Compact Disc 499audio bandwidth of the incoming signal. However, in practice, additional digital “ bits ”
will be added to this signal for error correction and other purposes, which will extend the
required bandwidth even further.
16.1.3 Translation Nonlinearity
The conversion of an analogue signal both into and from its binary-coded digital
equivalent carries with it the problem of ensuring that the magnitudes of the binary
voltage steps are defi ned with adequate precision. If, for example, “ 16-bit ” encoding is
used, the size of the “ most signifi cant bit ” (MSB) will be 32,768 times the size of the
“ least signifi cant bit ” (LSB). If it is required that the error in defi ning the LSB shall be
not worse than 0.5%, then the accuracy demanded of the MSB must be at least within
0.0000152% if the overall linearity of the system is not to be degraded.
The design of any switched resistor network, for encoding or decoding purposes, that
demanded such a high degree of component precision would be prohibitively expensive
and would suffer from great problems as a result of component aging or thermal drift.
Fortunately, techniques are available that lessen the diffi culty in achieving the required
accuracy in the quantization steps. The latest technique, known as “ low bit ” or “ bit-stream ”
decoding, side steps the problem entirely by effectively using a time-division method, since it
is easier to achieve the required precision in time, rather than in voltage or current, intervals.
16.1.4 Detection and Correction of Transmission Errors
The very high bandwidths needed to handle or record PCM-encoded signals means
that recorded data representing the signal must be very densely packed. This leads to
the problem that any small blemish on the surface of the CD, such as a speck of dust, a
scratch, or a thumb print, could blot out, or corrupt, a signifi cant part of the information
needed to reconstruct the original signal. Because of this, the real-life practicability of all
digital record/replay systems will depend on the effectiveness of electronic techniques
for the detection, correction, or, if worst comes to worst, masking of the resultant errors.
Some very sophisticated systems have been devised, which are also examined later.
16.1.5 Filtering for Bandwidth Limitation and Signal Recovery
When an analogue signal is sampled and converted into its PCM-encoded digital equivalent,
a spectrum of additional signals is created, of the kind shown in Figure 16.3(a) , where