Digital Audio Recording Basics 549Every bit period begins with a transition, irrespective of the value of the bit. If the bit is
a one, an additional transition is placed in the center of the bit period. If the bit is a zero,
this transition is absent. As a result, the waveform is always self-clocking irrespective of
the values of the data bits. Additionally, the waveform spends as much time in the low
state as it does in the high state. This means that the signal has no DC component and
will pass through capacitors, magnetic heads, and transformers equally well. However
simple FM may be, it is not very effi cient because it requires two transitions for every bit
and jitter of more than half a bit cannot be rejected.
More recent products use a family of channel codes known as group codes. In group
codes, groups of bits, commonly eight, are associated together into a symbol for
recording purposes. Eight-bit symbols are common in digital audio because two of
them can represent a 16-bit sample. Eight-bit data have 256 possible combinations, but
if the waveforms obtained by serializing them are examined, it will be seen that many
combinations are unrecordable. For example, all ones or all zeros cannot be recorded
because they contain no transitions to lock the clock and they have excessive DC content.
If a larger number of bits is considered, a greater number of combinations is available.
After the unrecordable combinations have been rejected, there will still be 256 left which
can each represent a different combination of eight bits. The larger number of bits are
channel bits; they are not data because all combinations are not recordable. Channel
bits are simply a convenient way of generating recordable waveforms. Combinations of
channel bits are selected or rejected according to limits on the maximum and minimum
periods between transitions. These periods are called run-length limits and as a result
group codes are often called run-length-limited codes.
In RDAT, an 8/10 code is used where 8 data bits are represented by 10 channel bits.
Figure 17.18 shows that this results in jitter rejection of 80% of a data bit period: rather
better than FM. Jitter rejection is important in RDAT because short wavelengths are used
and peak shift will occur. The maximum wavelength is also restricted in RDAT so that
low frequencies do not occur.
In CD, an 8/14 code is used where 8 data bits are represented by 14 channel bits. This
only has a jitter rejection of 8/14 of a data bit, but this is not an issue because the rigid
CD has low jitter. However, in 14 bits there are 16K combinations, and this is enough to
impose a minimum run length limit of 3 channel bits. In other words, transitions on the
disc cannot occur closer than 3 channel bits apart. This corresponds to 24/14 data bits.