510 Chapter 16
An accurate crystal-controlled clock regeneration circuit then causes the signal data
blocks to be withdrawn in correct order from a sequential memory “ shift register ” circuit
and reassembled into precisely timed and numerically accurate replicas of the original
pairs of 16-bit (left and fi ght channel) digitally encoded signals. The timing information
from this stage is also used to control the speed of the disc drive motor and ensure that
signal data are recovered at the correct bit rate.
The remainder of the replay process consists of the stages in which the signal is converted
back into analogue form, fi ltered to remove the unwanted high-frequency components,
and reconstructed, as far as possible, as a quantization noise-free copy of the original
input waveform. As noted earlier, the fi ltering and the accuracy of reconstruction of this
waveform are helped greatly by the process of “ oversampling ” in which the original
sampling rate is increased, on replay, from 44.1 kHz to some multiple of this frequency,
such as 176.4 kHz or even higher. This process can be done by a circuit in which the
numerical values assigned to the signal at these additional sampling points are obtained
by interpolation between the original input digital levels. As a matter of convenience,
the same circuit arrangement will also provide a steep-cut fi lter having a near-zero
transmission at half the sampling frequency.
16.3.2.1 The “ Eight to Fourteen Modulation ” Technique
This is a convenient shorthand term for what should really be described as “ 8-bit to
14-bit encoding/decoding ” and is done for considerations of mechanical convenience in
the record/replay process. As noted earlier, the ‘ 1’s in the digital signal fl ow are generated
by transitions from low to high, or from high to low, in the undulations on the refl ecting
surface of the disc. On a statistical basis, it would clearly be possible, in an 8-bit encoded
signal, for a string of eight or more ‘ 1’s to occur in the bit sequence, the recording of
which would require a rapid sequence of surface humps with narrow gaps between them,
making this inconvenient to manufacture. Also, in the nature of things, because these pits
or humps will never have absolutely square, clean-cut edges, transitions from one sloping
edge to another, where there is such a sequence of closely spaced humps, would also lead
to a reduction in the replay signal amplitude and might cause lost data bits.
However, a long sequence of ‘ 0’s would leave the mirror surface of the disc unmarked by
any signal modulation at all, and, bearing in mind the precise track and focus tolerances
demanded by the replay system, this absence of signals at the receiver photocell would
embarrass the control systems that seek to regulate the lateral and vertical position of the