430 Chapter 14
is formed along tracks on the medium. Random errors due to noise are corrected by the
inner code and do not impair the burst-correcting power of the outer code. Burst errors are
declared uncorrectable by the inner code, which fl ags the bad samples on the way into the
deinterleave memory. The outer code reads the error fl ags in order to locate erroneous data.
As it does not have to compute the error locations, the outer code can correct more errors.
The interleave, deinterleave, time-compression, and time base-correction processes
inevitably cause delay.
14.8 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
optical or magnetic disks, have only one active track, so it must be totally self-contained.
Tape-based recorders may have several tracks read or written simultaneously. At high
recording densities, physical tolerances cause phase shifts, or timing errors, between
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, samples may contain strings of identical bits. For example,
silence in digital audio is represented by samples in which all the bits are zero. If a
shift register is loaded with such a sample and shifted out serially, the output stays at a
constant level for the period of the identical bits, and nothing is recorded on the track.
On replay there is nothing to indicate how many bits were present or even how fast to
move the medium. Clearly, serialized raw data cannot be recorded directly, they must 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 them 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.
Spectrum shaping is used in optical disks to prevent data from interfering with the focus
and tracking servos and in hard disks and in certain tape formats to allow rerecording
without erase heads.