502 Chapter 16
some 3000 °, which would be clearly audible. (In the recording equipment it is possible
to employ steep-cut fi lter systems in which the phase and group delay characteristics
are controlled more carefully than would be practicable in a mass-produced CD replay
system where both size and cost must be considered.)
Similarly, because the frequency spectrum produced by a PCM-encoded 20-kHz
bandwidth audio signal will look like that shown in Figure 16.3(a) , it is necessary, on
replay, to introduce yet another equally steep-cut low-pass fi lter to prevent the generation
of spurious audio signals that would result from the heterodyning of signals equally
disposed on either side offs /2.
An improved performance in respect to both relative phase error and group delay in
such “ brick wall ” fi lters can be obtained using so-called “ digital ” fi lters, particularly
when combined with prefi ltering phase correction. However, this problem was only
fully solved, and then only on replay (because of the limitations imposed by the original
Philips CD patents), by the use of “ oversampling ” techniques in which, for example, the
sampling frequency is increased to 176.4 kHz ( “ four times oversampling ” ), which moves
the aliasing frequency from 22.05 kHz up to 154.35 kHz, giving the spectral distribution
shown in Figure 16.3(b). It is then a relatively easy matter to design a fi lter, such as that
shown in Figure 16.14 , having good phase and group delay characteristics, which has a
transmission near 100% at all frequencies up to 20 kHz, but near zero at 154.35 kHz.
16.2 The Record-Replay System ..................................................................................
16.2.1 The Recording System Layout
How the signal is handled, on its way from the microphone or other signal source to the
fi nal CD, is shown in the block diagram of Figure 16.6. Assuming that the signal has by
now been reduced to a basic L–R stereo pair, this is amplitude limited to ensure that no
signals greater than the possible encoding amplitude limit are passed on to the
analogue-to-digital converter (ADC) stage. These input limiter stages are normally cross
linked in operation to avoid disturbance of the stereo image position if the maximum
permitted signal level is exceeded, and the channel gain reduced in consequence of this,
in only a single channel.
The signal is then passed to a very steep-cut 20-kHz antialiasing fi lter (often called a
“ brick wall fi lter ” ) to limit the bandwidth offered for encoding. This bandwidth limitation