Z Technology 's RF Newsletter - DTV Edition

Broadcast News, Edition 5

DTV RF Transmission Path Measurements

Introduction

Digital television transmissions have started in every major U.S. market and television viewers can now receive programs of higher technical quality than ever before. The first viewers to access DTV transmissions are the ones appreciating this quality the most, but this demand for superior technical quality comes before the expected groundswell of commercial success. The requirement, then, is to economically implement and maintain a technically superior system in anticipation of future acceptance.

This article describes a staged measurement approach that can quickly verify digital television signal coverage over the city of license and identify any locations requiring further attention. Once the initial DTV plant is installed, the emphasis is on the best possible signal performance into every viewing location.

The success of the DTV transition will be measured by viewer acceptance; and how long we must keep old NTSC transmission plants in parallel operation.

 

Analog vs. Digital

An analog NTSC baseband video signal and an NTSC occupied RF transmission spectrum

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:

  1. NTSC is the analog RF transmission of a compressed, multiplexed analog audio/video signal. The weak link is the analog RF transmission path.
  2. DTV is the analog RF transmission of a compressed, multiplexed digital audio/video signal. The weak link is the analog RF transmission path.

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.

A digital baseband video signal and a DTV occupied RF spectrum

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:

RF Signal Properties

Integrated Power

The RF power received in the 6 MHz Channel in dBm, dBuV, or dBuV/Meter

Peak Power

Peak of the RF power received in the 6 MHz Channel in dBm, dBuV, or dBuV/Meter

Tilt

Indicator of flatness of received signal excluding the band edges in dB

High Low Difference

Indicator of depth of notches in dB

Standard Deviation

Indicator of dispersion of notches

Decoded Signal Properties

Main Tap

Energy a decoder equalizer sees from strongest signal source

Tap Energy

Energy a decoder equalizer sees from linear distortions of strongest signal, i.e. reflections (excludes energy from strongest signal, all taps normalized to a strongest signal of 1)

Signal to Noise after Equalizer (SNR)

Above 15.2 dB generally indicates good reception, calculated from MSE

Segment Error Rate(SER)

Number of Segments found in error per second in decoded data stream

Mean Square Error out of Equalizer (MSE)

Error in signal after correction

Sync Lock

Sync circuits locked on signal

Equalizer Lock

Equalizer locked on signal and producing decoded data

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.

Summary

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.

About the author

Guy Lewis studied physics, mathematics, and communications at Baylor and Texas A&M Universities and has served as a television station chief engineer and television group director of engineering. He is Director of Sales & Marketing for Z Technology, Inc. Beaverton, Oregon. He also consults with other digital television clients and is co-author of the Tektronix publication "A Guide to Standard and High Definition Digital Video Measurements". Mr. Lewis has more than 40 years experience in the broadcast industry including positions with Z Technology, Tektronix and RCA Corporation.

 

Z Technology DVS5100 RF Measurement system

Drive-test map screen with measurement data,

The DVS5100 System is a self contained Windows™ based RF field strength and digital parameter drive-test mapping system. The system package includes hardware and software for both NTSC and DTV broadcast transmission applications where it is desired to measure standard received signal strength plus RF figures-of-merit parameters for such digitally modulated systems as DTV, DVB-T, ISDB, and DAB.

The DVS5100 provides new and unique measurement parameters for DTV coverage analysis, emphasizing parameters which directly correlate with the viewer’s ability to receive, decode and utilize data being transmitted.

Key figure-of-merit parameters include values collected from the RF spectrum of the off-air received DTV signal. The most basic of these values is digital signal integrated power. The FCC specifies this value and the broadcaster is required to meet minimum UHF Channel field intensity levels of 41dBuV/Meter within the coverage area.  [The actual number for Ch.14 - 69 is determined by the equation:  41dBuV/Meter -20 x log(615/F0)] Peak in-band signal power is an important measurement which in an 8VSB DTV signal usually represents the peak power in the pilot carrier. Field intensity survey results can easily be mapped over an entire service area.

In-band frequency response tilt across the channel evaluates a combination of transmitter tuning, including bandpass filtering and antenna effects, and selective transmission path losses.

Several additional values give the user important clues as to the "receivability" of a DTV signal. For example a received signal can appear to be quite strong, yet have large frequency response notches within the 6 MHz spectrum. These notches may make decoding impossible.

All of these parameters characterize the available DTV signal and provide confidence that you are delivering acceptable and receivable signals.

The DVS5100 integrates data collection, dot plotting, swept spectrum analysis and RF figures-of-merit parameter measurements into an efficient, portable system.

 

Z Technology DSS5800 DTV measurement system

DSS5800 measurement screen, displaying characteristics of a DTV transmission

The DSS5800 system includes hardware and software to address two important measurement groups: RF Signal Properties and Decoded Signal Properties.

The DSS5800 provides all of the RF measurement capabilities of the DVS5100 system. In addition, a DTV receiver/decoder and data analysis software is included in the DSS5800 system.

Decoding the 8VSB transmitted signal provides an additional group of measurement parameters which can be used to determine what "margin" a signal has before it becomes unusable. These parameters include Signal-to-noise ratio (SNR), segment error rate (SER), EQ lock and SYNC lock. In addition, multipath performance can be studied by recording tap energy values and total tap energy being captured at the received site. All of these decoded values are good quality indicators well worth tracking over time and across the coverage area.

Both the DVS5100 and the DSS5800 provide the complete RF measurement capability required for drive-test measurements. Mapping allows the user to easily visualize signal reception over the coverage area. By substituting a portable reference antenna (for example, a tuned dipole mounted on a tripod) for the vehicle antenna, either system may be used to provide Static reference RF measurements. The DSS5800 system’s built-in 8VSB decoder provides the additional confidence that data can be recovered from the DTV signal.

 

Table 1. Comparison of Z Technology RF Measurement Systems

FEATURE

S5007GPS*

DSS5100

DSS5800

Record 15/150KHz power (analog)

X

X

X

Plot analog power on map

X

X

X

View spectrum display

X

X

X

Capture spectrum display to file

X

X

X

Log 6 MHz RF parameters

 

X

X

Plot RF parameters on map

 

X

X

View tap display

   

X

Capture taps display to file

   

X

Log decoded figures of merit

   

X

Plot decoded figures of merit on map

   

X

*The S5007GPS system is not specifically configured for television measurements