Audio Engineering

Compact Disc 503

is a specifi c requirement of the digital encoding/decoding process, for the reasons already
considered. It is necessary to carry out this fi ltering process after the amplitude limiting
stage because it is possible that the action of peak clipping may generate additional
high-frequency signal components. This would occur because “ squaring off ” the peaks of
waveforms will generate a Fourier series of higher frequency harmonic components.

The audio signal, which is still at this stage in analogue form, is then passed to two
parallel operating 16-bit ADCs and, having now been converted into a digital data stream,
is fed into a temporary data-storage device—usually a “ shift register ” —from which the
output data stream is drawn as a sequence of 8-bit blocks, with the ‘ L ’ and ‘ R ’ channel
data now arranged in a consecutive but interlaced time sequence.

From the point in the chain at which the signal is converted into digitally encoded blocks
of data, at a precisely controlled “ clock ” frequency, to the fi nal transformation of the
encoded data back into analogue form, the signal is immune to frequency or pitch errors
as a result of motor speed variations in the disc recording or replay process.

The next stage in the process is the addition of data for error correction purposes.
Because of the very high packing density of the digital data on the disc, it is very likely
that the recovered data will have been corrupted to some extent by impulse noise or

Clock

ADC

ADC

STORE CIRC EFM

ROM

SYNC

LPF RAM

LPF

Limiters

1.411 MB/s

44.1 kHz

Inputs

1.882 MB/s

4.123 MB/s4.322 MB/s

L

R

CD

Figure 16.6 : Basic CD recording system.