Processing for compression - The MPEG - page 100

Figure 3.72 shows a simple digital dithering system for shortening sample wordlength. The output of a two's
complement pseudo-random sequence generator of appropriate wordlength is added to input samples prior to
rounding. The most significant of the bits to be discarded is examined in order to determine whether the bits to be
removed sum to more or less than half a quantizing interval. The dithered sample is either rounded down, i.e. the
unwanted bits are simply discarded, or rounded up, i.e. the unwanted bits are discarded but one is added to the
value of the new short word. The rounding process is no longer deterministic because of the added dither which
provides a linearizing random component.
Figure 3.72: In a simple digital dithering system, two's complement values from a random number generator are
added to low-order hits of the input. The dithered values are then rounded up or down according to the value of the
bits to be removed. The dither linearizes the requantizing.
The probability density of the pseudo-random sequence is important. Lipshitz et al. [ 25 ] found that uniform probability
density produced noise modulation, in which the amplitude of the random component varies as a function of the
amplitude of the samples. A triangular probability density function obtained by adding together two pseudo-random
sequences eliminated the noise modulation to yield a signal-independent white-noise component in the least
significant bit.
[ 24 ] Vanderkooy, J. and Lipshitz, S.P., Digital dither. Presented at the 81st Audio Engineering Society Convention
(Los Angeles, 1986), Preprint 2412 (C-8)
[ 25 ] Lipshitz, S.P., Wannamaker, R.A. and Vanderkooy, J., Quantization and dither: a theoretical survey. J. Audio
Eng. Soc., 40, 355-375 (1992)