Audio Engineering

548 Chapter 17

laterally, in order to reject the maximum amount of jitter. At each decision point along the
time axis, the waveform is above or below the point, and can be returned to a binary signal.

Occasionally, noise or jitter will cause the waveform to pass the wrong side of a decision
point, and this will result in an error that will require correction.

Figure 17.17 shows an extremely simple channel code known as frequency modulation
(FM), which is used for the AES/EBU digital interface and for recording time code on tape.

Additional

transition in

bit period centre

indicates data one

Clock transitions

occur at every

bit period

Serial data 01011001100011100

FM encoded

waveform

No transition in

bit period centre

indicates data zero

Figure 17.17 : FM channel code, also known as Manchester code or biphase mark, is used in

AESEBU interface and for time code recording. The waveform is encoded as shown here. See

text for details.

Slice

level

Noise

 Decision point

Jitter

Figure 17.16 : At the decision points, the receiver must make binary decisions about the

voltage of the signal, whether it is above or below the slicing level. If the eyes remain open,

this will be possible in the presence of noise and jitter.