520 Chapter 16
process was applied—an option that would not incur signifi cant problems with accuracy
of quantization—this would provide low-bit resolution values as good as the 16-bit
equivalents at a lower manufacturing cost and with greater reproducibility. Ultimately, the
limit to the resolution possible with a multiple sampling decoder is set by the time “ jitter ”
in the switching cycles and the practicable operating speeds of the digital logic elements
used in the shift registers and adders. In the case of the 1024 times oversampling “ Sony ”
system, a 44.1584-MHz clock speed is required, which is near the currently available limit.
16.4 Error Correction
The possibility of detecting and correcting replay errors offered by digital audio
techniques is possibly the largest single benefi t offered by this process because it allows
the click-free, noise-free background level in which the CD differs so obviously from
its vinyl predecessors. Indeed, were error correction not possible, the requirements for
precision of the CD manufacturing and replay process would not be practicable.
Four possible options exist for the avoidance of audible signal errors once these have
been detected. These are the replacement of the faulty word or group of words by
correct ones, the substitution of the last correct word for the one found to be faulty, on
the grounds that an audio signal is likely to change relatively slowly in amplitude in
comparison with the 44.1-kHz sample rate, linear interpolation of intermediate sample
values in the gaps caused by the deletion of incorrectly received words, and, if worst
comes to worst, the muting of the signal for the duration of the error.
Of these options, the replacement of the faulty word, or group of words, by a correct
equivalent is clearly the fi rst preference, although it will, in practice, be supplemented
by the other error-concealment techniques. The error correction system used in the CD
replay process combines a number of error correction features and is called the cross-
interleave Reed–Solomon code system. It is capable of correcting an error of 3500 bits
and of concealing errors of up to 12,000 bits by linear interpolation. I will look at the
CIRC system later, but, meanwhile, it will be helpful to consider some of the options that
are available.
16.4.1 Error Detection
Errors likely to occur in a digitally encoded replay process are described as “ random ”
when they affect single bits and “ burst ” when they affect whole words or groups of