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Appendix I
Dithered noise shaping technology has been incorporated into a handful of hardware devices.
While all are based on the same concepts, some perform better than others. After simulating
and listening to all available public domain algorithms, Lavry Engineering came to some
conclusions in forming a basis for Acoustic Bit Correction. The principal conclusions are:
a. The practice of greatly amplifying low level signals to determine triangular flat PDF
(probability density function) dither reveals the effectiveness of distortion and noise
modulation elimination. This practice yields misleading results when testing unflattened
dithers and/or noise shapers. It conflicts directly with L. Fielder's findings showing
completely different threshold delectability curves for quiet and loud levels. Noise
shaping listening tests must be done at "reasonable" volume levels.
b. Given the above requirement, our listening tests concluded a strong preference for
"triangle high pass" dither (this dither is produced by simultaneously adding a new
random number and subtracting the previous value). Such dither is frequency-shaped to
carry more high frequency energy (the energy content at low frequencies is minimal).
c. Listening tests revealed a preference for smoothly varying noise-shaping curves. Peaks
and notches seem to irritate the listener (admittedly while turning the volume up). In
addition, despite the temptation to optimize the noise shaping curve to the average
listener's hearing threshold, given a significant variation from listener to listener requires
reasonable compromises in tailoring such a curve. In other words, smooth the curve.
The improvements offered by dither and noise shaping vary with source material and final word
length. An A/B/X test at 16 bit level requires a quiet environment and low level (loudness)
audio. The listener must resist the temptation to turn the volume up to unreasonable levels.
The practice of truncating to short word length (8-12 bits) should be avoided, although the
Model 3000S performs this admirably. The ideal noise-shaping curve may be irritating at loud
levels. (Model 3000S's 8 bit noise shaping curves are based on a different curve).
Lavry Engineering's listening tests were based on test tones and repeating loops of quiet
passages of various material (mostly classical music) with flat amplifier response. Listening to
test tones was straightforward: we used the Model 3000S test tone generator mode switching
the Acoustic Bit Correction on and off. The frequency and amplitude programmability was very
useful.
Listening to music required 18 to 20 bit material. Distortion present at the 16 bit input word can
not be removed by dither. The algorithm was functional (to a lesser degree) from a noise
shaping standpoint, but distortion removal did not take place (the data distortion content due to
the previous truncation was interpreted as signal). Acoustic Bit Correction is aimed at
correcting truncation problems associated with the shortening of word length.
20
LE3000S
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