3</a>].
    Perception falls off steeply at the edges.  Beyond these
    approximate limits, the light power needed for the slightest
    perception can fry your retinas. Thus, this is a generous
    span even for young, healthy, genetically gifted
    individuals, analogous to the generous limits of the audible
    spectrum.</p>

  <p>In our hypothetical Wide Spectrum Video craze, consider a
    fervent group of Spectrophiles who believe these limits
    aren't generous enough.  They propose that video represent
    not only the visible spectrum, but also infrared and
    ultraviolet.  Continuing the comparison, there's an even
    more hardcore [and proud of it!] faction that insists this
    expanded range is yet insufficient, and that video feels so
    much more natural when it also includes microwaves and some
    of the X-ray spectrum.  To a Golden Eye, they insist, the
    difference is night and day!</p>

  <p>Of course this is ludicrous.</p>

  <p>No one can see X-rays (or infrared, or ultraviolet, or
    microwaves).  It doesn't matter how much a person believes
    he can.  Retinas simply don't have the sensory hardware.</p>

  <p>Here's an experiment anyone can do: Go get your Apple IR
    remote.  The LED emits at 980nm, or about 306THz, in the
    near-IR spectrum.  This is not far outside of the visible
    range. Take the remote into the basement, or the darkest
    room in your house, in the middle of the night, with the
    lights off.  Let your eyes adjust to the blackness.</p>

<img src="https://www.xiph.org/~xiphmont/demo/apple-ir.jpg"/>
<div class="caption">
<p>Above: Apple IR remote photographed using a digital
camera.  Though the emitter is quite bright and the
frequency emitted is not far past the red portion of
the visible spectrum, it's completely invisible to the
eye.</p>
</div>

  <p>Can you see the Apple Remote's LED flash when you press a
    button [<a href="#foot4">4</a>]?  No?  Not even the tiniest
    amount?  Try a few other IR remotes; many use an IR
    wavelength a bit closer to the visible band, around
    310-350THz.  You won't be able to see them either.  The rest
    emit right at the edge of visibility from 350-380 THz and
    may be just barely visible in complete blackness with
    dark-adjusted eyes [<a href="#foot5">5</a>].  All would be
    blindingly, painfully bright if they were well inside the
    visible spectrum.</p>

  <p>These near-IR LEDs emit from the visible boundry to at most
    20% beyond the visible frequency limit.  192kHz audio
    extends to 400% of the audible limit.  Lest I be accused of
    comparing apples and oranges, auditory and visual perception
    drop off similarly toward the edges.</p>
</div>

<div id="toc_1ch">
  <h3>192kHz considered harmful</h3>

  <p>192kHz digital music files offer no benefits. They're not
    quite neutral either; practical fidelity is slightly worse.
    The ultrasonics are a liability during playback. </p>

  <p>Neither audio transducers nor power amplifiers are free of
    distortion, and distortion tends to increase rapidly at the
    lowest and highest frequencies.  If the same transducer
    reproduces ultrasonics along with audible content, any
    nonlinearity will shift some of the ultrasonic content down
    into the audible range as an uncontrolled spray of
    intermodulation distortion products covering the entire
    audible spectrum.  Nonlinearity in a power amplifier will
    produce the same effect. The effect is very slight, but
    listening tests have confirmed that both effects can be
    audible.</p>

<img src="https://www.xiph.org/~xiphmont/demo/intermod.png"/>
<div class="caption">
<p>Above: Illustration of distortion products resulting
from intermodulation of a 30kHz and a 33kHz tone in a
theoretical amplifier with a nonvarying total harmonic
distortion (THD) of about .09%.  Distortion products
appear throughout the spectrum, including at frequencies
lower than either tone.</p>

<p>Inaudible ultrasonics contribute to intermodulation
distortion in the audible range (light blue area).
Systems not designed to reproduce ultrasonics typically
have much higher levels of distortion above 20kHz, further
contributing to intermodulation.  Widening a design's
frequency range to account for ultrasonics requires
compromises that decrease noise and distortion performance
within the audible spectrum.  Either way, unneccessary
reproduction of ultrasonic content diminishes
performance.</p>
</div>

  <p>There are a few ways to avoid the extra distortion:</p>
  <ol>
    <li>
<p>A dedicated ultrasonic-only speaker, amplifier, and 
crossover stage to separate and independently reproduce
the ultrasonics you can't hear, just so they don't mess
up the sounds you can.</p>
    </li>
<li><p>Amplifiers and transducers designed for wider
frequency reproduction, so ultrasonics don't cause audible
intermodulation.  Given equal expense and complexity, this
additional frequency range must come at the cost of some
performance reduction in the audible portion of the
spectrum.</p></li>
    <li>
<p>Speakers and amplifiers carefully designed not to reproduce
ultrasonics anyway.</p>
    </li>
    <li>
<p>Not encoding such a wide frequency range to begin with. You can't
and won't have ultrasonic intermodulation distortion in the audible
band if there's no ultrasonic content.</p>
    </li>
  </ol>
  <p>They all amount to the same thing, but only 4) makes any sense.</p>

<p>If you're curious about the performance of your own system,
the following samples contain a 30kHz and a 33kHz tone in a
24/96 WAV file, a longer version in a FLAC, some tri-tone
warbles, and a normal song clip shifted up by 24kHz so that
it's entirely in the ultrasonic range from 24kHz to 46kHz:</p>


<p>Assuming your system is actually capable of full 96kHz
playback [<a href="#foot6">6</a>], the above files should be
completely silent with no audible noises, tones, whistles,
clicks, or other sounds.  If you hear anything, your system
has a nonlinearity causing audible intermodulation of the
ultrasonics.  Be careful when increasing volume; running into
digital or analog clipping, even soft clipping, will suddenly
cause loud intermodulation tones.</p>

