Word Length Reduction
The AD2404-96 and the SONIC AD2K+ are equipped with two state of the art world length reduction
systems: The Benchmark NN™ (Near Nyquist) system, and the Benchmark NS™ (Noise
Shaped) system. Unlike most competitive systems, the Benchmark NS™ system is
based upon the most current psycho-acoustic models. Furthermore, both Benchmark
systems are unique in that they were optimized while factoring in the noise
contribution of the recording environment.
The Benchmark NN™ and NS™ word length reduction systems
are the result of a cooperative effort between Benchmark Media Systems,
Inc., and the Audio Lab at the University of Waterloo in Ontario Canada.
We would especially like to thank Stanley P. Lipshitz Ph.D., John
Vanderkooy Ph.D., and Robert A. Wannamaker Ph.D. for their pioneering
research and for their significant contributions to the early stages of
this project. Special thanks are also in order to Robert Wannamaker for
creating and modifying the mathematical algorithms and filter
coefficients which are at the core of the Benchmark NN™ and
NS™ systems.
We have taken a new approach by optimizing word length reduction for
use in three different levels of ambient noise. The ambient noise in a
live recording situation is very different from ambient noise level in a
studio, and neither can be considered insignificant when reducing 24-bit
recording to 16-bits. All prior word-length-reduction systems ignored
the effects of this ambient noise and were optimized for noise-free
input signals. The Benchmark NN™ and NS™ systems were
mathematically optimized while calculating the effects of ambient noise.
Three levels of input noise were used and three different curves were
produced. NN3™ and NS3™ represent optimal solutions where
system noise is limited only by the 24-bit A/D conversion process.
NN2™ and NS2™ represent optimal solutions where the ambient
noise is 6 dB higher than the converter noise floor. NN1™ and
NS1™ represent optimal solutions where the ambient noise is 12 dB
higher than the converter noise floor. When properly used, this
optimization can improve the dynamic range of a finished 16-bit
recording by several decibels.
What is the Appropriate Setting to Use?
In general, NN3™ or NS3™ should be used for extremely
low-noise studio recording environments, NN2™ or NS2™ should
be used for live recording, while NN1™ and NS1™ should be
reserved for noisy recording environments. The greatest possible dynamic
range will be achieved when the proper function is selected.
The choice of NN™ versus NS™ is mostly a matter of
preference while the choice of curves 1, 2 or 3 is largely dependent
upon the dynamic range of the source material. Here are a few general
guidelines that should be followed:
1. If very high playback levels are anticipated, (i.e. playback gain will be
high enough for the noise floor to be heard), use the NN™ settings as these
produce natural sounding noise floors.
2. If the source material has been subjected to a prior 16-bit word length
reduction process, select NN3™ for subsequent processing.
3. If low to moderately high playback levels are anticipated (i.e. playback gain
will be low enough that the noise floor is inaudible), use the NS™ settings as
these yield the greatest dynamic range.
4. The NS™ functions can achieve lower psycho-acoustic noise levels than the
corresponding NN™ functions.
5. When in doubt concerning ambient noise levels, use a higher numbered
function.
6. When in doubt concerning anticipated playback levels, use an NN™ function.
7. When totally in doubt, use NN3™ and then try other settings as you gain
familiarity with the system.
8. The mathematically inclined can use the charts in appendix 1 to calculate
dynamic range, and the audibility of the various NN™ and NS™ functions.
While each of the Benchmark NN™ and NS™ processes have
been optimized for certain levels of ambient noise contribution, it is
important to point out that the processes do not rely on this noise for
dithering. TPDF dither is always applied to the 24-bit signal prior to
word length reduction. The NN™ and NS™ processes are always
fully dithered to insure full randomization of the quantization noise,
and to insure that the quantization noise is de-correlated from the
audio signal. Any of the NN™ and NS™ processes can be used
on any source without the risk of distortion that can result from an
inadequate dither process. Furthermore, the NN™ and NS™
processes can be used in cascade without any ill effects other than a
slight increase in the noise floor.
What Happens when NN™ or NS™ Processes are used in Cascade?
