518 Chapter 16
would be attributed to level 12 and that 40% would be attributed to level 13 so that, on
averaging, the fi nal analogue output from the ADC/DAC process would have the correct
value of 12.4.
A further benefi t is obtained by the addition of dither at the output of the replay DACs
(most simply contrived by allowing the requisite amount of noise in the following
analogue low-pass fi lters) in that it will tend to mask the quantization “ granularity ” of the
recovered signal at low bit levels. This defect is particularly noticeable when the signal
frequency happens to have a harmonic relationship with the sampling frequency.
16.3.2.5 The “ Bitstream ” Process and “ Noise Shaping ”
A problem in any analogue-to-digital or digital-to-analogue converter is that of obtaining
an adequate degree of precision in the magnitudes of the digitally encoded steps. It has
been seen that the accuracy required, in the most signifi cant bit in a 16-bit converter, was
better than 0.00305% if ‘ 0 ’ –’1 ’ transitions in the LSB were to be signifi cant. Similar,
although lower, orders of accuracy are required from all the intermediate step values.
Achieving this order of accuracy in a mass-produced consumer article is diffi cult and
expensive. In fact, differences in tonal quality between CD players are likely to be due, in
part, to inadequate precision in the DACs.
As a means of avoiding the need for high precision in the DAC converters, Philips took
advantage of the fact that an effective improvement in resolution could be achieved
merely by increasing the sampling rate, which could then be traded-off against the
number of bits in the quantization level. Furthermore, whatever binary encoding system is
adopted, the fi rst bit in the received 16-bit word must always be either a ‘ 0 ’ or a ‘ 1 ’ , and
in the “ two’s complement ” code used in the CD system, the transition in the MSB from
‘ 0 ’ to ‘ 1 ’ and back will occur at the midpoint of the input analogue signal waveform.
This means that if the remaining 15 bits of a 16-bit input word are stripped off and
discarded, this action will have the effect that the input digital signal will have been
converted—admittedly somewhat crudely—into a voltage waveform of analogue
form. Now, if this ‘ 0/1 ’ signal is 256 times oversampled, in the presence of dither, an
effective 9-bit resolution will be obtained from two clearly defi ned and easily stabilized
quantization levels: a process for which Philips coined the term “ bit stream ” decoding.
Unfortunately, such a low-resolution quantization process will incur severe quantization
errors that manifest as a high background noise level. Philips ’ solution to this is to