<p>In summary, it's not certain that intermodulation from
ultrasonics will be audible on a given system.  The added
distortion could be insignificant or it could be noticable.
Either way, ultrasonic content is never a benefit, and on
plenty of systems it will audibly hurt fidelity.  On the
systems it doesn't hurt, the cost and complexity of handling
ultrasonics could have been saved, or spent on improved audible range
performance instead.</p>
</div>

<div id="toc_sfam">
  <h3>Sampling fallacies and misconceptions</h3>

  <p>Sampling theory is often unintuitive without a signal processing
    background. It's not surprising most people, even brilliant PhDs in
    other fields, routinely misunderstand it.  It's also not
    surprising many people don't even realize they have it wrong.</p>

  <img src="https://www.xiph.org/~xiphmont/demo/jaggy.png"/>
<div class="caption">
<p>Above: Sampled signals are often depicted as a rough
stairstep (red) that seems a poor approximation of the
original signal.  However, the representation is
mathematically exact and the signal recovers the exact
smooth shape of the original (blue) when converted back to
analog.</p>
</div>

  <p>The most common misconception is that sampling is
    fundamentally rough and lossy.  A sampled signal is often
    depicted as a jagged, hard-cornered stair-step facsimile of
    the original perfectly smooth waveform.  If this is how you
    envision sampling working, you may believe that the faster
    the sampling rate (and more bits per sample), the finer the
    stair-step and the closer the approximation will be.  The
    digital signal would sound closer and closer to the original
    analog signal as sampling rate approaches infinity.</p>

  <p>Similarly, many non-DSP people would look at the following:</p>

  <img src="https://www.xiph.org/~xiphmont/demo/jaggy2.png"/>

  <p>And say, "Ugh!&amp;quot It might appear that a sampled
signal represents higher frequency analog waveforms
badly. Or, that as audio frequency increases, the sampled
quality falls and frequency response falls off, or becomes
sensitive to input phase.</p>

  <p>Looks are deceiving. These beliefs are incorrect!</p>

<p class="aside">
<span class="caption">added 2013-04-04:</span><br/> As a
followup to all the mail I got about digital waveforms and
stairsteps, I demonstrate actual digital behavior on real
equipment in our
video <a href="https://video.xiph.org/vid2.shtml">Digital
Show &amp; Tell</a> so you need not simply take me at my
word here!
</p>

  <p>All signals with content entirely below the Nyquist
    frequency (half the sampling rate) are captured perfectly
    and completely by sampling; an infinite sampling rate is not
    required.  Sampling doesn't affect frequency response or
    phase. The analog signal can be reconstructed losslessly,
    smoothly, and with the exact timing of the original analog
    signal.</p>

<p>So the math is ideal, but what of real world complications?
The most notorious is the band-limiting requirement. Signals
with content over the Nyquist frequency must be lowpassed
before sampling to avoid aliasing distortion; this analog
lowpass is the infamous antialiasing filter.  Antialiasing
can't be ideal in practice, but modern techniques bring it
very close. ...and with that we come to oversampling.</p>

</div>

<div id="toc_o">
  <h3>Oversampling</h3>

  <p>Sampling rates over 48kHz are irrelevant to high fidelity
    audio data, but they are internally essential to several
    modern digital audio techniques.  <em>Oversampling</em> is the
    most relevant example [<a href="#foot7">7</a>].</p>

<p>Oversampling is simple and clever.  You may recall from my
    <a href="http://www.xiph.org/video/vid1.shtml">A Digital
      Media Primer for Geeks</a> that high sampling rates
    provide a great deal more space between the highest
    frequency audio we care about (20kHz) and the Nyquist
    frequency (half the sampling rate).
<a href="http://www.xiph.org/video/vid1.shtml?time=678.1">
This allows for simpler, smoother, more reliable analog
      anti-aliasing filters, and thus higher fidelity</a>.  This
    extra space between 20kHz and the Nyquist frequency is
    essentially just spectral padding for the analog
    filter.</p>

<img src="https://www.xiph.org/~xiphmont/demo/filters.png"/>
<div class="caption">
<p>Above: Whiteboard diagram from <u>A Digital Media
Primer for Geeks</u> illustrating the transition band
      width available for a 48kHz ADC/DAC (left) and a 96kHz
      ADC/DAC (right).</p>
</div>


  <p>That's only half the story.  Because digital filters have
    few of the practical limitations of an analog filter, we can
    complete the anti-aliasing process with greater efficiency
    and precision digitally.  The very high rate raw digital
    signal passes through a digital anti-aliasing filter, which
    has no trouble fitting a transition band into a tight space.
    After this further digital anti-aliasing, the extra padding
    samples are simply thrown away.  Oversampled playback
    approximately works in reverse.</p>

  <p>This means we can use low rate 44.1kHz or 48kHz audio with
    all the fidelity benefits of 192kHz or higher sampling
    (smooth frequency response, low aliasing) and none of the
    drawbacks (ultrasonics that cause intermodulation
    distortion, wasted space).  Nearly all of today's
    analog-to-digital converters (ADCs) and digital-to-analog
    converters (DACs) oversample at very high rates. Few people
    realize this is happening because it's completely automatic
    and hidden.</p>

  <p>ADCs and DACs didn't always transparently
    oversample. Thirty years ago, some recording consoles
    recorded at high sampling rates using only analog filters,
    and production and mastering simply used that high rate
    signal.  The digital anti-aliasing and decimation steps
    (resampling to a lower rate for CDs or DAT) happened in the
    final stages of mastering.  This may well be one of the
    early reasons 96kHz and 192kHz became associated with
    professional music production [<a href="#foot8">8</a>].</p>
</div>

<div id="toc_1bv2b">
  <h3>16 bit vs 24 bit</h3>

  <p>OK, so 192kHz music files make no sense. Covered, done.  What about
    16 bit vs. 24 bit audio?</p>

