cable tech facts issue 106

In This Issue

• Making Differential Gain Measurement
• Making Differential Phase Measurements
• Making Chroma To Luma Delay Measurements
• Other Tests For Video Quality
• Industry News

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In this issue of the Cable Tech Facts, we will be covering the remaining system performance tests, including Differential Gain, Differential Phase, and Chroma To Luma Delay. We will also be discussing some additional tests that will prove valuable in determining the quality of signals being delivered at your headend.

Differential Gain

Testing the Differential Gain characteristics of the headend is a required test by the FCC. These measurements must be made and documented every three years, although good engineering practices would warrant testing at a more regular basis. Differential Gain is a measurement of the variation in gain of the color frequencies as the luminance level varies. Poor differential gain can cause poor color picture reproduction. Newer CATV video analyzers provide an automated test which automatically takes the measurement, makes the calculations for the complex measurement, and displays the reading.

FCC Specifications
76.601 (c)(1): For cable systems with 1,000 or more subscribers, but with 12,500 subscriber or less, proof of performance tests conducted pursuant to this section shall include measurements taken at six widely separated points with 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 subscriber terminals most distant from the system input in terms of cable length . . . an identification of the instrumentation, 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.

76.601 (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's upper frequency limit . . . the channels selected for testing must be representative of all the channels within the cable television system.

76.605 (a)(12): The Differential Gain for the color subcarrier of the television signal, which is measured as the difference in amplitude between the largest and the smallest segments of the chrominance signal (divided by the largest), shall not exceed 20%. Again, while the FCC requires documenting only a few channels and only every three years for the "color test", you are responsible for all channels at all times should a complaint arise. Although this specification is typically easy to meet, careful attention must be given to the Percent Modulation. Over modulation will result in severe degradation to Differential Gain.

Definition: Differential Gain is the measure of the system's ability to linearly reproduce the high frequency signals at all levels of luminance. This amplitude distortion of the chrominance signal is dependent on the luminance level. Differential Gain is measured as the percentage of the difference between the largest and the smallest burst packet divided by the largest packet amplitude, when the packet levels are varied at all luminance levels. Both peaking and attenuation can occur as the luminance level is varied. For the CATV definition, the maximum variation is used and not referenced to the blanking level. Typical system deficiencies occur at the higher luminance levels.

Picture Effects: High Differential Gain results in color saturation variation dependent on the picture luminance level. This manifests itself as unwanted changes in color saturation as the brightness of the picture changes.

Measurement Procedure: The total Differential Gain includes the effects of the headend processing equipment, demodulators, descramblers, encoders, video switches, modulators, etc... If the programmer supplies a VITS signal, which you will use to test your system, each chroma burst at each luminance level must be measured at the satellite receiver and be used in calculating the Differential Gain of your system. This may not be necessary if you pass the FCC requirements without this calculation, since it is unlikely that the uplink and the satellite receiver will "help" your system, but they will definitely contribute to errors. If you use the programmers test signals and fail the test by a small margin, or get marginal results, measuring the satellite receiver output and subtracting these measurements from the system measurements will give you a better measure of your system's performance.

Remember to keep the data for each packet separate, since the distortion may be different in each piece of equipment and at different stair step levels. You must also keep the phase in mind - subtract and add vectorially. If the satellite receiver output measures 5% on the highest step and the total system measures 10% on the same step, then the actual system performance is 15%. This is a good case for inserting your own test signals.

To measure the Differential Gain of a channel without interference to the system's operation, you will need to insert a VITS test signal in the VBI of the channel being tested. This should be done at the satellite receiver output or follow the decoder, if used. Note the diagram in Fig. 1.

You will want to choose a VITS test signal with a Modulated Staircase pattern. The FCC Composite Test Signal Show in Fig. 2 is preferred since it can be used to make other key tests. Other VITS patterns may be used, such as the 5 Step Multiburst, 10 Step Multiburst, or Modulated Ramp. Be sure that the test signal chosen is compatible with the measurement device and the equipment under test.

Test signals such as the NTC-7 Composite, which have signals above the 100 IRE level, can overmodulate the carrier, generating excessive Differential Gain and Phase errors. Modulators and demodulators generally contribute most of the Differential Gain to any system. Strip amps and processors can also contribute Differential Gain, but typically much less than modulators and demodulators. Video switching and commercial insertion equipment typically contribute very little Differential Gain. Differential Phase and Differential Gain are normally found together: if you have one, you will likely have the other.

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Differential Phase

As with Differential Gain, Differential Phase measurements must be made and documented every three years. Yet you are still responsible for meeting the requirements on all channels at all times. Differential Phase is a measurement of the variation in phase of the color frequencies as the luminance level varies. Poor Differential Phase will cause poor color picture reproduction. The following sections provide detailed information on the FCC rules and regulations.

FCC Specifications: 76.601 (c)(1): For cable systems with 1,000 or more . . . same as listed for Differential Gain.