Every time the number of generations is doubled, the noise-floor will
increase by 3 dB. For example, two passes through a NN™ or
NS™ process will reduce the dynamic range by 3 dB (as compared to
the results obtained after only one pass). After 4 passes through a
NN™ or NS™ process, the dynamic range will have decreased by
an additional 3 dB for a total of 6 dB. And after the 8th pass, the
dynamic range will have decreased by a total of 9 dB. The charts below
show the results of cascaded processes. Please note that the noise of a
first generation 16-bit process is at or slightly above the threshold of
audibility. Multiple passes through 16-bit word length reduction
processes will raise the noise floor above the threshold of audibility,
and should therefore be avoided when possible. If there is no
alternative, and 16-bit word length reduction must be cascaded,
NN3™ or NS3™ should be used for the first process, while
NN3™ should be used for all subsequent processing steps.
Why is NN3™ Recommended for Cascaded 16-bit Processes?
The NN™ processes produce a noise floor that sounds very much
like white noise. On the other hand, the NS™ processes produce a
colored noise floor and are best suited for applications where this
noise floor is below the threshold of hearing. The advantage of using a
NS™ process is that the dither noise will remain inaudible at
higher playback levels, than if a corresponding NN™ process were
used. IF the NS™ dither noise exceeds the threshold of audibility
(due to cascaded processing or very high playback levels), the NS™
process will yield better results. The reason for this is that the
natural sound of white noise is less distracting than colored noise even
when the colored noise is at a slightly lower level.
Dither
Reducing a 20-bit or 24-bit audio to 16-bits always requires the addition of
dither noise. Failure to add a source of dither prior to each truncation process
will create distortion in the output. Remember that dither noise is of a very
low level, and remains inaudible or nearly inaudible until the gain of a
playback system is made extremely high.
A good word length reduction system will remain inaudible during quiet portions
of a recording, even when the playback system is adjusted to achieve high peak
sound pressure levels. Dither that is audible will tend to mask musical details.
This masking effect of dither increases as the audibility of the dither
increases.
While it is desirable to keep the dither inaudible, it is also necessary to
apply enough dither to fully randomize the noise added by word length reduction.
The Benchmark NN™ and NS™ systems provide full randomization and are carefully
designed for minimum audibility. Benchmark word length reduction systems never
add distortion. They only add very low-level random noise. Unlike other noise
reduction systems, this low-level noise is not effected by the musical signal.
TPDF dither (white noise dither), will always be more audible than the dither
noise produced by a well-designed word length reduction system. The Benchmark
16-bit 44.1-kHz NS3™ system yields a whopping 14 dB improvement over 16-bit
TPDF. 16-bit 44.1 kHz NS3™ will remain inaudible unless play back levels are
adjusted such that a 0 dBFS signal exceeds 107 dB SPL. In contrast, 16-bit TPDF
dither will become audible when playback levels are adjusted such that a 0 dBFS
signal exceeds 93 dB SPL. Please note, at these gain settings, the dither will
only begin to be audible when there is a point of full silence in the recording,
and then only when the room itself is also sufficiently quiet. Dither noise is
never audible in the presence of a 0 dBFS signal.
16-bit NS™ Word Length Reduction Curves for 44.1 kHz
The 16-bit 44.1-kHz NS3™ noise shaping, shown above, provides
a 14-dB improvement over 16-bit TPDF, in terms of noise audibility.
16-bit NS™ Word Length Reduction Curves for 48 kHz
The 16-bit, 48-kHz NS3™ has a 17-dB advantage over 16-bit
TPDF.
16-bit NS™ Word Length Reduction Curves for 96 kHz
The 16-bit, 96-kHz NS3™ provides a 28-dB improvement over 16-bit TPDF.
Will Dither Noise Damage My Speakers?
Please note that nothing bad happens when the gain of a playback
system is increased enough to hear dither noise. Dither will not blow
out your speakers, unless possibly someone inadvertently turns on an
audio source while the amplifier is in this high gain state! Remember
that dither is an extremely low-level signal (much like tape hiss, only
of a much lower level).
How is the Performance of a Word Length Reduction System Measured?
The audibility of dither can be expressed in terms of "F-Weighted Noise Power".
The F-weighting function is derived from measurements of the ear's sensitivity
to very low level signals. At 16-bits, 44.1-kHz the F-weighted noise power of
TPDF dither is -93.3 dBFS and the F-weighted noise power of NS3™ is -107.5 dBFS.
In other words, 16-bit TPDF dither provides a 93.3 dB noise-free dynamic range,
while NS3™ provides a much greater 107.5 dB noise-free dynamic range.