  <p>It's true that 16 bit linear PCM audio does not quite cover
    the entire theoretical dynamic range of the human ear in
    ideal conditions.  Also, there are (and always will be)
    reasons to use more than 16 bits in recording and
    production.</p>

  <p>None of that is relevant to playback; here 24 bit audio is
    as useless as 192kHz sampling.  The good news is that at
    least 24 bit depth doesn't harm fidelity.  It just doesn't
    help, and also wastes space.</p>
</div>

<div id="toc_rye">
  <h3>Revisiting your ears</h3>

  <p>We've discussed the frequency range of the ear, but what
    about the dynamic range from the softest possible sound to
    the loudest possible sound?</p>

  <p>One way to define absolute dynamic range would be to look
    again at the absolute threshold of hearing and threshold of
    pain curves.  The distance between the highest point on the
    threshold of pain curve and the lowest point on the absolute
    threshold of hearing curve is about 140 decibels for a
    young, healthy listener.  That wouldn't last long though;
    +130dB is loud enough to damage hearing permanently in
    seconds to minutes.  For reference purposes, a jackhammer at
    one meter is only about 100-110dB.</p>

  <p>The absolute threshold of hearing increases with age and
    hearing loss.  Interestingly, the threshold of pain decreases
    with age rather than increasing.  The hair cells of the cochlea
    themselves posses only a fraction of the ear's 140dB range;
    musculature in the ear continuously adjust the amount of sound
    reaching the cochlea by shifting the ossicles, much as the iris
    regulates the amount of light entering the eye
    [<a href="#foot9">9</a>].  This mechanism stiffens with age,
    limiting the ear's dynamic range and reducing the effectiveness
    of its protection mechanisms [<a href="#foot10">10</a>].</p>
</div>

<div id="toc_en">
  <h3>Environmental noise</h3>

  <p>Few people realize how quiet the absolute threshold of
    hearing really is.</p>

  <p>The very quietest perceptible sound is about -8dbSPL
[<a href="#foot11">11</a>].  Using an A-weighted scale, the
hum from a 100 watt incandescent light bulb one meter away
is about 10dBSPL, so about 18dB louder.  The bulb will be
much louder on a dimmer.</p>

  <p>20dBSPL (or 28dB louder than the quietest audible sound) is
    often quoted for an empty broadcasting/recording studio or
    sound isolation room.  This is the baseline for an
    exceptionally quiet environment, and one reason you've
    probably never noticed hearing a light bulb.</p>
</div>

<div id="toc_tdro1b">
  <h3>The dynamic range of 16 bits</h3>

  <p>16 bit linear PCM has a dynamic range of 96dB according to
the most common definition, which calculates dynamic range
as (6*bits)dB.  Many believe that 16 bit audio cannot
represent arbitrary sounds quieter than -96dB. This is
incorrect.</p>

  <p>I have linked to two 16 bit audio files here; one contains
    a 1kHz tone at 0 dB (where 0dB is the loudest possible tone)
    and the other a 1kHz tone at -105dB.</p>


<img src="https://www.xiph.org/~xiphmont/demo/-105dB.png"/>
<div class="caption">
<p>Above: Spectral analysis of a -105dB tone encoded as 16
bit / 48kHz PCM. 16 bit PCM is clearly deeper than 96dB,
else a -105dB tone could not be represented, nor would
it be audible.</p>
</div>

<p>How is it possible to encode this signal, encode it with no
distortion, and encode it well above the noise floor, when
its peak amplitude is one third of a bit?</p>

<p>Part of this puzzle is solved by proper dither, which
renders quantization noise independent of the input signal.
By implication, this means that dithered quantization
introduces no distortion, just uncorrelated noise. That in
turn implies that we can encode signals of arbitrary depth,
even those with peak amplitudes much smaller than one bit
[<a href="#foot12">12</a>].  However, dither doesn't change
the fact that once a signal sinks below the noise floor, it
should effectively disappear.  How is the -105dB tone
still clearly audible above a -96dB noise floor?</p>

<p>The answer: Our -96dB noise floor figure is effectively
wrong; we're using an inappropriate definition of dynamic
range.  (6*bits)dB gives us the RMS noise of the entire
broadband signal, but each hair cell in the ear is sensitive
to only a narrow fraction of the total bandwidth. As each
hair cell hears only a fraction of the total noise floor
energy, the noise floor at that hair cell will be much lower
than the broadband figure of -96dB.</p>

<p>Thus, 16 bit audio can go considerably deeper than 96dB.
With use of shaped dither, which moves quantization noise
energy into frequencies where it's harder to hear, the
effective dynamic range of 16 bit audio reaches 120dB in
practice [<a href="#foot13">13</a>], more than fifteen times
deeper than the 96dB claim.</p>

  <p>120dB is greater than the difference between a mosquito
    somewhere in the same room and a jackhammer a foot
    away.... or the difference between a deserted 'soundproof'
    room and a sound loud enough to cause hearing damage in
    seconds.</p>

  <p>16 bits is enough to store all we can hear, and will
    be enough forever.</p>
</div>

<div id="toc_stnr">
  <h3>Signal-to-noise ratio</h3>

  <p>It's worth mentioning briefly that the ear's S/N ratio is
    smaller than its absolute dynamic range.  Within a given
    critical band, typical S/N is estimated to only be about 30dB.
    Relative S/N does not reach the full dynamic range even when
    considering widely spaced bands.  This assures that linear
    16 bit PCM offers higher resolution than is actually
    required.</p>

<p>It is also worth mentioning that increasing the bit depth
of the audio representation from 16 to 24 bits does not
increase the perceptible resolution or 'fineness' of the
audio. It only increases the dynamic range, the range
between the softest possible and the loudest possible sound,
by lowering the noise floor. However, a 16-bit noise floor is
already below what we can hear.</p>
</div>

<div id="toc_wd2bm">
  <h3>When does 24 bit matter?</h3>

  <p>Professionals use 24 bit samples in recording and
    production [<a href="#foot14">14</a>] for headroom, noise
    floor, and convenience reasons.</p>