76.601 (c)(2): Proof of performance tests . . . shall be made on a minimum of four channels plus one additional channel for every 100 MHz, . . . same as listed for Differential Gain.

76.605 (c)(13): The Differential Phase for the color subcarrier of the television signal, which is measured as the largest phase difference in degrees between each segment of the chrominance signal and the reference segment (the segment at the blanking level of 0 IRE), shall not exceed ±10 degrees.

Again, while the FCC requires documenting only a few channels, and only every three years for the "color tests", you are responsible for all channels at all times should a complaint arise. The Differential Phase test is often referred to as one of the "new or Color Tests" along with Differential Gain and Chroma To Luma Delay. Although this specification is typically easy to meet, careful attention must be paid to the % Modulation. Over modulation will result in severe degradation to differential Phase.

Definition: Differential Phase is the unwanted change in phase of the chrominance signal as the amplitude of the luminance signal changes. Differential Phase is measured as the greatest change in phase at any luminance level expressed in degrees, referenced to the burst at blanking level. Both lead and lag errors can exist at different luminance levels, positive is lagging and negative is leading. Measurements should state the maximum phase error from the burst reference and the polarity. Note that to meet the requirements, 12 degrees positive and 5 degrees negative would not be considered <=10 degrees.

Picture Effect: High Differential Phase will result in color hue variation that is dependent on the picture luminance level. This is seen as unwanted changes in color hue as the brightness of the picture changes. Typically the higher luminance levels of the picture are the most affected by Differential Phase distortions.

Measurement Procedure: The total Differential Phase includes the effects of the headend processing equipment, demodulators, descramblers, encoders, video switches, modulators, etc. If the programmer supplies a VITS signal which you would use to test your system, each chroma burst at each luminance level must be measured at the satellite receiver and be used in calculating the Differential Phase of your system. This may not be necessary if you pass the FCC requirements,since it is unlikely that the uplink and satellite receiver will "help" your system, but they definitely will contribute to errors. This again, is a good cause for inserting your own test signals, but if you don't own a VITS insertion generator, make the end of the system tests first. If you pass, you will not have to do the other tests at the satellite receiver. You must keep phase in mind; subtract and add vectorially. If the satellite receiver output measures -5 degrees and the total system measures 10 degrees, then the actual system performance is 15 degrees.

To measure the Differential Phase of a channel without interference to the system's operation, you will need to insert a VITS test signal in the VBI of the channel to be tested. This should be done at the satellite receiver output or decoder output if used. Note the insertion diagram in Fig. 3

You will want to use the FCC Composite VITS test signal as shown in Fig. 4, since it will permit making most of our other tests. Other VITS patterns may be used, such as the 5 Step Multiburst, 10 Step Multiburst, or Modulated Ramp test signals. Be sure that the test signal chosen is compatible with the measurement device and the equipment under test. Test signals such as the NTC-7 Composite, which have signals above the 100 IRE level can over modulate the carrier generating excessive Differential Gain and Phase errors. As mentioned earlier, if you have Differential Phase distortions, you will likely find Differential Gain distortions also.

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Chroma To Luma Delay

Chroma To Luma Delay is, as of June 30, 1995, an FCC required performance measurement that is to be tested through the headend every three years. Operators are still responsible for meeting the requirements on all channels at all times. Chroma To Luma Delay is a measurement of the variation in phase (timing) of the color and the luminance information. Chroma to Luma Delay causes poor color to luminance picture registration. Here is the detailed information on the FCC Rules and Regulations.

FCC Specifications: 76.601 (c)(1): For cable systems with 1,000 or more subscribers . . . basically the same as for Differential Gain.

76.601 (c)(2): Proof of performance tests . . . shall be made on a minimum of four channels . . . same as was listed for Differential Gain and Phase.

76.605 (a)(11): The Chrominance to Luminance Delay inequality or chroma delay, which is the change in delay time of the chrominance component of the signal relative to the luminance component after it passes through the system, shall be within 170 nanoseconds.

Definition: Chrominance to Luminance Delay, sometimes called Chroma Delay, is the measurement, in time, that the chrominance component of the video signal is delayed through the system, referenced to the luminance signal and stated in nanoseconds. The NTSC system "M" uses 170 nanoseconds of pre-correction. Modulators and receivers both anticipate this correction; delay measurements are made for times in excess of or referenced to the 170 nSec. intentional delay. The specification is 170 nSec by coincidence.

Picture Effect: Chroma To Luma Delay causes a mis-registration in the color and luminance information in the picture. Picture distortions appear as color smearing or bleeding at the edges of the objects in the picture. Sharp luminance transitions may also become fuzzy.

