Hamlet 503 EM, Руководство оператора, Страница: 17 / 22

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SERIAL DIGITAL BASICS
The period between EAV and SAV is not used by normal video and is available for other
purposes e.g.: error checking, timecodes or embedded audio.
Illegal Values: The values 0 and 3FF hex are used solely by TRS pulses (EAV and SAV) they
must not appear anywhere in the active video area.
Out of Gamut: Values apart from the illegal values, which should not be used.
Luminance is defined as being between peak white, 700mV 3AC, hex and
black, 0mV 040 hex.
Chroma is defined as being between max positive, 350mV 3C0, hex and
max negative, -350mV 040 hex.
The values above and below these are termed out of gamut.
The data is serialised using an NRZ (None Return to Zero) code to produce a 270 Mb/s
signal. This coding method removes any low frequency component and is insensitive to
polarity. The data has to be scrambled first to avoid the possible transmission of all '0's.
This data is output at 800mV p-p to normal 75 ohm video coaxial cable.
Due to the high frequencies, the cable losses are quite high, typically 10dB per 100 metres at
270 MHz. To allow acceptable cable lengths, automatic cable equalises are used at the
receiver, which usually allow up to 300 metres of cable to be used. It is important that
standard cable is used, otherwise the equaliser will not compensate correctly.
Suitable cable is:
PSF 2/3 BELDEN 8281 F&G 1.0/6.6
DIGITAL ERROR DETECTION OVERVIEW
In order to check if the digital video signal has been received correctly a Cyclic Redundancy
Check (CRC) can be made on each frame in the generating equipment, this four digit number
is then placed in a 'packet' and put in the EAV-SAV space of one line of each field.
At the receiving equipment the incoming video field also has a Cyclic Redundancy Check
number calculated, this value is then compared with the 4 digit number sent in the packet. If
the two numbers are not identical an error has occurred between transmission and reception
of the signal.