  <p>16 bits is enough to span the real hearing range with room
    to spare. It does not span the entire possible signal range
    of audio equipment.  The primary reason to use 24 bits when
    recording is to prevent mistakes; rather than being careful
    to center 16 bit recording-- risking clipping if you guess
    too high and adding noise if you guess too low-- 24 bits
    allows an operator to set an approximate level and not worry
    too much about it.  Missing the optimal gain setting by a
    few bits has no consequences, and effects that dynamically
    compress the recorded range have a deep floor to work
    with.</p>

  <p>An engineer also requires more than 16 bits during mixing
    and mastering.  Modern work flows may involve literally
    thousands of effects and operations.  The quantization noise
    and noise floor of a 16 bit sample may be undetectable
    during playback, but multiplying that noise by a few
    thousand times eventually becomes noticeable. 24 bits keeps
    the accumulated noise at a very low level.  Once the music
    is ready to distribute, there's no reason to keep more than
    16 bits.</p>
</div>

<div id="toc_lt">
  <h3>Listening tests</h3>

  <p>Understanding is where theory and reality meet. A matter is
    settled only when the two agree.</p>

  <p>Empirical evidence from listening tests backs up the
    assertion that 44.1kHz/16 bit provides highest-possible
    fidelity playback.  There are numerous controlled tests
    confirming this, but I'll plug a recent paper,
<a href="http://www.aes.org/e-lib/browse.cfm?elib=14195"><u>Audibility
of a CD-Standard A/D/A Loop Inserted into High-Resolution
Audio Playback</u></a>, done by local folks here at the
    <a href="http://www.bostonaudiosociety.org/">Boston Audio
      Society</a>.</p>

  <p>Unfortunately, downloading the full paper requires an AES
    membership. However it's been discussed widely in articles
    and on forums, with the authors joining in.  Here's a few
    links:</p>


  <p>This paper presented listeners with a choice between
    high-rate DVD-A/SACD content, chosen by high-definition
    audio advocates to show off high-def's superiority, and that
    same content resampled on the spot down to 16-bit / 44.1kHz
    Compact Disc rate.  The listeners were challenged to
    identify any difference whatsoever between the two using an
    ABX methodology.  BAS conducted the test using high-end
    professional equipment in noise-isolated studio listening
    environments with both amateur and trained professional
    listeners.</p>

  <p>In 554 trials, listeners chose correctly 49.8% of the
    time. In other words, they were guessing.  Not one listener
    throughout the entire test was able to identify which was
    16/44.1 and which was high rate [<a href="#foot15">15</a>],
    and the 16-bit signal wasn't even dithered!</p>

  <p>Another recent study [<a href="#foot16">16</a>] investigated
    the possibility that ultrasonics were audible, as earlier studies had
    suggested.  The test was constructed to maximize the possibility of
    detection by placing the intermodulation products where they'd be most
    audible. It found that the ultrasonic tones were not audible... but
    the intermodulation distortion products introduced by the loudspeakers
    could be.</p>

  <p>This paper inspired a great deal of further research, much
    of it with mixed results.  Some of the ambiguity is
    explained by finding that ultrasonics can induce more
    intermodulation distortion than expected in power amplifiers
    as well. For
    example, <a href="http://www.davidgriesinger.com/intermod.ppt">David
      Griesinger reproduced this experiment</a>
    [<a href="#foot17">17</a>] and found that his loudspeaker
    setup did not introduce audible intermodulation distortion
    from ultrasonics, but his stereo amplifier did.</p>
</div>

<div id="toc_cl">
<h3>Caveat Lector</h3>

  <p>It's important not to cherry-pick individual papers or
    'expert commentary' out of context or from self-interested
    sources. Not all papers agree completely with these results
    (and a few disagree in large part), so it's easy to find
    minority opinions that appear to vindicate every imaginable
    conclusion.
<em>Regardless, the papers and links above are
      representative of the vast weight and breadth of the
      experimental record.</em>  No peer-reviewed paper that has
    stood the test of time disagrees substantially with these
    results.  Controversy exists only within the consumer and
    enthusiast audiophile communities.</p>

<p>If anything, the number of ambiguous, inconclusive, and
outright invalid experimental results available through
Google highlights how tricky it is to construct an accurate,
objective test.  The differences researchers look for are
minute; they require rigorous statistical analysis to spot
subconscious choices that escape test subjects' awareness.
That we're likely trying to 'prove' something that doesn't
exist makes it even more difficult. Proving a null
hypothesis is akin to proving the halting problem; you
can't. You can only collect evidence that lends overwhelming
weight.</p>

<p>Despite this, papers that confirm the null hypothesis are
especially strong evidence; confirming inaudibility is far
more experimentally difficult than disputing
it. Undiscovered mistakes in test methodologies and
equipment nearly always produce false positive results (by
accidentally introducing audible differences) rather than
false negatives.</p>

<p>If professional researchers have such a hard time properly
testing for minute, isolated audible differences, you can
imagine how hard it is for amateurs.</p>
</div>

<div id="toc_htisualc">
  <h3>How to [inadvertently] screw up a listening comparison</h3>

  <p>The number one comment I heard from believers in super high
    rate audio was [paraphrasing]: <i>"I've listened to high
      rate audio myself and the improvement is obvious.  Are you
      seriously telling me not to trust my own ears?"</i></p>

  <p>Of course you can trust your ears.  It's brains that are
    gullible.  I don't mean that flippantly; as human beings,
    we're all wired that way.</p>
</div>

<div id="toc_cbtpeadb">
  <h3>Confirmation bias, the placebo effect, and double-blind</h3>

