cable tech facts issue 103

In This Issue

• Video Terms And Their Definitions
• Typical System Configuration
• VITS Test Signal Insertion
• Test Your Knowledge
• Industry News

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In this issue of the Cable Tech Facts, we will do a quick run down of common terms and their definition. Many of these terms may be very familiar, others may not. We hope that this information will be useful while learning to perform the "color or video " tests.

Video Terms And Their Definitions:

Composite Video Signal:
The composite video signal illustrated in Figure 1 contains all of the information that is necessary to produce a complete color video picture.

The TV Channel:
The standard TV channel consists of two RF carriers: the video carrier and the audio carrier. Figure 1 shows the carrier frequency allocations within the 6 MHz channel bandwidth. The audio carrier is FM modulated and is placed 4.5 MHz above the video carrier frequency. The carrier FM deviation is 25 kHz and is typically operated 15 to 17 dB below the video carrier level to reduce system loading and avoid interference with the upper adjacent video signals. The video carrier is 1.25 MHz above the channel's lower boundary and is AM modulated with the luminance information up to ~4.2 MHz. The color information is modulated on a 3.58 MHz subcarrier.

Video Modulated Carrier:
The video carrier is a type M negative AM vestigial sideband modulated. That is, the lower sideband is limited to .75 MHz below the video carrier frequency at full amplitude and to 1.25 MHz after attenuation by filtering as shown in Figure 1. Vestigial modulation creates an inequality in the frequency response of the modulated signal if a simple detector were used to demodulate the signal. The inequality of this type of modulation, when detected, is compensated by the IF filter's shape in the receiver and is a "mirror image" of the vestigial frequency response. The video modulation ranges from the near DC to 4.2 MHz and includes both the luminance and color information.

The Video Bandwidth:
As mentioned earlier, the luminance signal includes frequencies ranging from near DC to 4.2 MHz and color signals slightly above and below 3.58 MHz. Therefore all equipment which processes the composite video signal must have a sufficient bandwidth to correctly reproduce all the signals from near DC to 4.2 MHz without amplitude or phase distortion. High frequency roll-off will reduce the clarity and fine definition of the picture.

Transporting Video:
While not as high in frequency as the RF signals used on the distribution system, composite video signals must be considered RF and careful attention must be given in interconnecting between the various components in the headend or studio. Proper grounding, shielding and impedance matching are very important.

Cable and Connectors:
Grounding and shielding of the video signals and processing equipment are very important. As more video processing, in the form of commercial insertion, syndex switching and local origination in used, the video path becomes longer and longer between the satellite receiver, demodulator, or modulator. As these video paths become more complex, the importance of shielding the signal from other signals and noise becomes more and more important. We know from our system work that an interfering signal 50 dB down from the desired signal causes visible picture distortion. High quality coaxial cable and carefully installed connectors will minimize ingress of undesired signal in the baseband video signal paths. Proper grounding is also very important, as ground loops between equipment can exist even though they are connected by the shield of the coaxial cable. Just 10 millivolts developed across the ground between the video processing equipment and the modulator can generate 1% hum in the picture. Equipment ground straps and strapping between equipment racks will reduce the possibility of hum generated into the baseband video signal processing system. Clamping circuits or DC restorers found in some equipment inputs effectively eliminate minor ground loop problems. If you are unsure, refer to the headend equipment operation manual for specific equipment capabilities.

Impedance:
As with the distribution systems, proper impedance matching must be maintained throughout the video path. All video equipment should be terminated into 75 ohms and be interconnected using 75 ohm cables and connectors. This is not as simple as it may appear. Some equipment is fixed with 75 ohm internal impedance, while other equipment has a switchable 75 ohm terminator, some will have loopthrough connectors that require an external terminator, or the equipment may be high impedance with no provision for termination. This can become confusing, but is very important. In some complex configurations two terminators can be as bad as none. Check each piece of equipment or the operator's manual carefully. Without proper termination, the signal level will be too high, if un-terminated, and too low if it is double terminated. This can severely effect the S/N and the % Modulation, thus effecting many other elements of your customer's picture quality.

Signal generators are typically loopthrough type devices that require you to interrupt the signal path. (The Sencore VIG791, however, includes a low cost insertion routing device that may be left permanently installed in the video path for future use without service interruption). Measurement devices are either high impedance, or switchable from high impedance to 75 ohms. High impedance devices may be connected to the video path without affecting operation. Before connecting any device, be sure it is in the high impedance mode. If a 75 ohm impedance is connected to a video path which is already properly terminated in 75 ohms, the signal level will be "loaded down" by -3 dB or approximately 70% of the proper voltage level. This may reduce the % Modulation, increase S/N and cause the operator to reset the % Modulation to a level that will result in severe over modulation when the measurement device is removed.

Typical System Configuration:
Figure 3 illustrates a portion of a typical headend. Notice the various video paths that are used in different programming situations. When testing such systems,the operator should test each of the possible paths to be sure that each video source provides the required 1 V of video with acceptable noise and distortion performance. Each device in series with the video path should also be tested in all modes of operation. This includes video switches, encoders, decoders, etc.

