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NOT 557-2002-en



it is present when a single tv carrier is modulated by the other channels being carried by the coaxial cable.

look t the pictures toidentify this phenomena in the tv screen.

picture free of distortions

picture interfered

the information of one channel is crossing over into another channel.

when the operating signal levels are higher than in normal conditions, the 15625 hz side bands from the modulated channels become so strong that interfere with the video content of other carriers.

15625 hz side bands are present in every standard tv channel. they are a result of synchronising information transmitted along with the video information.

the synchronising information is transmitted at the rate of:

50 fields/sec* 312.5 lines/field =15625 hz

a tv picture consists of 625 lines, with a synch pulse at the end of each line.
50 fields are transmitted during 1 second,
and a field contains 312.5 lines

the 15625 hz is the strongest side band in the transmitted channel and can cause cross modulation to other channels on the system.

as the c/n, carrier/xmod calculations are made in two steps.

  1. first of all it is calculated the distortion figure for one amplifier
  2. and then for the whole system.

carrier/xmod 1 amp = xmod + 2*(max. output level - output level)

xmod and max. output level are manufacturer specifications,
e.g. an amplifier can obtain 58 db xmod
for a max. output level = 47dbmv.

let us suppose an output level = 34 dbmv


carrier/xmod 1 amp = 58 +2(47 – 34) = 84 db

as it can be seen, the output level follows a 1:2 ratio:1 db more in the output level decreases 2 db the carrier/xmod ratio.

for n amplifiers in cascade the formula is:

carrier/xmod n amp = carrier/xmod 1 amp - 20 log n

example 1:

two amplifiers in cascade

carrier/xmod 2 amp= 84 – 20 log2 = 78 db

example 2:

four amplifiers in cascade

carrier/xmod 4 amp = 84 – 20 log4 = 72 db

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