Z Technology 's RF Newsletter - DTV Edition
Broadcast News, Edition 5
DTV RF Transmission Path Measurements
In the broadcast studio, baseband video signals are well controlled. Both analog and digital video signals can be very good. While digital video is easier to handle and process, either NTSC or digital studio formats provide a quality higher than can be maintained in the traditional analog television channel.
NTSC transmission of television signals to the home has been the norm for almost 50 years, and remains important part of the broadcast industry revenue stream. While most television station engineers consider transmission of the NTSC signal to be simple and well understood, it is in fact a relatively complex process. In the studio, a bandwidth-limited matrix of three color video channels is quadrature modulated to provide chroma information sidebands which is added to a derived luma to produce an NTSC baseband video signal. The "analog" NTSC transmitter then band-limits the baseband video, amplitude modulates it onto an RF carrier, and filters the modulation products to create a vestigial sideband AM signal which is then transmitted along with an FM sound carrier to provide a complete video/audio RF program transmission. The weak link in this process is the analog RF transmission system to the viewer.
Recite the following as necessary:
Digital video signals also a represent three color video channels, and there is lots of room for multi-channel audio and other ancillary data. Each of the video and audio channels is separately digitized and time-division-multiplexed into a data stream. In the transmission channel, this is an analog signal; except that the analog fluctuations in the digital transmission signal represent data rather than video or audio signal levels directly. The video, audio, and ancillary information is finally converted to numeric data formatted to industry standards and compliant with FCC guidelines. By recovering these numbers, the receiver knows how to recreate the video and audio information from the source. So all we need in the analog transmission system is a stream of numbers.
In studio digital formats, baseband video is represented by two levels with numeric data contained in the transitions between high and low levels. The FCC-defined Digital Television (DTV) broadcast transmission system uses 8 voltage levels (8VSB) with an RF bandwidth of 6 MHz. By processing the numeric video data into an MPEG-2 data stream, several standard-definition or one high-definition video signals, plus multiple channels of audio and ancillary data can be contained within the allowed 19.38 Mb/s data stream; which fits into the spectrum of a 6 MHz bandwidth broadcast television channel. The weak link in this process is still the analog RF transmission system.
Why, then, DTV?
Digital television (DTV) transmission provides the home viewer a number of very important benefits. The data signal, when correctly received, is free from the effects of transmission path ghosting and noise common in the NTSC transmission system. MPEG-2 compression artifacts are managed at the origination point, and subjectively, the television program video and audio can be as good in the home as leaving the studio. To the viewer, DTV reception is remarkably cleaner and clearer than NTSC. Although numeric data must be received exactly as transmitted recreate useful video and audio, the DTV transmission system is designed to be very tolerant of transmission errors.
Why be concerned with the RF transmission path?
If the numeric data is perfectly reproduced in the receiver, there is no concern. Unlike, NTSC transmission, however, program content is often lost completely when the DTV receiver receives corrupted data. What happens if the data is not perfectly received?
If the analog RF transmission path is unable to convey perfect data, the data receiver may fail completely. An incomplete number is a completely different number….. usually meaningless to the receiver. Paraphrasing the old nursery rhyme, "When data is good, it is very, very good; but when it is bad it can be horrid". So it is very important the viewer be able to recover perfect data from the DTV signal.
We want to know whether the program data can be perfectly received; and how close the data is to failure.
Where do we start?
The road to DTV is driven simultaneously by business and technical considerations. Viewer benefits come from DTV’s relative immunity to analog characteristics of the transmission channel, and the availability of additional content through digital compression techniques. The television broadcast business benefits when viewers and advertisers accept the transition to DTV, and the NTSC transmitters can be turned off. The FCC benefits when empty NTSC channels can be reassigned. And the technical community benefits from the reduced maintenance of a properly designed and installed DTV service.
In many cases, the initial DTV transmission system will defer to the revenue generating NTSC transmission system. The antenna will often be a temporary, sometimes a lower-power design, side-mounted on the NTSC channel transmission tower. The DTV transmitter may be a low-power version of the final system. This temporary system provides DTV service to a key set of viewers and serves as a technical model for the final installation. The key is to ultimately provide a system that will cover your service area in accordance with your business needs and the expectations of the FCC. Those with a clear map through the transition will most enjoy the ride.
Characterizing the RF transmission path
The reception quality of a DTV signal can be characterized by measuring both its RF properties and its digitally decoded properties. The following table gives a brief explanation of measurable properties, also called Figures-of-Merit:
As a practical matter, it is impossible to measure the signal at every location, and all of the variables of a viewer’s installation. This situation can be efficiently addressed, however, by a staged approach.
