Audio Engineering

Compact Disc 513

Since the input “ 16-bit ” encoded signal is divided into two “ 8-bit ” words in the CD
replay process, representing the MS and LS sections from e 1 to e 8 and from e 9 to e 16 ,
these two 8-bit digital words can be separately D/A converted, with the outputs added in
an appropriate ratio to give the fi nal 16-bit D/A conversion.

16.3.2.3 Digital Filtering and “ Oversampling ”

It was noted previously that Philips ’ original choice of sampling frequency (44.1 kHz) and
of signal bandwidth (20 Hz to 20 kHz) for the CD imposed the need for steep-cut fi ltering
both prior to the ADC and following the DAC stages. This can lead to problems caused
by propagation delays and phase shifts in the fi lter circuitry, which can degrade the sound
quality. Various techniques are available that can lessen these problems, of which the most
commonly used come under the headings of “ digital fi ltering ” and “ oversampling. ” Because
these techniques are interrelated, I have lumped together the descriptions of both of these.

There are two practicable methods of fi ltering used with digitally encoded signals. For
these signals, use can be made of the effect that if a signal is delayed by a time interval,
Ts , and this delayed signal is then combined with the original input, signal cancellation—
partial or complete—will occur at those frequencies whereTs is equal to the duration of
an odd number of half cycles of the signal. This gives what is known as a “ comb fi lter ”
response, shown in Figure 16.11 , and this characteristic can be progressively augmented
to approach an ideal low-pass fi lter response (100% transmission up to some chosen
frequency, followed by zero transmission above this frequency) by the use of a number
of further signal delay and addition paths having other, carefully chosen, gain coeffi cients
and delay times. (Although, in principle, this technique could also be used on a signal
in analogue form, there would be problems in providing a nondistorting time delay
mechanism for such a signal—a problem that does not arise in the digital domain.)

Ideal low-pass filter

Frequency

Output

Figure 16.11 : Comb fi lter frequency response.