Measurement Procedure: The total Chrominance To Luminance Delay includes the effects of the headend processing equipment, demodulators, descramblers, encoders, video switches, modulators, etc. If the programmer supplies a VITS signal which you can use to test your system, you will want to measure the delay at the satellite receiver and subtract the delay from the total system delay measurement. Remember to subtract vectorially. If the satellite receiver output measures -50 nSec. and the total system measures 100 nSec, then the actual system performance is 150 nSec. In addition, Chroma To Luma gain inequality will limit the ability to measure Chroma To Luma Delay. A value of 2 dB of amplitude inequality will limit the minimum delay measurement to 50 nSec. This too, is a good case for inserting your own test signals.

To measure the Chroma To Luma Delay of a channel without interference to the system's performance, you will need to insert a VITS tests signal in the VBI of the channel to be tested. This should be done at the satellite receiver output or the encoder if one is used. For an example, see Fig. 4.

You will want to use the FCC Composite Test Signal with a Modulated 12.5T sine squared pulse in the VITS, as shown in Fig. 4. Other test signals can also be used, but be sure that the test signal is compatible with the measurement device and equipment under test. Test signals with over 100 IRE can over modulate the carrier, and generate excessive Differential Gain or Phase errors. Modulators and demodulators generally contribute the most Chroma To Luma Delay to any system.

Strip amps and processors can also contribute to the problem, typically much less than modulator and demodulators. Filters and traps used to reduce TI prevent pre-amp overload, separate broadcast signals, etc., are the second most common cause of Chroma to Luma Delay. Video switching and commercial insertion equipment typically add very little to the problem. High Differential Gain and/or Phase Distortions will make Chroma To Luma Delay Measurements nearly impossible.

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Other Tests For Video Quality

As we mentioned at the beginning of this article, there are other tests that are not required by the FCC, but do provide additional information regarding your headend performance.

One test is Line Time Distortion, those distortions which are 1 to 64 µs in duration and manifest themselves as tilt in the reproduction of medium length pulses in the 1 to 64 µs duration range. The percentage of line time distortion can be measured by viewing the FCC Composite test signal, setting the white reference pulse to 100 IRE, and comparing the tilt of the white reference bar to the 100 IRE reference. Typical performance for headend equipment will be <15%. Like phase distortions, filters and traps will increase the line time distortions of the video signal.

Another performance test is Luminance Linearity. Luminance Linearity is a measurement of the distortion of the video signal as the luminance level increases from 0 to 100 IRE in the waveform monitor mode. Luminance non-linearity typically occurs at the higher luminance levels and manifests itself as poor resolution in the brightest portion of the picture. Measuring luminance linearity is similar to measuring Differential Gain, except that a staircase without modulation is used. The step-to-step height is compared on each step to identify the largest and smallest steps as noted in Fig. 5. The difference in amplitude is divided by the largest step amplitude resulting in the percent of non-linearity. Modulators are the most likely cause of luminance non-linearity. Acceptable performance should be <10% distortion. Measurements should be made with the demodulator in the synchronous mode.

Although there is no FCC requirement for Chrominance Phase Distortion, it is another important test for your headend performance. Chrominance Phase Distortion is the result of a phase shift in chrominance signals as the level of chrominance signal changes. The end result is changes in the hue as the color saturation changes. This is typically evident at higher chrominance signal levels. Chrominance Phase Distortion should be <10 degrees in the headend. Like Differential Phase Distortions, filters and traps increase the Chrominance Phase Distortion of the video signal.

Another performance test is the Chrominance Gain Distortion. Again this is not an FCC required test, but it will give you a better understanding of your headend performance. Chrominance Gain Distortion results from the nonlinear reproduction of the amplitude of the chrominance signal, The picture effect is improper color saturation in the picture. This is typically evident at higher chrominance signal levels.

The last additional headend test is the Chrominance To Luminance Intermodulation. This condition is the result of the non linear reproduction of the amplitude of the luminance signal as the chrominance signal level varies. Chrominance To Luminance Intermodulation occurs in variations of brightness as color saturation changes in the picture.

Top Performance
While only Chroma To Luma Delay, In-Channel Frequency Response, and Differential Gain and Phase are required by the FCC, many of the previously mentioned tests will be helpful in troubleshooting or evaluating your headend performance and insuring that only quality pictures are being delivered to your customers. Following these guidelines should be helpful in making your "color" tests easier and more accurate. Further information is available from many sources. Refer to the SCTE or NCTA for reference materials.

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Industry News

HBO Picks B-Frames
The HBO network made a decision to transmit its digital programming using main level, main profile B-frames when it changes over to MPEG-2 digital encoding in the fourth quarter. The move was made after the network determined that its affiliates would be buying digital set-tops compatible with B-frame coding scheme. The B-frame name is derived from their bi-directional coding capabilities. Standard MPEG-2 in the dual prime mode uses information from previous frames to build a current video frame. B-frame technology takes a step further by looking at the next frame as well as the previous frame to build a central video frame. The use of B-frames will allow operators to receive higher signal quality at lower transmission bit rates.

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