1. System Acceptance
You need to know that the system is installed correctly and performing to its design specifications. RF field strength measurements should be taken along six to eight radials, depending on the antenna design, to confirm the horizontal antenna pattern and system power gains/losses. This first set of measurements should be made equidistant, and as near the same elevation (both above ground and above mean sea level) as possible, to avoid vertical pattern effects. An additional set of measurements should be made at different distances from the tower to confirm the antenna pattern in the vertical direction. These initial measurements are easy to make with a field strength meter or spectrum analyzer. A programmable field strength meter with a spectrum display enhances the confidence of the measurement and, along with GPS location information, provides for later analysis. Additionally, the programmable field strength meter can be set up to also record data, using appropriate antenna factors, for your NTSC channel and for other DTV channels in your area. If decoding equipment is available, you’ll also want to know if the DTV signal can be decoded. The decoder’s equalizer lock gives a go/no-go indication. An indication of margin can be obtained by looking at the decoder’s signal-to-noise ratio, tap energy, and segment error rate values.
Discovering installation problems early will avoid many hours of puzzlement later, and having a signal strength record of the initial system will provide valuable information when important viewers have problems receiving the signal from the final system.
2. Problem Discovery
You will want to "drive test" the coverage area to discover any problems. This would be expected to reveal any pattern nulls not detected in the radial tests; signal nulls due to reflections; and areas shielded by terrain or buildings. A drive test measures signal strength, at a lower elevation, but mostly in the clear, from a moving vehicle. The test is done with a programmable field strength meter and includes GPS location information. A calibrated dipole antenna is located about 24" above the center of the vehicle roof. Valid readings are obtained with the antenna generally oriented towards the transmitter, and the antenna direction must be corrected whenever the path of the vehicle changes by about 30 degrees. Except for being aware of general antenna direction, the measurement requires little operator attention. (Operation of the system by the vehicle driver is specifically not recommended.) Measurement parameters of interest in this moving environment are integrated power, peak power, and if a decoder is available, sync lock.
Since drive test results can be plotted on a map, problem locations can be revealed immediately.
3. Detailed Analysis
A detailed analysis should be performed at any problem location revealed by the drive test. Detailed testing should be done while stationary, to allow analysis with a swept field strength meter or spectrum analyzer. The calibrated antenna should be at a standard height (for example 30 ft) and oriented with the aid of the spectrum or Tap value display from an instrumentation quality DTV decoder. The decoder included in the Z Technology DSS5800 DTV Measurement System also provides other valuable information on the ability of a receiver to lock onto and decode signal data. This test is standardized to provide comparable information from site-to-site, avoiding as many variables as possible. Since this test is stationary, the operator can record many values for later analysis. It becomes more useful if the operator only has to be concerned with the physical antenna setup and the test equipment records all of the results in a standard format file.
4. Solving Reception Problems
Fourth, despite all of your planning and testing, a very important viewer (i.e., advertiser, contributor, or your boss) will have a reception problem. Being able to rapidly diagnose the problem at this viewer’s home is most efficient and economical; but the problem might also be affecting other viewers who have simply fallen back to your NTSC transmission. Being able to take portable field strength and decoding test equipment from your vehicle into the home can be a great advantage, and provides the consistency of using the same equipment in both tests.
5. Final System Checkout
As DTV becomes more prevalent, and as the importance of your NTSC transmission fades, you will be upgrading to the final DTV antenna, antenna location, and transmitter. (In some case, to your final DTV channel assignment.) It is important to determine if the DTV transmission system provides an adequate signal over the city of license. To avoid reception problems, and to provide the best possible service to your customers, it is extremely useful to have good records of signal coverage before, during, and after the transition.
DTV: Little different, but better
It is important to note that the equipment and procedures described above are really little different for DTV than for the traditional NTSC transmission systems. In fact, if you measure your NTSC system along with your new DTV plant, you may discover some interesting things about your traditional broadcast coverage.
The simultaneous move to DTV by many broadcasters has generated new activity in the design of programmable test systems. New signal measurement systems are smaller, easily transportable, more reliable, and much more capable than the collection of individual instruments used to measure NTSC signals just a few years ago. Modern systems are GPS mapped, and benefit from modern PC operator interfaces. The system PC coordinates instrument functions, including the selection of frequencies to be measured or swept, and the application of antenna correction factors. The data recorded is used for immediate analysis and is available for comparison as the RF transmission system changes in the future.
DTV offers many advantages to the broadcaster and to the viewer. It provides a broader data path to the home, with the potential of a greater number of services. It is more precise than NTSC analog transmission, and, with modern test equipment, the DTV signal is easy to analyze and understand. Everything is exactly as the numbers define and little is left to the imagination. DTV may be new, both to the station engineer, and to the viewer. There will be concerns, but if these concerns are addressed early in the implementation phase, there will be a clear path to follow as the system is finalized.
The weak link will always be the analog RF transmission path transporting the data. By measuring received signal field intensity and determining other RF and decoded figures of merit and plotting them over the licensed area, an operator can reliably gauge signal receivability in the coverage area. With this information, the engineer has real measured data from a transmitted DTV signal for review and analysis. Modern RF transmission equipment and measurement systems make it easy to maintain the operational margins that provide high-quality, reliable service to your viewers and advertisers.
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