COMMUNITY ANTENNA TELEVISION (CABLE TELEVISION)
Minimum noise model.
Let the bandwidth,
B, of an NTSC TV signal be rounded to 4 MHz. The open circuit
noise voltage for a 75-
= (4 × 75 × 4 × 10
= 2.2 µV rms.
Figure 17.4 shows a 2.2-
µV noise generating source (resistor) connected to a 75-
(noiseless) load. Only half of the voltage (1
.1 µV) is delivered to the load. Thus the
noise input to 75
.1 µV or -59 dBmV. This is the basic noise level, the minimum
that will exist in any part of a 75-
CATV system. The value
-59 dBmV will be used
repeatedly below (Ref. 3).
The noise figure of a typical CATV amplifier ranges between 7 and 9 dB (Ref. 3).
Signal-to-Noise Ratio (S/N) Versus Carrier-to-Noise Ratio (C/N)
in CATV Systems
We have been using S/N and C/N many times in previous chapters. In CATV systems
S/N has a slightly different definition as follows (Ref. 2):
This relationship is expressed by the "signal-to-noise ratio," which is the difference between
the signal level measured in dBmV, and the noise level, also measured in dBmV, both levels
being measured at the same point in the system.
S/N can be related to C/N on CATV systems as
= S/N + 4.1 dB
This is based on Carson (Ref. 4), where the premise is "noise just perceptible" by a
population of TV viewers, with an NTSC 4.2-MHz TV signal. Adding noise weighting
improvement (6.8 dB), we find
= C/N + 2.7 dB.
It should be noted that S/N is measured where the signal level is peak-to-peak
noise level is rms. For C/N measurement, both the carrier and the noise levels are rms.
Weighting (IEEE). The artificial adjustment of measurements in order to account for factors that in normal
use of the device would otherwise be different from the conditions during measurement. In the case of TV,
the lower baseband frequencies (i.e., from 20 Hz to 15 kHz) are much more sensitive to noise than the higher
Peak-to-peak voltage refers, in this case, to the measurement of voltage over its maximum excursion, which
is the voltage of the "sync tips." See Figures 16.3 and 16.4.