  <p>In any test where a listener can tell two choices apart via
    any means apart from listening, the results will usually be
    what the listener expected in advance; this is
    called <a href="http://en.wikipedia.org/wiki/Confirmation_bias">
      confirmation bias</a> and it's similar to
    the <a href="http://en.wikipedia.org/wiki/Placebo_effect">placebo
      effect</a>.  It means people 'hear' differences because of
    subconscious cues and preferences that have nothing to do
    with the audio, like preferring a more expensive (or more
    attractive) amplifier over a cheaper option.</p>

<p> The human brain is designed to notice patterns and
    differences, even where none exist. This tendency can't just
    be turned off when a person is asked to make objective
    decisions; it's completely subconscious.  Nor can a bias be
    defeated by mere skepticism. <em>Controlled experimentation
      shows that awareness of confirmation bias can increase
      rather than decreases the effect!</em> A test that doesn't
    carefully eliminate confirmation bias is worthless
    [<a href="#foot18">18</a>].</p>

<p>In <em>single-blind</em> testing, a listener knows nothing
    in advance about the test choices, and receives no feedback
    during the course of the test. Single-blind testing is
    better than casual comparison, but it does not eliminate
    the <a href="http://en.wikipedia.org/wiki/Experimenter%27s_bias">
      experimenter's bias</a>.  The test administrator can easily
    inadvertently influence the test or transfer his own
    subconscious bias to the listener through inadvertent cues
    (eg, "Are you sure that's what you're hearing?", body
    language indicating a 'wrong' choice, hesitating
    inadvertently, etc).  An experimenter's bias has also been
    experimentally proven to influence a test subject's
    results.</p>

  <p><em>Double-blind</em> listening tests are the gold
    standard; in these tests neither the test administrator nor
    the testee have any knowledge of the test contents or
    ongoing results.  Computer-run ABX tests are the most famous
    example, and there are freely available tools for performing
    ABX tests on your own computer[<a href="#foot19">19</a>].
    ABX is considered a minimum bar for a listening test to be
    meaningful; reputable audio forums such
    as <a href="http://www.hydrogenaudio.org/">Hydrogen
      Audio</a>
    often <a href="http://www.hydrogenaudio.org/forums/index.php?showtopic=3974#entry149481">do
      not even allow discussion of listening results unless they
      meet this minimum objectivity requirement</a>
    [<a href="#foot20">20</a>].</p>

<img src="https://www.xiph.org/~xiphmont/demo/squishyball.png"/>
<div class="caption">
<p>Above: Squishyball, a simple command-line ABX tool, running in an xterm.</p>
</div>

  <p>I personally don't do any quality comparison tests during
    development, no matter how casual, without an ABX
    tool. Science is science, no slacking.</p>
</div>

<div id="toc_lt">
  <h3>Loudness tricks</h3>

<p>The human ear can consciously discriminate amplitude
differences of about 1dB, and experiments show subconscious
awareness of amplitude differences under .2dB.  Humans
almost universally consider louder audio to sound better,
and .2dB is enough to establish this preference. Any
comparison that fails to carefully amplitude-match the
choices will see the louder choice preferred, even if the
amplitude difference is too small to consciously notice.
Stereo salesmen have known this trick for a long time.</p>

<p>The professional testing standard is to match sources to
within .1dB or better.  This often requires use of an
oscilloscope or signal analyzer. Guessing by turning the
knobs until two sources sound about the same is not good
enough.</p>
</div>

<div id="toc_c">
<h3>Clipping</h3>

<p>Clipping is another easy mistake, sometimes obvious only in
retrospect.  Even a few clipped samples or their aftereffects
are easy to hear compared to an unclipped signal.</p>

  <p>The danger of clipping is especially pernicious in tests
    that create, resample, or otherwise manipulate digital signals
    on the fly.  Suppose we want to compare the fidelity of 48kHz
    sampling to a 192kHz source sample.  A typical way is to
    downsample from 192kHz to 48kHz, upsample it back to 192kHz,
    and then compare it to the original 192kHz sample in an ABX
    test [<a href="#foot21">21</a>].  This arrangement allows us
    to eliminate any possibility of equipment variation or sample
    switching influencing the results; we can use the same DAC to
    play both samples and switch between without any hardware mode
    changes.</p>

  <p>Unfortunately, most samples are mastered to use the full
    digital range. Naive resampling can and often will clip
    occasionally.  It is necessary to either monitor for clipping
    (and discard clipped audio) or avoid clipping via some other
    means such as attenuation.</p>
</div>

<div id="toc_dmdm">
  <h3>Different media, different master</h3>

  <p>I've run across a few articles and blog posts that declare
    the virtues of 24 bit or 96/192kHz by comparing a CD to an
    audio DVD (or SACD) of the 'same' recording.  This comparison
    is invalid; the masters are usually different.</p>
</div>

<div id="toc_ic">
  <h3>Inadvertent cues</h3>

  <p>Inadvertant audible cues are almost inescapable in older
    analog and hybrid digital/analog testing setups. Purely
    digital testing setups can completely eliminate the problem in
    some forms of testing, but also multiply the potential of
    complex software bugs.  Such limitations and bugs have a long
    history of causing false-positive results in testing
    [<a href="#foot22">22</a>].</p>

  <p><a href="http://www.bostonaudiosociety.org/bas_speaker/abx_testing2.htm"><u>The
Digital Challenge - More on ABX Testing</u></a>, tells a
    fascinating story of a specific listening test conducted in
    1984 to rebut audiophile authorities of the time who asserted
    that CDs were inherently inferior to vinyl.  The article is
    not concerned so much with the results of the test (which I
    suspect you'll be able to guess), but the processes and
    real-world messiness involved in conducting such a test.  For
    example, an error on the part of the testers inadvertantly
    revealed that an invited audiophile expert had not been making
    choices based on audio fidelity, but rather by listening to
    the slightly different clicks produced by the ABX switch's
    analog relays!</p>

  <p>Anecdotes do not replace data, but this story is
    instructive of the ease with which undiscovered flaws can bias
    listening tests.  Some of the audiophile beliefs discussed
    within are also highly entertaining; one hopes that some
    modern examples are considered just as silly 20 years from
    now.</p>
</div>

