Newnes Audio and Hi-Fi Engineer's Pocket Book, 3rd Edition

101

Decoding

the

signal

Demodulator and buffer

The

synchronising bits identify

the

start

of

each frame from the data

bit

stream from

the

photodiodes, which

is

then demodulated, converting

the

14-bit

symbols back

to 8.

Next the frames

are fed

into

a

buffer

memory and extracted from the other end. The input rate may

vary slightly due

to

disc speed irregularities,

but

the output rate

is regulated

by a

quartz-controlled clock

so no

wow

can

appear

in

the

reproduced signal. The buffer

is

kept nominally 50% full

to permit regulation

in

both directions,

but

the amount

in the

buffer

at

any instant

is an

indication

of

whether the motor

is

slow

or

fast ...