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VITS Testing:
VITS is an acronym for Vertical Interval Test Signals, but is often used to describe the method of inserting test signals into the vertical blanking interval. The VBI (vertical blanking interval) is the portion of the signal which produces the black edge at the top and bottom of the TV screen. It is also responsible for blanking the electron beam during the time that is reuired to move the beam from the bottom back to the top of the screen. Since the lines are not viewed or a part of the actual picture, additional information can be added to these lines without effecting the picture or TV receiver operation. This area is most often used for closed caption signals, VITS, VIRs and data signals. Many test signals, such as the FCC Composite, FCC Multiburst, Modulated Ramp, or others can be added to the VBI and used for testing the quality of the video signal without interference to the normal operation of the system. A frame of the video signal contains two fields, the even and the odd fields are interlaced to prevent flicker in the reproduction of the picture. The illustration in Figure 4 shows one field of video with the VBI containing VITS in addition to all the other normal video components.

VITS Test Signal Insertion

Waveform monitors and vector scopes must have a line select feature, which permits the operator to select a single line in either field for display in order to use the VITS test signals. VITS generators also require normal sync to properly insert the test signals.

Conclusion
This completes our review of the video signal and its components. As you can see there is a lot of information packaged in the video information. Each individual component is essential to provide a good clean picture for the customer. That is why it is important to run a performance test of the signal you are providing on a regular basis. Any change in the video information can greatly affect the end results. As many technicians are aware, the FCC is requiring cable system operations to test for Differential Gain, Differential Phase and Chrominance-to-Luminance. The mandate also states that the measurements must be made on at least four channels. (plus one channel for every 100 MHz upper frequency limit above 100 MHz).

These three color measurements are relatively easy to make and are only mandatory once every three years. Many operators already make these measurements and find that it does help maintain a higher quality picture for their customers. In the next issue of the Cable Tech Facts, we will begin to review how to perform the different color tests and their effects on your system's end result. This will include using test instruments and any shortcuts that can be used to help you in performing these tests. We will also show you some additional tests that are not required by the FCC but are more critical to the quality of your system's performance.

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Test Your Knowledge
In order to give you a sample of the information that you will be learning in the next issue, we like you to try to answer these few simple questions to find out how prepared you are to start making the color performance tests. They are true and false questions.

1. While these measurements are similar to the measurements commonly made by broadcasters, there are differences in measurement
   definitions and criteria used for testing. You may not need to purchase high end, expensive broadcast instruments to do your testing.
   
2. The are a lot of other applications for a waveform monitor and vector scope capabilities in setting up and maintaining your headend.
   
3. While you only have to document a few channels every three years, the system is responsible for all the channels at all times.
   
4. No sweep systems really fulfill the frequency response testing requirements.
   
5. There is a way to avoid system interruption, when making the new color tests.
   
6. The headend performance can contribute to C/N and Hum distortions, decreasing picture quality-and common testing methods will never show the problem.

Call your Area Sales Representative at
1-800-SENCORE with your answers. They
will be glad to help in any way they can.

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Industry News

FCC Approves DBS Transfer
A new Direct Broadcast Satellite system could be launched as early as this fall, adding the first direct competitor to the Hughes/Hubbard Direct TV service that is currently operating. The FCC recently approved the sales of SSE Telecom's DirectSat Corp. to Echostar Communications. The acquisition means that the construction permit and orbital slot assignment for 119 degrees West, a prime slot for U.S. coverage, are now under the control of a single entity. Echostar has stated it intends to move ahead with its DBS plans, using 22 frequencies from that location. The company's first satellite, EchoStar 1, is currently under construction by Martin Marietta and is scheduled for launch this fall. EchoStar II, slated for launch in 1996 is expected to allow 250 channels of programming.

Most Cable Subscribers Would Switch
More than half of U.S. cable households would switch from their current operator to a telco or satellite carrier that supplies TV service, according to a new survey from Odyssey, a market-research company. Only one-third of the survey's 4,000 respondents rated the image of their cable operator as "very good" while 55% said they would be very likely to switch to another provider if it offered competitive prices. The group least satisfied with cable service: people who said they're open to new technologies.

Equipment For Interactive Service
Sony Corp. has plans to develop a range of equipment for interactive services viewed over TV sets, which might include video servers and switching gear. Sony's reasoning behind the development of the new equipment is the growing demand of cable operators for new cable converter boxes that will receive hundreds of channels. The set-top-box market is expected to grow from about $550 million today to $1 billion in 1996. Sony expects the first box to be ready late this year.

"Ghost" Market
Philips Consumer Electronics Company has formed a new business unit, the Specialty Television products Group, that will handle the worldwide marketing and manufacturing of the Philips Ghost Cancellation Reference (GCR) technology. The Philips GCR signal has been recommended as a world standard by the International Telecommunications Union. The new division will be responsible for the television ghost cancellation system sales and installation assistance to terrestial and cable networks, and local stations. The Philips ghost canceling decoders should be available to consumers early this year and are expected to improve television picture and solve multi-path problems such as color fading and mis-registration.

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