<div id="toc_ftgn">
  <h2>Finally, the good news</h2>

  <p>What actually works to improve the quality of the digital
    audio to which we're listening?</p>
</div>

<div id="toc_bh">
  <h3>Better headphones</h3>

  <p>The easiest fix isn't digital.  The most dramatic possible
    fidelity improvement for the cost comes from a good pair of
    headphones.  Over-ear, in ear, open or closed, it doesn't much
    matter. They don't even need to be expensive, though expensive
    headphones can be worth the money.</p>

  <p>Keep in mind that some headphones are expensive because
    they're well made, durable and sound great.  Others are
    expensive because they're $20 headphones under a several
    hundred dollar layer of styling, brand name, and marketing.  I
    won't make specfic recommendations here, but I will say you're
    not likely to find good headphones in a big box store, even if
    it specializes in electronics or music.  As in all other
    aspects of consumer hi-fi, do your research (and caveat
    emptor).</p>

</div>

<div id="toc_lf">
  <h3>Lossless formats</h3>

  <p>It's true enough that a properly encoded Ogg file (or MP3,
    or AAC file) will be indistinguishable from the original at a
    moderate bitrate.</p>

  <p>But what of badly encoded files?</p>

  <p>Twenty years ago, all mp3 encoders were really bad by
    today's standards.  Plenty of these old, bad encoders are
    still in use, presumably because the licenses are cheaper and
    most people can't tell or don't care about the difference
    anyway.  Why would any company spend money to fix what it's
    completely unaware is broken?</p>

  <p>Moving to a newer format
    like <a href="http://www.vorbis.com">Vorbis</a> or AAC doesn't
    necessarily help.  For example, many companies and individuals
    used (and still
    use) <a href="http://xiphmont.livejournal.com/51160.html">FFmpeg's
      very-low-quality built-in Vorbis encoder</a> because it was
    the default in FFmpeg and they were unaware how bad it
    was. AAC has an even longer history of widely-deployed,
    low-quality encoders; all mainstream lossy formats do.</p>

  <p>Lossless formats
    like <a href="http://flac.sourceforge.net/">FLAC</a> avoid any
    possibility of damaging audio fidelity
    [<a href="#foot23">23</a>] with a poor quality lossy encoder,
    or even by a good lossy encoder used incorrectly.</p>

  <p>A second reason to distribute lossless formats is to avoid
    generational loss.  Each reencode or transcode loses more
    data; even if the first encoding is transparent, it's very
    possible the second will have audible artifacts.  This matters
    to anyone who might want to remix or sample from downloads. It
    especially matters to us codec researchers; we need clean
    audio to work with. </p>
</div>

<div id="toc_bm">
  <h3>Better masters</h3>

  <p>The <a href="http://www.aes.org/e-lib/browse.cfm?elib=14195">
      BAS test I linked earlier</a> mentions as an aside that the
    SACD version of a recording <em>can</em> sound substantially
    better than the CD release. It's not because of increased
    sample rate or depth but because the SACD used a
    higher-quality master.  When bounced to a CD-R, the SACD
    version still sounds as good as the original SACD and
    better than the CD release because the original audio used to
    make the SACD was better. Good production and mastering
    obviously contribute to the final quality of the music
    [<a href="#foot24">24</a>].</p>

  <p>The recent coverage of 'Mastered for iTunes' and similar
    initiatives from other industry labels is somewhat
    encouraging.  What remains to be seen is whether or not Apple
    and the others actually 'get it' or if this is merely a hook
    for selling consumers yet another, more expensive copy of
    music they already own.</p>
</div>

<div id="toc_su">
  <h3>Surround</h3>

<p>Another possible 'sales hook', one I'd enthusiastically buy
into myself, is surround recordings.  Unfortunately, there's
some technical peril here.</p>

<p>Old-style discrete surround with many channels (5.1, 7.1,
etc) is a technical relic dating back to the theaters of the
1960s.  It is inefficient, using more channels than competing
systems.  The surround image is limited, and tends to collapse
toward the nearer speakers when a listener sits or shifts out of
position.</p>

<p>We can represent and encode excellent and robust
localization with systems like Ambisonics.  The problems are
the cost of equipment for reproduction and the fact that
something encoded for a natural soundfield both sounds bad
when mixed down to stereo, and can't be created artificially
in a convincing way.  It's hard to fake ambisonics or
holographic audio, sort of like how 3D video always seems to
degenerate into a gaudy gimmick that reliably makes 5% of
the population motion sick.</p>

<p>Binaural audio is similarly difficult. You can't simulate
it because it works slightly differently in every person.
It's a learned skill tuned to the self-assembling system of
the pinnae, ear canals, and neural processing, and it never
assembles exactly the same way in any two individuals.
People also subconsciously shift their heads to enhance
localization, and can't localize well unless they do.
That's something that can't be captured in a binaural
recording, though it can to an extent in fixed surround.</p>

<p>These are hardly impossible technical hurdles. Discrete
surround has a proven following in the marketplace, and I'm
personally especially excited by the possibilities offered
by Ambisonics.</p>

</div>

<div id="toc_outro">
  <h2>Outro</h2>

<blockquote>
"I never did care for music much.<br/>
It's the high fidelity!"<br/>
     —Flanders &amp; Swann, <u>A Song of Reproduction</u>
</blockquote>

<p>The point is enjoying the music, right?  Modern playback
fidelity is incomprehensibly better than the already excellent
analog systems available a generation ago.  Is the logical
extreme any more than just
another <a href="http://www.youtube.com/watch?v=M3w1_E1V46M">first
world problem</a>? Perhaps, but bad mixes and
encodings <em>do</em> bother me; they distract me from the
music, and I'm probably not alone.</p>

