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KRAMER ELECTRONICS LTD.
1.3
Several Points to Consider When Working with Digital Signals
The minimum noise introduced by quantizing is 1 bit (1 level), so the higher the number of bits, the lower the
inherent noise, and the higher the resolution.
Since A/D and D/A conversions create artifacts, it is important not to convert back and forth. In a “mixed”
outfit, the video should be digitized, and
all
the digital processing done before converting back to analog.
“Multi-media” systems usually quantize the video to 8 bit (ie. 2
8
= 256) levels. Some “levels” are dedicated to
special codes (SAV, EAV – similar to syncs in analog video), leaving 220 quantization levels (~3mV steps) for
the luminance signals. Audio is usually quantized to 16 bits.
Broadcast systems use 10 bit video, and 20 bit audio.
A digital signal may be transported very easily, and saved and retrieved reliably with no generation losses.
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 referred to simply as a “reclocker”).
“Equalization” is a process of amplifying the input signal to overcome losses on the cable. This possible for
digital signals, since the correct amplitude of the signal is known.
“Reclocking” is a process of “cleaning up” the signal in the time-domain, ie, 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 a 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).
