mere mention of "truncation" seems to instill fear in the hearts of
audio men and women. Dither sounds a bit more genteel, but I can't seem
to get it out of my head that it rhymes with "whatever," and
of course spoken with a valley accent. Oh well.
That doesn't undermine the seriousness of a discussion of truncation
vs. dither. Oh no. But
after all is said and done, after you slice it and/or dice it and the
smoke clears, you're going to be left with a decision to make that is
mostly subjective in nature. Sorry about that.
The Playing Field
When an audio file or digital recording is created with a particular
bit resolution and is transferred to another device or application with
a lower bit resolution, this change in bit
width must be resolved in some way. Do nothing, and the receiving
device will truncate, or chop off the bits that it can not handle. In
other words, if your recording has been made at 24 bits and you're transferring
to a device that can only handle 16 bits, the "extra" 8 bits
get chopped off and are lost in oblivion. Gone. Kaput. Buh bye.
Only if your audio
device or software has an option to add dither can the decision to choose
dither be made. You may be able to add dither after the fact, alleviating
the need to make a permanent irreversible decision on the spot. Either
way, the 8 dits are lost. Dither is a way of compensating for the truncation's
inevitable increase in audible quantization error before the truncation
A Truncation Situation
The implication? Bit depth represents dynamic
range in digital recordings. A recording that is done in 24 bits will
have a greater dynamic range than a recording done in 16 bits. Dynamic
range is the difference, from softest to loudest, that a recording device
is capable of. Dynamic range specs will include system noise, so that
signal to noise ratio will always be less than dynamic range. In other
words, s/n is a subset of dynamic range.
Analog systems have
some inherent system noise. Digital systems introduce no system noise,
so where the noise factor enters in is with something we call quantization
error. Quantization, simply put, is the amplitude component of the digitized
error occurs when the program signal becomes so quiet that the lowest,
or "least significant bit," can no longer adequately process
the audio. There is dynamic range between the 0 and the 1, but 0 and
1 can't express that -- it's one or the other (no pun intended). The
resultant square wave is the equivalent of a digital noise floor, which
in effect is a random array of "artifacts", often expressed
as "granulation noise," since the perception of these digital
artifacts is that the audio has attained a grainy quality. Understand
that this happens at the quietest level -- when you make your final
Truncation vs. Dither decision, you're going to have to listen with
your critical ears.
In terms of digital noise, the greater the bit depth, the quieter
the digital noise. Each added bit reduces the audible level of quantization
error by 6dB, since each bit adds 6dB to the dynamic range. This is
a substantial reduction in perceived noise when the bit depth is maintained
throughout the recording, but if audio CD is the ultimate goal, 16 bits
is the limit in the digital domain.
As stated, truncation is inevitable in a transfer to a lower bit depth.
The real question is not so much truncation vs. dither, but rather,
doing nothing vs. adding dither.
To Dither or Not to Dither
Quantization error is a digital reality, and also applies to the analog
to digital conversion of audio. Since the answer to "what came
first, the A/D or the D/D?" is "A/D," dither was initially
an analog white noise that was added to the signal to displace the audio
signal level above the least significant bit. So we're actually adding
analog noise to prevent digital noise? Yup, that's right.
The Fletcher-Munson curves show higher responsiveness to sounds
in the 3 kHz to 4 kHz area. Intensity in level varies while the
ear perceives equal volume at all frequencies (Click image for
As insane as that
might sound, you might consider this analog noise to be a more known
quantity that those nasty random artifacts, and in that sense more desirable.
A little system noise or tape hiss is something that our ears have been
used to for years, and perhaps far more digestible than the unknown.
Also, quantization error is considered distortion, whereas a noise is
just a noise (but is a sigh just a sigh?). In the realm of digital to
digital, this analog noise is created or synthesized using an algorithm,
generally specific to the device or software.
a few permutations to this dither signal. "Noise shaping"
adds an EQ filter to the white noise signal so that the noise is shaped
according to the frequencies that the ear is most sensitive to (as in
the Fletcher-Munson curve, or some more modern variation). This noise
shaping can also be offered by some systems without dither, and rather
is calculated as a function of the errors introduced by truncation.
If you have the time and the luxury, try transferring your digital audio
first by merely truncating. Then, if your audio source device provides
these options, transfer using dither, dither with noise shaping, and
noise shaping without dither. Listen. Is there a result that sounds
most desireable to you?
This may be the bottom
line. There are other considerations, however, that would influence
your decision not only whether or not you should, but when to do so.
Quantization error occurs at the softest or quietest points in the recorded
signal, so the content of this recorded signal is something to consider.
If are no quiet moments in the program, there may be no audible artifacts
until the end of the fade-out, for example.
Also, if your final
objective is to pre-master your CD with a variety of cuts, cross fading
two cuts that already have dither added can have a double-dither effect
(to coin a phrase). Adding dither to the final edited version would
be a better choice. Noise shaping may be undesireable if your final
product is a sound that may wind up in a sampler, subject to pitch shifting.
The audio quality of the noise shaping would then be subject to this
pitch shifting. Lastly, noise shaping without dither, which functions
in relation to the quantization error, can have varying results dependent
on the dynamics of the program material. A "pumping" effect
may result from unusual modulation in the quantization.
The choices are yours. Hopefully, the choices that you now make are
a "bit" more informed. Pun intended.
JD Mars is the
Producer of DigitalProSound.com