<p>Why push back against 24/192?  Because it's a solution to a
problem that doesn't exist, a business model based on
willful ignorance and scamming people.  The more that
pseudoscience goes unchecked in the world at large, the
harder it is for truth to overcome truthiness... even if
this is a small and relatively insignificant example.</p>

<blockquote>
"For me, it is far better to grasp the Universe as it really
is than to persist in delusion, however satisfying and
reassuring."
<br/>     —Carl Sagan
</blockquote>
</div>

<div id="toc_more">
<h2>Further reading</h2>
<p>Readers have alerted me to a pair of excellent papers of
which I wasn't aware before beginning my own article. They
tackle many of the same points I do in greater detail.</p>

<ul>
<li>
<p><a href="http://www.meridian.co.uk/ara/coding2.pdf"><u>Coding
High Quality Digital Audio</u></a> by Bob Stuart
of Meridian Audio is beautifully concise despite
its greater length.  Our conclusions differ
somewhat (he takes as given the need for a
slightly wider frequency range and bit depth
without much justification), but the presentation
is clear and easy to follow.  <i>[Edit: I may not
agree with many of Mr. Stuart's other articles,
but I like this one a lot.]</i></p></li>

<li><p><a href="http://lavryengineering.com/pdfs/lavry-sampling-theory.pdf">
<u>Sampling Theory For Digital Audio</u></a> [Updated link 2012-10-04] by Dan
Lavry of Lavry Engineering is another article that several
readers pointed out.  It expands my two pages or so about
sampling, oversampling, and filtering into a more detailed
27 page treatment.  Worry not, there are plenty of graphs,
examples and references.</p></li>
</ul>

<p>Stephane Pigeon
of <a href="http://www.audiocheck.net/">audiocheck.net</a>
wrote to plug the browser-based listening tests featured on
his web site.  The set of tests is relatively small as yet,
but several were directly relevant in the context of this
article. They worked well and I found the quality to be
quite good.</p>
</div>

<div id="toc_fn">
  <h2>Footnotes</h2>

  <ol>


    <li id="foot2">
<p>If it wasn't the most boring
party trick ever, it was pretty close.
    </p></li>

    <li id="foot3">
<p>It's more typical to speak of
visible light as wavelengths measured in nanometers or
angstroms.  I'm using frequency to be consistent with
sound.  They're equivalent, as frequency is just the
inverse of wavelength.</p>
    </li>

    <li id="foot4">
<p>The LED experiment doesn't work
with 'ultraviolet' LEDs, mainly because they're not really
ultraviolet. They're deep enough violet to cause a little
bit of fluorescence, but they're still well within the
visible range. Real ultraviolet LEDs cost anywhere from
$100-$1000 apiece and would cause eye damage if used for
this test.  Consumer grade not-really-UV LEDs also emit
some faint white light in order to appear brighter, so
you'd be able to see them even if the emission peak really
was in the ultraviolet.</p>
    </li>

    <li id="foot5">
      <p>The original version of this article stated that IR
LEDs operate from 300-325THz (about 920-980nm),
wavelengths that are invisible.  Quite a few readers wrote
to say that they could in fact just barely see the LEDs in
some (or all) of their remotes.  Several were kind enough
to let me know which remotes these were, and I was able to
test several on a spectrometer.  Lo and behold, these
remotes were using higher-frequency LEDs operating from
350-380THz (800-850nm), just overlapping the extreme
edge of the visible range.
      </p>
    </li>

    <li id="foot6">
<p>Many systems that cannot play back 96kHz samples will
silently downsample to 48kHz, rather than refuse to play
the file.  In this case, the tones will not be played at
all and playback would be silent no matter how nonlinear
the system is.</p>
</li>

    <li id="foot7">
<p>Oversampling is not the only
application for high sampling rates in signal
processing.  There are a few theoretical advantages to
producing band-limited audio at a high sampling rate
eschewing decimation, even if it is to be downsampled
for distribution.  It's not clear what if any are used
in practice, as the workings of most professional
consoles are trade secrets.</p>
    </li>

    <li id="foot8">
<p>Historical reasoning or not,
there's no question that many professionals today use high
rates because they mistakenly assume that retaining
content beyond 20kHz sounds better, just as consumers
do.</p>
    </li>

    <li id="foot9">
<p>The sensation of eardrums
'uncringing' after turning off loud music is quite
real!</p>
    </li>

    <li id="foot10">
<p>Some nice diagrams can be found
at the HyperPhysics site:<br/>
<a href="http://hyperphysics.phy-astr.gsu.edu/hbase/sound/protect.html#c1">http://hyperphysics.phy-astr.gsu.edu/hbase/sound/protect.html#c1</a></p>
    </li>

    <li id="foot11">
<p>20ÂµPa is commonly defined to be
0dB for auditory measurement purposes; it is approximately
equal to the threshold of hearing at 1kHz.  The ear is as
much as 8dB more sensitive between 2 and 4kHz however.</p>
    </li>

<li id="foot12">
<p>The following paper has the best explanation of dither
that I've run across.  Although it's about image dither,
the first half covers the theory and practice of dither in
audio before extending its use into images:</p>
<p>Cameron Nicklaus Christou, 
<a href="http://uwspace.uwaterloo.ca/bitstream/10012/3867/1/thesis.pdf">
<u>Optimal Dither and Noise Shaping
in Image Processing</u></a>
</p>
</li>

    <li id="foot13">
<p>DSP engineers may point out, as
one of my own smart-alec compatriots did, that 16 bit
audio has a theoretically infinite dynamic range for a
pure tone if you're allowed to use an infinite Fourier
transform to extract it; this concept is very important to
radio astronomy.</p>
      <p>Although the ear works not entirely unlike a Fourier transform, its
resolution is relatively limited.  This places a limit on the maximum
practical dynamic depth of 16 bit audio signals.</p>
    </li>

