Audio Engineering

Digital Audio Recording Basics 545

data. As it does not have to compute the error locations, the outer code can correct more
errors.

An alternative to the product block code is the convolutional cross interleave, shown in
Figure 17.14(c). In this system, data are formed into an endless array and the code words
are produced on columns and diagonals. The compact disc and DASH formats use such a
system because it needs less memory than a product code.

The interleave, deinterleave, time-compression, and time base-correction processes cause
delay and this is evident in the time taken before audio emerges after starting a digital
machine. Confi dence replay takes place later than the distance between record and replay
heads would indicate. In DASH format recorders, confi dence replay is about one-tenth of
a second behind the input. Synchronous recording requires new techniques to overcome
the effect of the delays.

The presence of an error-correction system means that the audio quality is independent
of the tape/head quality within limits. There is no point in trying to assess the health of a

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Burst errors

Interleave

delays

(c)

Figure 17.14(c) : Convolutional interleave is shown. Instead of assembling samples in blocks,

the process is continuous and uses RAM delays. Samples are formed into columns in an

endless array. Each row of the array is subject to a different delay so that after the delays,

samples in a column are available simultaneously which were previously on a diagonal. Code

words which cross one another at an angle can be obtained by generating redundancy before

and after the delays.