Acoustic Bit Correction - Lavry AD122-96 Mark III Operation Manual

Operations Manual
Appendix I: Acoustic Bit Correction™
High quality digital audio sound often incorporates long words (many bits). Both high-resolution A/D
converters (22 bits or more) and signal processing equipment (digital mixers, equalizers, reverb and
more) generate longer words than those supported by standards (such as the 16-bit CD format). A
straightforward word length reduction severely deteriorates the musical fine detail of low level signals
by introducing distortions and a noise floor that is modulated by the signal.
Proper dithering serves to eliminate the distortions and noise modulation effects. The desirability of
such improvement is unquestionable but is coupled with increasing noise floor. Noise shaping is based
on shifting the noise from high sensitivity frequency regions to less sensitive ones. That process is
based on psychoacoustic research. A "stand alone" noise shaper does not improve distortions and noise
modulation problems. Lavry Engineering's Acoustic Bit Correction incorporates both concepts to
reduce distortion and noise modulation and shape the noise psychoacoustically.
Customer feedback and further studies taught us two interesting facts:
a. Various data compression schemes require the dither to have flat frequency response
characteristics.
b. Recording engineers' preference of dither type (high pass or flat) and noise shaping curves
often depends on characteristics of the music involved.
The Model AD122-96 MKIII offers High Pass or Flat Dither type and four noise-shaping curves.
Model AD122-96 MKIII allows the user to use dither with or without noise shaping. Operating noise
shapers without dither is undesirable so the unit automatically shuts the noise shaper off when dither is
off. Using dither (with or without noise shaping) requires user discretion. Ideal data transfer and
processing should retain long word length. A single word length reduction (and therefore dithering and
noise shaping) should take place last.
Figure 1 shows a -100 dB 1 kHz 24 bit tone truncated to 16 bits. The distortion is severe. Figure 2
shows the same tone with high-pass dither only. The distortion is gone, and the tone's level appears at
a true -100 dB. Figure three shows the same tone with Acoustic Bit Correction™ applied. Notice
what the addition of noise shaping to the dither does to the noise floor in the ear's most sensitive mid-
range area (approx. 2kHz to 8 kHz).
AUDIO PRECISION FFT AMP1(dBFS) vs FREQ(Hz)
-100.0
-105.0
-110.0
-115.0
-120.0
-125.0
-130.0
-135.0
-140.0
-145.0
-150.0
20.00 2.02k 4.02k 6.01k 8.01k 10.0k 12.0k 14.0k 16.0k
Fig. 1- truncation only
A number of mathematical and psychoacoustic researchers introduced two concepts for sonic
improvement: dithering and noise shaping. Credit is due to L. Fielder of Dolby Labs for providing the
basis for various noise shaping curves. The reasons for triangle PDF (probability density function)
dither have been explained by S. P. Lipshitz and J. Vanderkooy of the University of Waterloo.
13 NOV 93 20:02:38 AUDIO PRECISION FFT AMP1(dBFS) vs FREQ(Hz)
-100.0
-105.0
-110.0
-115.0
-120.0
-125.0
-130.0
-135.0
-140.0
-145.0
-150.0
20.00 2.02k 4.02k 6.01k 8.01k
18.0k 20.0k
Fig. 2 - high-pass dither
13 NOV 93 20:04:55 AUDIO PRECISION FFT
-100.0
-105.0
-110.0
-115.0
-120.0
-125.0
-130.0
-135.0
-140.0
-145.0
-150.0
10.0k 12.0k 14.0k 16.0k 18.0k 20.0k
20.00
Fig. 3 - Acoustic BitCorrection
High Pass- NS2
AD122-96 MKIII
AMP1(dBFS) vs FREQ(Hz) 13 NOV 93 20:05:49
2.02k 4.02k 6.01k 8.01k 10.0k 12.0k 14.0k 16.0k 18.0k 20.0k
13