CARRIER-TO-NOISE RATIO (C/N)
it indicates the noise present within the useful video bandwidth of a tv channel.
see picture1 to identify how it looks like in the tv screen, and compare to picture 2, which is a free of distortions good quality picture.
a tv channel contains video, audio and guardband.
video bandwidth is part of this channel that for a pal b/g is approximately 5 mhz.
for a cascading system of identical amplifiers the c/n is calculated in two steps:
for one amplifier the formula to be applied is:
that depends on the input level to the amplifier following a ratio 1:1.
increasing the input level 1 db, the c/n improves 1 db (ratio 1:1)
for more than one amplifier, the formula is:
two amplifiers in cascade like the above mentioned.
remark: in a logarithmic scale, +-3 db, doubles or halves the level
then for this particular case the c/n has been impair to the half.
let us take now 4 amplifiers in cascade
therefore each time the cascade doubles, the c/n gets worse by 3 db
as cascades increase, input level also must increase for compensating the contribution of noise of each amplifier.
the use of amplifiers with low nf is a very important factor as the number of amplifiers in cascade increases.