Fundamentals - The MPEG - page 38

Figure 2.29: Dither can be applied to a quantizer in one of two ways. In (a) the dither is linearly added to the
analog input signal, whereas in (b) it is added to the reference voltages of the quantizer.
The addition of dither means that successive samples effectively find the quantizing intervals in different places on
the voltage scale. The quantizing error becomes a function of the dither, rather than just a function of the input
signal. The quantizing error is not eliminated, but the subjectively unacceptable distortion is converted into
broadband noise which is more benign. An alternative way of looking at dither is to consider the situation where a
low-level input signal is changing slowly within a quantizing interval. Without dither, the same numerical code
results, and the variations within the interval are lost. Dither has the effect of forcing the quantizer to switch
between two or more states. The higher the voltage of the input signal within the interval, the more probable it
becomes that the output code will take on a higher value. The lower the input voltage within the interval, the more
probable it is that the output code will take the lower value. The dither has resulted in a form of duty cycle
modulation, and the resolution of the system has been extended indefinitely instead of being limited by the size of
the steps.
Dither can also be understood by considering the effect it has on the transfer function of the quantizer. This is
normally a perfect staircase, but in the presence of dither it is smeared horizontally until with a certain minimum
amplitude the average transfer function becomes straight.
The characteristics of the noise used are rather important for optimal performance, although many sub-optimal but
nevertheless effective systems are in use. The main parameters of interest are the peak-to-peak amplitude, and
the probability distribution of the amplitude. Triangular probability works best and this can be obtained by summing
the output of two uniform probability processes.
The use of dither invalidates the conventional calculations of signal-to- noise ratio available for a given wordlength.
This is of little consequence as the rule of thumb that multiplying the number of bits in the wordlength by 6 dB gives
the SNR a result that will be close enough for all practical purposes.
It has only been possible to introduce the principles of conversion of audio and video signals here. For more details
of the operation of convertors the reader is referred elsewhere. [ 2 ][ 14 ]
[ 12 ] Roberts, L.G., Picture coding using pseudo-random noise. IRE Trans. Inform. Theory , IT-8 145-154 (1962)
[ 13 ] Lipshitz S.P., Wannamaker, R.A. and Vanderkooy, J., Quantization and dither: A theoretical survey. J. Audio
Eng. Soc. , 40 , 355-375 (1992)
[ 2 ] Watkinson, J.R., The Art of Digital Audio , third edition, Oxford: Focal Press (2001)
[ 14 ] Watkinson, J.R., The Art of Digital Video , second edition, Oxford: Focal Press (1994)
2.14 Introduction to digital processing
However complex a digital process, it can be broken down into smaller stages until finally one finds that there are
really only two basic types of element in use, and these can be combined in some way and supplied with a clock to