To Test at the Tap — or the Converter?
That is the question!
revised: 6/22/03
A lot of systems run all of the FCC Proof of Performance tests "at the tap". Others run some of the tests at the tap and others at the output of a set-top converter. So which is correct? As with so many aspects of the proofs ………. It depends!
First, the rules say that certain tests should be conducted at the "subscriber terminal". A quick check in Part 76 provides the FCC's definition of subscriber terminal. Its "The cable television terminal to which a subscriber's equipment is connected". Another way of saying this is its the point at which your subscriber's TV, VCR or ?? connects to your cable system, be it at the tap or at the output of the set-top converter that you provide.
For systems that don't provide converter boxes, all tests can be conducted at the tap. If your system does provide converter boxes, it gets more complicated.
One approach is to run tests both at the tap and at the converter. That way, you can be certain that you are meeting the testing requirements both for those that have only basic service and those that use set-top converters. This gets a little awkward and can be a real waste of time.
Let's review the items in Part 76 that specify tests at "the subscriber terminal."
(In the following text, all references to the subscriber tap or subscriber terminal include a 100 ft cable connected to the tap)
Aural offset frequency (4.5 MHz +/- 5 kHz): The rules say that this should be measured in the headend and at the subscriber terminal. The frequencies at the tap and at the output of the set-top converter — depending on the type of converter being used — may be different. So, is it necessary to measure the aural offset frequencies on all channels? The answer is no! The rules also say that the aural offset frequency is one of the tests that are only required on the test channels.
As a practical matter, we run this test by measuring the frequencies of all channels in the headend, then, in the field, we measure only the test channels — both at the tap and at the output of a set-top converter. For most systems, frequencies measured at the tap will be no different than those in the headend. The aural offset frequencies measured at the output of a set-top converter will also be the same as those in the headend — except when a baseband type of converter is used. For a baseband converter, the aural offset frequency is essentially constant. There's no need to go beyond the minimal required tests.
Visual signal level (at least 3 dBmV at the tap, at least 0 dBmV at the output of the converter): According to the rules, this should be measured on all channels at the subscriber terminal. For most systems, this means at the tap and at the output of the converter. With the automated test capabilities available today, tests at the tap are a simple matter of running a carrier survey. Tests at the output of the converter are not so simple because the test must be paused long enough to change channels on the converter.
Here's the way we approach this test. We measure all levels at the tap. If the converter being used is a baseband converter (demod, remod type), the levels at the output of the converter don't change. So, rather than test all channels at the converter's output, we only check the test channels and put a note in the report indicating that the level doesn't change at the output of the converter as demonstrated by the samples. For other converters, we go ahead and run the tests on all channels. We have a simple program to perform the tests using our signal level meter and a notebook computer.
Aural signal level (-10 to -17 dBc unless a baseband converter is used, then –6.5 to –17 dBc): This is another case in which the test should be conducted on all channels at the tap and at the output of a converter. It is also required in the headend. As with the visual signal level test, we run the test on all channels at the tap and, if a baseband converter is being used, only on the test channels at the output of the converter — with the appropriate comment in the report. For non-baseband converters, we run the test on all channels at the tap and at the converter. Its not unusual for the frequency response of non-baseband converters to roll off the aural carrier a dB or two. This can cause the aural carrier levels for some channels (those that were marginal at the headend or tap) to fail to meet the requirement.
Amplitude characteristic (in-channel response) (+/- 2 dB from 500 kHz below to 3.75 MHz above the visual carrier frequency): This test can be performed at the tap until Dec 31, 1999. After Dec 31, 1999 this test must be performed at the subscriber terminal. It is only required on the "test channels". An advantage of running the test at the output of the converter is that it can easily be done on scrambled channels. The converter will descramble the signal. Be aware that some scrambling systems "trash" in-channel response when using VITS. We inserter a multiburst test signal on scan line 24 for this test. For more information, refer to the paper on running tests on scrambled channels.
