cable tech facts issue 104
In This Issue
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In this issue of the Cable Tech Facts we will be taking a closer look at the required FCC color tests and the test procedures to be used in evaluating system performance. The quality of the color and luminance signals for each channel can be affected by almost any single component in the CATV system. This is why the FCC requires testing from satellite receivers through to the converter for many of the tests.
FCC Reporting Requirements
The color tests become effective June 30, 1995. As of June 30, systems subject to FCC reporting must have on file passing reports of their system's performance. The color tests and any repair or adjustment must take place prior to that date. The minimum number of channels that must be tested are listed here by system bandwidth utilization. 4 Channels minimum up to 101 MHz
5 Channels for 101 MHz to 216 MHz
6 Channels for 217 MHz to 300 MHz
7 Channels for 301 MHz to 400 MHz
8 Channels for 401 MHz to 500 MHz
9 Channels for 501 MHz to 600 MHz
10 Channels for 601 MHz to 700 MHz
11 Channels for 701 MHz to 800 MHz
12 Channels for 801 MHz to 900 MHz
13 Channels for 901 MHz to 1000 MHz
+1 Channel for each 100 MHz or portion thereof
Remember while you are only required to report on a few channels, the system is responsible for all NTSC channels carried by the system at all times. Good engineering practices would indicate that all channels be tested. In choosing channels to be reported to the FCC, you must choose channels that are representative of those used in the system. Choose one channel of each signal processing technique used in your headend. The other test channels should be based on the distribution of signal processing type. This means you should choose channels for testing that are generated by every process in the headend. If a few processors are used, at least one processor channel should be tested, even if most channels are generated from satellite receivers.
This information is considered to be accurate, but does not represent a legal recommendation or complete interpretation of the FCC rules. You can obtain a copy of the FCC rules, Part 76.605 from the NCTA along with additional information and recommendations on filing the FCC reports.
In the past two issues of the Cable Tech Facts, we have been discussing the basics of the video signal. We're now ready to take a step by step procedure on performing the video tests. We'll begin with the reference point that has been established by the FCC.
The video output of the satellite receiver is the reference point set by the FCC. From this point onward through the system, the operator is responsible for most of the performance tests. Problems can occur ahead of this point, however, in the receiver, in the LNA, dish alignment or other associated hardware which could result in poor picture quality. This can occur even if the system were meeting all of the other performance specifications. This point of reference was chosen due to the difficulty and cost of the necessary instrumentation to truly test the performance of the microwave satellite receiving equipment.
In many cases, the quality of the incoming satellite signals can be evaluated using a VITS test signal which the programmer or uplink facility has inserted for their testing purposes. Using the line select feature on waveform monitor/vector scope or CATV Video Signal Analyzer allows the operator to search the VBI for these test signals. The FCC composite signal will commonly be found between lines 11 and 20. In measuring the output of your satellite receiver, remember that you will be measuring the total noise and distortion of the programmer's video processing, distribution, uplink and satellite transponder. You should expect the performance to be roughly: > 60 dB S/N, < 2 degrees Differential Phase Distortion, < 3% Differential Gain, < .5 dB Frequency Response. This represents fairly good performance and is the main reason that commercial satellite receiving equipment is much more expensive than the typical home TVRO equipment. This performance, while good, could still add to your system measurements. Be sure to measure and record the raw data for each burst packet when measuring Frequency Response, Differential Gain; and the phase or sign of each measurement when testing Differential Phase and Chroma to Luma Delay, for each packet or phase vector. Typically these measurements are greatly simplified when a local test signal is injected using a VITS insertion generator.
If the satellite receiver includes a decoder, it is called an IRD (integrated receiver/decoder). No special provisions are needed for an integrated decoder, since the VITS test signal used for measurement during the VBI is unaffected by common scrambling and descrambling methods.
In systems which use a separate decoder, the output of the decoder may be measured just as the output of an IRD. These results show the satellite's delivery system's performance in addition to the decoder's performance.
The FCC Tests
The following information was taken from various sources and is believed to be accurate at the time of printing. Listed with each recommended test and the FCC required tests is information on the FCC Rules and Regulations, a definition of the test, a description of the picture effect, the measurement procedure, and other helpful hints when making and interpreting these measurements.
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In-Channel Frequency Response
In-Channel frequency response is an FCC test that was reinstituted in July of 1992. It is a measurement of the gain vs. frequency over the range of frequencies occupied by the video for a specific channel. Poor frequency response will cause low picture quality. The in-channel frequency response requirement includes both the headend and the distribution system (and in the year 2000, the converter). The headend may be tested separately and added to the worst case system frequency response for a single channel or the entire system measured using the following procedures. The following sections provide detailed information on the FCC Rules and Regulations, a test definition, and a description of the picture effect.
