USER’S MANUAL. PROMAX-10
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Page 50
05/2004
APPENDIX D.-
CARRIER TO NOISE RATIO (C/N)
-ANALOGUE CHANNELS-
The Carrier to Noise ratio is a signal quality measurement. The noise power
depends on the measurement filter bandwidth, in analogue TV it is usual to refer the
noise level to a bandwidth of 5 MHz (this is the case of the PROMAX-10
Premium
). If it
is desired to know the C/N measurement for a different bandwidth (BWx), the following
correction should be applied.
Starting from the C/N definition:
X
X
BW
BW
N
C
N
/
C
−
=
where C:
Video carrier level
N
BWX
:
Noise power referred to a BWx measurement bandwidth
and as the noise power referred to BWx could be related with the noise power referred
to 5 MHz according to:
)
MHz
5
BW
log(
10
N
N
X
MHz
5
BW
X
+
=
the C/N ratio referred to a noise measurement bandwidth BWx should be expressed as:
)
mhZ
5
BWx
log(
10
N
/
C
)
MHz
5
BWx
log(
10
N
C
N
C
N
/
C
10
PROMAX
MHz
5
BW
BW
X
X
−
=
−
−
=
−
=
−
That is to say, the factor 10 log (BWx / 5 MHz) should be subtracted to the
PROMAX-10
Premium
readout.
There exist some standards that determine the minimum C/N value for an
installation. In general, greater values than 40 dB are considered as good quality
signals. Under 40 dB snow should appear on the TV screen.
