Audio Engineering

564 Chapter 17

medium, but brings blocks of data to a computer memory where it is edited before being
sent back for storage.

In fact, this is the only way that digital audio recordings can be edited because of the use
of interleave and error correction.

Interleave reorders the samples on the medium, and so it is not possible to fi nd a physical
location on the medium that corresponds linearly to the time through the recording. Error
correction relies on blocks of samples being coded together. If part of a block is changed,
the coding will no longer operate.

Figure 17.31 shows how an audio edit is performed. Samples are played back, deinterleaved,
and errors are corrected. Samples are now available in their natural real-time sequence and
can be sent to a cross-fader where external material can be inserted. The edited samples are
then recoded and interleaved before they can be rerecorded. Deinterleave and interleave
cause delay, and by the time these processes have been performed, the tape will have moved
further through the machine. In simple machines, the tape will have to be reversed, and new
data recorded in a second pass. In more sophisticated machines, an edit can be made in a
single pass because additional record heads are placed further down the tape path.

In a stationary head machine, these are physically displaced along the head block. In a
rotary head machine, the extra heads are displaced along the axis of the drum.

Digital

to

analogue

Data

expansion

Error

correction

Data

separator

9

Analogue

to

digital

Data

reduction

Time

compression

 add

redundancy

Channel

coding

9

9

9

Figure 17.30 : In DCC, PCM data from the convertors are reduced to one-quarter of the

original rate prior to distribution over eight tape tracks (plus an auxiliary data track). This

allows a slow linear tape speed that can only be read with an MR head. The data reduction

unit is mirrored by the expansion unit on replay.