FCC Specification: 76.601 (c): The operator of each cable television system shall conduct complete performance tests of that system at least twice each calendar year (at intervals not to exceed seven months), and shall maintain the resulting test data on file at the operator's local business office for at least 5 years.
76.601 (c)(1): For cable systems with 1,000 or more subscribers, but with 12,500 subscribers or less, proof of performance tests conducted pursuant to this section shall include measurements taken at six widely separated points within each mechanically continuous set of cables within the cable television system. Within the cable system, one additional test point shall be added for every additional 12,500 subscribers or fraction there of. Such proof of performance test points shall be balanced to represent all geographic areas served by the cable system.
...at least one third of the test points shall be representative of the subscriber terminals most distant from the system input in terms of cable length.
... An identification of the instruments, including the make, model number, and most recent date of calibration, a description of the procedure utilized, and statement of the qualifications of the person performing the test shall be set forth.
67.601 (c)(2): Proof of performance tests... shall be made on a minimum of four channels plus one additional channel for every 100 MHz, or fraction thereof, of the cable distribution system upper frequency limit... The channels selected for testing must be representative of all channels within the cable television system.
76.605 (a)(6): The amplitude characteristics shall be within the range of ±2 dB from the .75 MHz to 5.0 MHz above the lower boundary frequency of the cable television channel, referenced to the highest and lowest amplitudes within these frequency boundaries. Note: Prior to December 30, 1999, tests do not include the converter frequency response; after that date the converter or other equipment must be included in the test, if it is provided or maintained by the cable operator.
76.609 (I): For systems using cable traps... measurements may be performed prior to the trap. The effects of the trap... must be attached to the proof of performance records.
Definition: Frequency response is the measurement of the amplitude versus frequency variation in an RF system (the video processing equipment contributes to the overall frequency response). Typically the frequency response is measured in dB (ratio of the maximum to the minimum gain variation)  and is stated as ±X dB where X is 1/2 of the total variation. In some circumstances the maximum total variation is stated as the peak-to-valley. The frequency response of the channel is measured from .75 to 5.0 MHz above the channel frequency boundary or from -.5 MHz below the video carrier frequency to + 3.75 MHz above the video carrier frequency. Across this frequency range the gain of the system must not exceed a difference of 4 dB peak-to-valley or ±2 dB from the mean level.
The frequency response's effect on the picture quality will depend on the particular signature of the response and the degree of gain inequity at various frequencies in the channel. Disturbances may include: fuzzy edges of the objects in the picture or un-intentional brightness variations from top to bottom or right to left in the picture. It can also cause color streaking and smearing and high frequency roll off. This will produce soft, slightly out of focus pictures with weak colors and low contrast.
The measurement procedure involves total channel frequency response including the effects of the headend processing equipment, demodulators, descramblers, encoders, video switches, modulators, and the distribution systems frequency response over a particular channel. The NTCA recommended practices outlines two methods for measuring in-channel frequency response. One provides for adding two individual measurements or making a total measurement. Both procedures have advantages and disadvantages which you'll need to evaluate for yourself. If we assume that we want the most accurate measurement without interfering with our system's operation, then the most desirable method is to use a VITS test signal inserted at the headend and measured at our distribution system test points utilizing a demodulator and waveform monitor/vector scope. This testing method is applicable to all channel configurations except those using RF processors to convert a channel from one carrier frequency directly to another, without demodulating the video signal. If the programmer supplies a VITS signal on the processor channel, using the supplied test signal avoids the difficult process of inserting a test signal which requires system interruption. The major drawback to testing the complete system is that two technicians are required; one to make the field measurements and another to set up the test signal generator in the headend. Setup time is saved if the FCC Composite and FCC Multiburst test signal are both provided by the generator and are programmable on any VITS line and field. Using either method, the test procedure is the same, whether measuring at the headend test point or at the subscriber drop. Using the Multiburst method, by documenting the amplitude of each frequency packet, will provide the most accurate and consistent information. System end-to-end testing will provide the most accurate measurements.
To measure the frequency response of a channel without interference to the  system's operation you will need to insert a VISTA test signal in the VBI of the channel to be tested. This should be done at the satellite receiver output ( or following the decoder if used). Note the insertion diagram in Fig. 2
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Percent Modulation
There are no FCC specifications for Percent Modulation, which is also known as Depth of Modulation. Percent Modulation is however, one of the most critical parameters in the headend. The proper level of modulation is 87.5% for NTSC type A5C negative modulation. Over modulation produces far more severe problems that under modulation. Over modulation almost ensures that your system will fail the otherwise easy to pass FCC specification for Differential Gain and Differential Phase. Over modulation also produces sparkles in the picture and sync clipping. The effects of clipped sync varies from TV to TV and VCR; some sets will not sync properly or will occasionally roll. Sparkles will be more noticeable on higher quality TV sets, especially large screen sets. Under modulation reduces the brightness of the picture. This will be especially noticeable when switching between channels with more than 5% difference in their percent modulation. A low percentage of modulation will produce a weak luminance signal at the video detector of the TV receiver. Turning up the TV set's brightness control to compensate for the weak signal enhances the noise that is present in the signal. Again, larger screen TV sets will display this problem more predominately.
