Audio Engineering

Digital Audio Recording Basics 547

Spectrum shaping is used in CD to prevent data from interfering with the focus and
tracking servos and in RDAT to allow rerecording without erase heads.

A self-clocking code contains a guaranteed minimum number of transitions per unit time, and
these transitions must occur at multiples of some basic time period so that they can be used
to synchronize a phase locked loop. Figure 17.15 shows a phase-locked loop that contains
an oscillator whose frequency is controlled by the phase error between input transitions and
the output of a divider. If transitions on the medium are constrained to occur at multiples of a
basic time period, they will have a constant phase relationship with the oscillator, which can
stay in lock with them even if they are intermittent. As the damping of the loop is a low-pass
fi lter, jitter in the incoming transitions, caused by peak-shift distortion or by speed variations
in the medium, will be rejected and the oscillator will run at the average frequency of the
off-tape signal. The phase-locked loop must be locked before data can be recovered, and
to enable this, every data block is preceded by a constant frequency recording known as a
preamble. The beginning of data is identifi ed by a unique pattern known as a sync pattern.

Irrespective of the channel code used, transitions always occur separated by a range of
time periods which are all multiples of the basic clock period. If such a replay signal is
viewed on an oscilloscope, a characteristic display called an eye pattern is obtained. Figure
17.16 shows an eye pattern, and in particular the regular openings in the trace. A decision
point is in the center of each opening, and the phase-locked loop acts to keep it centered

Phase

compare Damping VCO

Divide

byN

F in

Output

F in

input

N Fin

Phase

error

Control

voltage

Figure 17.15 : A typical phase-locked loop where the VCO is forced to run at a multiple of the

input frequency. If the input ceases, the output will continue at the same frequency until it drifts.