Carrier to Noise ratio (43 dB). C/N should be measured at the subscriber terminal. One interpretation is that it should be conducted both at the tap and at the output of the converter. If one assumes that the converter will degrade (not improve) C/N, testing only at the output of the converter makes sense.
A lot of systems run this test at the tap then apply "correction factors" to account for the converter performance (others don't even bother with the correction factors). Several reasons are given for testing at the tap:
1: Concern about accuracy. Testing at the tap is pretty basic and gives accurate results (for C/N at the tap). Among the concerns: "are these the results we need according to the rules?" and "how accurate are the correction factors".
2: It seems easier.
3: You don't need to worry about AGC or AFC effects if the test is done by turning off the carrier or removing the video input to the modulator.
4: Its faster. No connecting and disconnecting or tuning of the converter.
If C/N can be accurately measured with the channel in service, essentially all of the arguments in favor of testing at the tap become invalid.1: Gated and notch-filter methods give accurate results with channels in service. When making C/N tests in the headend, measurement dynamic range may be a concern. However, measurement range is no problem when measuring C/N to the typical levels encountered in the field (up to 50 or 55 dB).
2: The process for testing at the converter is identical to testing at the tap. Testing at the tap is no easier. Testing at the tap, then applying correction factors is actually much more complicated, and almost certainly less accurate than testing at the output of the converter to start with.
3: AGC and AFC circuits are operated at their normal operating points. This is a much better way to run the tests than attempting to estimate the effects of the AGC circuit when coming up with correction factors.
4: If you normally use a preselector filter, testing at the converter may be faster than testing at the tap. By running the tests at the output of the converter, no preselector is needed. Also, if a preamp is needed, it can be installed at the output of the converter and left in place for the duration of the tests.
Additional advantages to in-service tests at the output of the converter:
1: You don't need to be concerned about following the rules. No one will say you did the tests wrong by testing at the converter instead of at the tap!
2: You can run tests on scrambled channels — with the scrambler in service.
3: Tests are conducted under the same conditions that your subscribers normally see. This is the basic idea of running the tests to start with – to ensure your subscribers receive pictures meeting prescribed technical parameters.
Coherent Disturbances (at least -51 dBc for most systems): This is a whole topic by itself. See the Discussion about Coherent Disturbance tests. Regardless, according to the rules, coherent disturbance tests should be conducted at the subscriber terminal. In fact, all coherent disturbance tests except those at the visual carrier frequency can be conducted with the channel in service using gated measurement techniques. However, for tests at the visual carrier frequency, the carrier must be removed. When the carrier is removed, most set-top converters will lock to the lower adjacent channel's aural carrier and/or their AGC circuit will increase the gain — giving inaccurate test results. Consequently, most systems perform these tests at the tap. If a simple heterodyne converter is used (one without AGC or AFC circuits), all coherent disturbance tests can be conducted at the output of the converter. For converters with AGC/AFC circuits, or for baseband converters, it is possible to run the tests at the output of the converter by using offset substitute carrier techniques. However, few systems are actually doing this.
The typical specifications for most converters are good enough that, if measurements at the tap yield results well beyond the FCC requirements, and if the signal levels into the converter are in the 5 to 15 dB range, the converter will contribute little in the way of intermodulation products (CSO and CTB). If the visual carrier levels exceed those specified in the converters' specifications or, if the measurements at the tap are near the 51 dB FCC limit, its probably best to correct the levels or track down and repair the source of the disturbances. Otherwise a method to accurately measure coherent disturbances at the output of the converter should be used.
Testing at the output of the set-top converter tends to encourage non-disruptive types of tests. Once you commit to in-service C/N and in-channel response tests, service disruptions are held to an absolute minimum. Even coherent disturbances, including CTB, can be performed in service by using gated measurements - see the information about the TVMS 4200 In-Service Test Processor.
Beyond the obvious benefit of fewer upset subscribers, in service testing minimizes the amount of time spent running tests at night and essentially eliminates the need for a tech in the headend during proofs.
Gary Andrews
Television Measurement Services
garya@tvms.net
www.tvms.net