Percent Modulation is one of the parameters that has been typically disregarded or monitored by idiot lights on the modulator. % Modulation cannot be properly set using the indicators on most modulators, looking at a monitor, or turning up the modulation until you see sparkles, and then backing off the modulation slightly. Indicators on the modulator usually cannot respond properly to the high frequency color signals. There are three procedures described in the NCTA recommended practices. Any of these, used carefully, will provide the proper % Modulation setting.
Definition: % Modulation of a video carrier is the percentage of the difference between the maximum and the minimum of the RF envelope amplitude divided by the maximum RF envelope amplitude
% Mod = 100 X (A-B) / A%
The maximum RF envelope amplitude occurs at the sync tips. The minimum RF envelope occurs at the whitest part of the picture. To properly measure or set the % Modulation, the modulated carrier must contain both sync tip peaks and a 100% white signal. If suppressed or stripped sync is used in your scrambling system, the scrambling must be turned off or a procedure permitting measurement of the unsurpressed sync during VBI must be used.
As mentioned earlier, picture effects of over modulation are far more detrimental to picture quality than under modulation. The problems associated with over modulation are caused by the high level of luminance information and non linearity in the modulator. The non linearity is measured in terms of Differential Gain and Differential Phase. High Differential Phase produces changes in the hue of a color as the brightness changes, when only the picture brightness should change. Under modulated signals will produce a picture with low brightness. This low brightness or weak luminance signal will produce poor contrasting and fuzzy pictures.
There are three measurement procedures described in the NCTA Recommended Practices. Of the three, the preferred method is to use a precision demodulator and waveform monitor vector scope, or wide bandwidth oscilloscope. Using a spectrum analyzer requires taking the channel out of service or depending on the program content for the required white level signal to measure the minimum RF envelope amplitude. A VITS signal cannot be used since the spectrum analyzer cannot easily be triggered on a specific line containing the VITS test signal with a white reference. Compromises also must be made, either limiting the bandwidth of the signal being tested or temporarily lowering the sound carrier of the channel under test and the lower adjacent channel.
Another method that includes the use of a signal level meter has the same limitations as using the spectrum analyzer method. In addition, some signal level meters have additional characteristics that limit their ability to perform the demodulator function. Some meters have video AGC circuits that maintain a 1 V video output, which prevents linear changes in video output proportional to RF input level changes. Other signal level meters have an AGC IF system, which also prevents the desired video level amplitude change with RF level changes. Signal level meters are also typically designed to detect only the peak signal level amplitude and do not exhibit detector characteristics.
The precision demodulator and CATV Waveform Monitor/Vectorscope method is the more precise method and can be accomplished without interference to system operation. This method uses a demodulator with a zero carrier reference pulse that effectively turns off the carrier, simulating 0% modulation. As with the other % modulation testing procedures, it is best that a test signal with a white reference be used for testing. This can easily be done using a generator in the VITS mode, generating the FCC combination test signal as used for the other tests.
As you can see, these tests can provide a true evaluation of the quality of signal being delivered to your customer. In the next issue of the Cable Tech Facts, we pick-up with the remaining performance measurements that can make a difference in your system quality of service. If you would like to receive a complete hands-on training guide to making the color tests, call 1-800-SENCORE (736-2673) and talk with your Area Sales Representative.
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Industry News
SCTE Changes Their Name And Gets New Members
The SCTE will officially become the Society of Cable Telecommunications Engineers next month when the board of directors meets just before the opening of the annual Cable-Tec Expo in Las Vegas. The new name substitutes the word telecommunications for television which reflects the changes going on within the industry. The name change was approved by 93 percent of the SCTE members who voted in the latest board
elections.
MPEG-2 Market Heats Up At NAB
While cable operators are frustrated by the delayed introduction of digital set-tops, the fast-moving MPEG-2 market continues to heat up with new product announcements and news that an intellectual property rights group has reached agreement on royalty payments. During the last National Association of Broadcasters (NAB) in Las Vegas, several new encoders and servers made their debut, signaling the start of the MPEG-2 era. For example, announcements were made by Scientific-Atlanta, Micropolis, Compression Labs Inc., Hyundai, Optivision, C-Cube Microsystems, and others.
Bigger Bell
Bell South has chosen Northern Telecom to build a $100 million GSM-based personal communications network for the Carolinas and Eastern Tennessee. Construction should take about three years.
FCC Cable Bureau Splits Into New Units
The FCC Cable Services Bureau has split into four divisions; financial analysis and compliance, which will review rate complaints and "social contracts"; consumer protection and competition, which will handle must-carry and program-access complaints; policy and rules; and engineering and technical services.
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