All word length reduction systems add noise to the audio, it is a
law of mathematics. However, the noise can be placed anywhere within the
bandwidth of the digital system. If the noise is evenly spread out over
the entire bandwidth (as it is with TPDF dither), the system will yield
the lowest possible unweighted noise when measured on an audio analyzer.
But, a uniform noise distribution is not the best solution from an
audibility standpoint. Our ears are not equally sensitive to all
frequencies within the 0 to 22.05 kHz bandwidth of a 44.1 kHz digital
system. The audibility of the added noise is greatly reduced when it is
concentrated at frequencies where our ears are least sensitive.
Near-Nyquist systems reduce noise audibility by concentrating most of
the noise energy between 18 kHz and the Nyquist frequency (1/2 of the
sample rate) while maintaining a relatively flat and natural sounding
noise floor below 16 kHz. Noise-Shaped systems attempt to achieve the
greatest possible noise improvement by distributing the noise in a
function that is the inverse of the ear’s sensitivity. TPDF will
read the lowest on the meters, but will always sound louder than a good
word-length-reduction system. Don’t let the meters fool you!
Remember, unlike your ears, most meters “hear” equally well
at all frequencies.
Avoid Truncation without Dither
There are numerous potential sources of noise within an A/D
converter. These may include thermal noise, noise from a delta sigma
modulation process, cross talk, clock feed-through, etc. None of these
sources of noise are added intentionally, but in many cases these noise
sources may be of a high enough level to allow truncation without the
addition of a dither signal. For example, many 20-bit A/D converters
have enough self-noise to allow their outputs to be truncated to 16-bits
without ill effects. Similarly many 24-bit converters have enough
self-noise to allow truncation to 20-bits. Many recording engineers have
discovered that they can truncate the outputs of their A/D converters
without causing distortion. Do not try this with the AD2404-96 or the
AD2K+! These are very quiet 24-bit converters, and it do not
have enough self-noise to provide adequate dither for truncation to
20-bits (nor 16-bits). For this reason it is imperative to use one of
the 20-bit output settings when feeding 20-bit devices, and one of the
16-bit settings when feeding 16-bit devices. Remember that truncation
without adequate dither will cause distortion.
One additional caution concerning truncation: Each world length
reduction process requires a new source of dither noise. For example,
consider a signal starts out at 24-bits and is dithered down 16-bits.
Lets suppose we take this 16-bit signal and feed it into a 24-bit
digital audio workstation, and apply a minor gain change or a touch of
EQ. This 16-bit signal has now become a 24-bit signal inside the
workstation. If the final product is going to be 16-bits, a second 24 to
16-bit word length reduction process must be applied. Some have assumed
that it is not necessary to add dither in the second process because it
was already added in the first process. However, truncation to 16-bits
at the output of the workstation will add distortion to the audio.
Instead, use the digital to digital word length reduction feature in the
AD2404-96 and the AD2K+ to reduce the word length back to 16-bits.
Remember every word length reduction process requires a new source of
dither noise.
Digital Word Length Increases when a Signal is Processed
Every time a digital signal is processed, its word length increases.
"Processing" includes even simple operations such as level changes and the
mixing of two signals. Word lengths expand dramatically when more complex
operations such as equalization, sample rate conversion, and effects processing
are applied. Long word lengths created by digital signal processing must be
shortened before they can be re-recorded or sent to a DAC for monitoring.
Every time a digital word length is shortened, a new source of noise
must be added to the signal prior to truncation. Dither noise that was
applied for one truncation operation is not useful for dithering a
subsequent truncation operation. Failure to add a new source of noise
prior to each truncation process will create distortion. The self-noise
of an A/D converter, the noise of the mic-pre, and ambient noise. Again,
every word length reduction process requires a new source of dither
noise.
Additional Information
An article on dither and a web audio demonstration by Christopher M. Hicks of
the effects of three forms of word length reduction can be found at
http://www.mtsu.edu/~dsmitche/rim420/reading/rim420_Dither.html
Demonstrated are; rounding, TPDF dither and simple noise shaped dither. This is
an excellent and graphic demonstration of these issues.
Another educational overview of digital audio and dither by Christopher M. Hicks may be accessed at
http://www.benchmarkmedia.com/appnotes-d/hicks1994.htm
A more in depth educational paper by Mr. Hicks in PDF form is found at
http://www.benchmarkmedia.com/appnotes-d/ditherap.pdf
Another dither article entitled "The Secrets of Dither" authored by Bob Katz,
can be found at http://www.digido.com/ditheressay.html
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