546 Chapter 17
machine by listening to it, as this will not reveal whether the error rate is normal or within
a whisker of failure. The only useful procedure is to monitor the frequency with which
errors are being corrected and to compare it with normal fi gures. Professional digital
audio equipment should have an error rate display.
Some people claim to be able to hear error correction and misguidedly conclude that the
aforementioned theory is fl awed. Not all digital audio machines are properly engineered,
however, and if the DAC shares a common power supply with the error correction logic,
a burst of errors will raise the current taken by the logic, which loads the power supply
and can interfere with the operation of the DAC. The effect is harder to eliminate in small
battery-powered machines where space for screening and decoupling components is hard
to fi nd, but it is only a matter of design: there is no fl aw in the theory.
17.3.6 Channel Coding
In most recorders used for storing digital information, the medium carries a track that
reproduces a single waveform. Clearly data words representing audio samples contain
many bits and so they have to be recorded serially, a bit at a time. Some media, such as
CD, only have one track, so it must be totally self-contained. Other media, such as DCC,
have many parallel tracks. At high recording densities, physical tolerances cause phase
shifts, or timing errors, between parallel tracks and so it is not possible to read them in
parallel. Each track must still be self-contained until the replayed signal has been time
base corrected.
Recording data serially is not as simple as connecting the serial output of a shift register
to the head. In digital audio, a common sample value is all zeros, as this corresponds to
silence. If a shift register is loaded with all zeros and shifted out serially, the output stays
at a constant low level, and nothing is recorded on the track. On replay there is nothing
to indicate how many zeros were present or even how fast to move the medium. Clearly
serialized raw data cannot be recorded directly, it has to be modulated into a waveform
that contains an embedded clock irrespective of the values of the bits in the samples.
On replay a circuit called a data separator can lock to the embedded clock and use it to
separate strings of identical bits.
The process of modulating serial data to make it self-clocking is called channel coding.
Channel coding also shapes the spectrum of the serialized waveform to make it more
effi cient. With a good channel code, more data can be stored on a given medium.