Digital Audio Recording Basics 553on tape, so different edits can be made in the future, or simply recording the output of the
current edit so that the disc can be freed for another job.
The rapid access and editing accuracy of hard disc systems make them ideal for
assembling sound effects to make the sound tracks of motion pictures.
The use of data reduction allows the recording time of a disc to be extended considerably.
This technique is often used in plug-in circuit boards, which are used to convert a
personal computer into a digital audio recorder.
17.5 The PCM Adaptor .................................................................................................
The PCM adaptor was an early solution to recording the wide bandwidth of PCM audio
before high-density digital recording developed. The video recorder offered suffi cient
bandwidth at moderate tape consumption. While they were a breakthrough at the time
of their introduction, by modern standards PCM adaptors are crude and obsolescent,
offering limited editing ability and slow operation.
Figure 17.20 shows the essential components of a digital audio recorder using this
technique. Input analogue audio is converted to digital and time compressed to fi t into
the parts of the video waveform which are not blanked. Time-compressed samples are
then odd-even shuffl ed to allow concealment. Next, redundancy is added and data are
interleaved for recording. Data are serialized and set on the active line of the video signal
as black and white levels shown in Figure 17.21. The video is sent to the recorder, where
the analogue FM modulator switches between two frequencies representing the black and
white levels, a system called frequency shift keying (FSK). This takes the place of the
channel coder in a conventional digital recorder.
On replay, the FM demodulator of the video recorder acts to return the FSK recording to
the black/white video waveform, which is sent to the PCM adaptor. The PCM adaptor
extracts a clock from the video sync pulses and uses it to separate the serially recorded bits.
Error correction is performed after deinterleaving, unless the errors are too great, in which
case concealment is used after the deshuffl e. The samples are then returned to the standard
sampling rate by the time base expansion process, which also eliminates any speed
variations from the recorder. They can then be converted back to the analogue domain.
In order to synchronize playback to a reference and to simplify the circuitry, a whole
number of samples is recorded on each unblanked line. The common sampling rate of