Is it C/N or S/N?

9/9/02

A common question is whether or not C/N measurements using gated techniques actually result in the measurement of S/N rather than C/N.

The short response is that, when we measure C/N using gated methods, we first measure the visual carrier, then measure the noise and take the ratio of the two.  So, yes, we really are measuring Carrier to Noise ratio. 

Gated measurements typically take place during the vertical blanking interval using scan lines which have no video or other signals present.  The test equipment takes a sample of the RF signal during these “quiet lines”.   Since no video signals are present, this results in a measurement of the noise level.  This is basically the same technique as pulling the video input to the modulator in order to measure noise.  In both cases, noise is measured in the absence of video signals.

The confusion comes in when there is significant noise on the video signal (low video S/N ratio).  In that case, gated C/N measurement results without the use of a local quiet line inserter will include both the noise contribution of the incoming video and the cable system noise.  The test result is still C/N (as opposed to S/N) even though C/N is impacted by the S/N of the incoming video signal.

Test Methods

First, we need to decide whether we want to measure the C/N of just the cable system or the C/N that the subscribers are receiving.  In many cases, the numbers are the same.  However, if the programming received at the headend is noisy, there may be a large difference between system C/N and what the subscriber sees.  Its important to be aware of both numbers.  However, for system maintenance purposes and for proof of performance testing, it’s generally the system C/N – independent of incoming signal quality – that’s most important.

Lets look at some common measurement methods:

1:  Measure the carrier level, then turn off the modulator to measure noise.

This will provide C/N for the system downstream from the modulator.  A useful number, but it ignores noise contribution from the modulator.  It also ignores noise on the incoming video signal.

2:  Remove the video input to the modulator, then measure C/N. 

This method will include any noise generated in the modulator but, of course, ignores noise on the incoming video signal.

Note that methods 1 and 2 provide C/N test results that are always independent of the S/N of the incoming video signal.  This is usually the desired result.

3:  Measure the carrier level, then measure noise only during quiet lines in the vertical blanking interval (gated measurement).

This method includes both noise generated in the modulator and noise on the incoming video signal.  This is the only method that allows us to measure what the subscriber is actually receiving.  

If a quiet line inserter (sometimes called a line blanker) is used with this method, noise will only be measured during the locally blanked line.  This provides system C/N, independent of the incoming video. 

Note that method 3 can be used to measure either system noise alone or system noise plus incoming video noise, depending on whether or not a quiet line inserter is used.

There are several tradeoffs between commonly used C/N measurement techniques:

Method	Yields system C/N independent of incoming video?	Yields sub C/N?	In-service method?	Can be performed at output of STB?
Turn off modulator	Yes	No	No	No 1
Pull video input to modulator	Yes	No	No	Yes 2
Gated with quiet line inserter	Yes	No	Yes	Yes
Gated without quiet line inserter	No3	Yes3	Yes	Yes

1. If the set top box (STB) is a baseband type, or has automatic gain or automatic frequency control circuits, removing the visual carrier will result in erroneous C/N readings. 
2. Removing the video input to a modulator may cause the peak of the visual carrier to change resulting in erroneous test results.  Many modulators have built in AGC circuits that keep the peak of the visual carrier constant when video is removed, in that case, the C/N readings will be correct.
3. For channels that are received digitally, the VBI is generated in the digital receiver.  In that case, this test method results in C/N independent of the quality of the original video signal.

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