Kramer Electronics SD-7108 - Manual (Page 4)

KRAMER ELECTRONICS LTD. 3
Broadcast systems usually use 10-bit video, and 20- to 24- bit audio.
The advantages of working in a digital domain are obvious – a digital signal may be transported
very easily, and saved and retrieved reliably with no generation losses.
The transmission of a digital video signal is reliable up to a certain length of cable. Beyond this
length, the signal is destroyed. This phenomenon is known as the “cliff-effect”. To avoid the “crash” at
the cliff, an “equalizer and reclocker” should be inserted at a distance less than the “cliff” length. From
this point it is again possible to drive a cable up to the “cliff” length. This is similar to the “repeater”
analogy for analog signals. (Note: the term “equalizer” is usually dropped, and the “equalizer and
reclocker” is often referred to simply as a “reclocker”).
“Equalization” is a process of amplifying the input signal to overcome losses on the cable. For
digital signals, this is possible, since the correct amplitude of the signal is known!
“Reclocking” is a process of “cleaning up” the signal in the time-domain, i.e., removing the jitter
which was introduced as a result of the long cable. To do this, the timing source must be recovered from
the signal, and the signal is regenerated with stable timing.
A graphic representation of the timing and amplitude distortions is shown in an “eye diagram”, as
below:
Figure 1: Digital “eye” Diagram
Increase in jitter and decrease in amplitude cause the eye to “close”. It is clear that jitter of more
than ±50% would result in an irretrievable signal (cliff effect).