    <li id="foot14">
<p>Production increasingly uses 32
bit float, both because it's very convenient on modern
processors, and because it completely eliminates the
possibility of accidental clipping at any point going
undiscovered and ruining a mix.</p>
    </li>

<li id="foot15">
<p>Several readers have wanted to know how, if ultrasonics
can cause audible intermodulation distortion, the Meyer
and Moran 2007 test could have produced a null result.</p>
<p>It should be obvious that 'can' and 'sometimes' are not
the same as 'will' and 'always'. Intermodulation
distortion from ultrasonics is a possibility, not a
certainty, in any given system for a given set of
material.  The Meyer and Moran null result indicates that
intermodulation distortion was inaudible on the systems
used during the course of their testing.</p>
<p>Readers are invited to <a href="#toc_intermod">try the
simple ultrasonic intermodulation distortion test
above</a> for a quick check of the intermodulation
potential of their own equipment.</p>
</li>

    <li id="foot16">
<p>Karou and Shogo, <u>Detection of
Threshold for tones above 22kHz</u> (2001). Convention paper
5401 presented at the 110th Convention, May 12-15 2001,
Amsterdam.</p>
    </li>

    <li id="foot17">
<p>Griesinger, <a href="http://www.davidgriesinger.com/intermod.ppt"><u>Perception
            of mid-frequency and high-frequency intermodulation
            distortion in loudspeakers, and its relationship to
            high definition audio</u></a></p>
    </li>

<li id="foot18">
<p>Since publication, several commentators wrote to me with
similar versions of the same anecdote [paraphrased]: "I
once listened to some headphones / amps / recordings
expecting result [A] but was totally surprised to find 
[B] instead!  Confirmation bias is hooey!"</p>
<p>I offer two thoughts.</p>  

<p>First, confirmation bias does not replace all correct
results with incorrect results.  It skews the results in
some uncontrolled direction by an unknown amount.  How
can you tell right or wrong <em>for sure</em> if the
test is rigged by your own subconscious?  Let's say you
expected to hear a large difference but were shocked to
hear a small difference. What if there was actually no
difference at all?  Or, maybe there <em>was</em> a
difference and, being aware of a potential bias, your
well meaning skepticism overcompensated? Or maybe you
were completely right?  Objective testing, such as ABX,
eliminates all this uncertainty.</p>

<p>Second, "So you think you're not biased? Great!
Prove it!"  The value of an objective test lies not only
in its ability to inform one's own understanding, but
also to convince others.  Claims require proof.
Extraordinary claims require extraordinary proof.</p>
</li>

    <li id="foot19">
<p>The easiest tools to use for ABX testing are
probably:</p>

    </li>

    <li id="foot20">
<p>At Hydrogen Audio, the objective testing requirement is
abbreviated <em>TOS8</em> as it's the eighth item in the
      Terms Of Service.</p>
    </li>

    <li id="foot21">
<p>It is commonly assumed that resampling irreparably
damages a signal; this isn't the case.  Unless one makes
an obvious mistake, such as causing clipping, the
downsampled and then upsampled signal will be audibly
indistinguishable from the original.  This is the usual
test used to establish that higher sampling rates are
unneccessary.</p>
    </li>

    <li id="foot22">
<p>It may not be strictly audio related,
but... faster-than-light neutrinos, anyone?</p>
    </li>

    <li id="foot23">
<p><a href="http://www.wired.com/gadgetlab/2012/02/why-neil-young-hates-mp3-and-what-you-can-do-about-it/">Wired
          magazine implies that lossless formats like FLAC
          are not always completely lossless</a>:</p>

      <blockquote>
"Some purists will tell you to skip FLACs altogether
and just buy WAVs. [...] By buying WAVs, you can avoid
the potential data loss incurred when the file is
compressed into a FLAC. This data loss is rare, but it
happens."
      </blockquote>

      <p>This is false.  A lossless compression process never
alters the original data in any way, and FLAC is no
exception.</p>

      <p>In the event that Wired was referring to hardware
corruption of data files (disk failure, memory failure,
sunspots), FLAC and WAV would both be affected. A FLAC
file, however, is checksummed and would detect the
corruption. The FLAC file is also smaller than the WAV,
and so a random corruption would be less likely because
there's less data that could be affected.</p>

    </li>

    <li id="foot24">
<p>
The <a href="http://en.wikipedia.org/wiki/Loudness_war">'Loudness
War'</a> is a commonly cited example of bad mastering
practices in the industry today, though it's not the
only one.  Loudness is also an older phenomenon than the
Wikipedia article leads the reader to believe; as early
as the 1950s, artists and producers pushed for the
loudest possible recordings. Equipment vendors
increasingly researched and marketed new technology to
allow hotter and hotter masters. Advanced vinyl
mastering equipment in the 1970s and 1980s, for example,
tracked and nested groove envelopes when possible in
order to allow higher amplitudes than the groove spacing
would normally permit.</p>

<p>Today's digital technology has allowed loudness to be
pumped up to an absurd level.  It's also provided a
plethora of automatic, highly complex, proprietary DAW
plugins that are deployed en-masse without a wide
understanding of how they work or what they're really
doing.</p>
</li>

  </ol>
</div>

<hr/>

<div class="author">
  <address>—Monty
    (<a href="mailto:monty@xiph.org">monty@xiph.org)</a> March
    1, 2012

<br/><i>last revised March 25, 2012 to add improvements
suggested by readers.
<br/>Edits and corrections made after this date are marked inline, except for spelling errors 
<br/>spotted on Dec 30, 2012 and March 15, 2014, and an extra 'is' removed on April 1, 2013]
</i></address>

</div>

<div class="et">
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<div class="etcenter">
<div class="etcontent">

<p>Monty's articles and demo work are sponsored by Red Hat Emerging Technologies.
<br/>(C) Copyright 2012 Red Hat Inc. and Xiph.Org
<br/>Special thanks to Gregory Maxwell for technical
contributions to this article</p>

</div>
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