Z Technology 's RF Newsletter - DTV Edition

Broadcast News, Edition 2

Welcome to this issue of Z Technology, Inc.'s DTV Newsletter. We've had lots of great response to our first issue and will try to keep each issue brief and to the point. Let us know how we are doing (as a NYC mayor once said many times).

U.S. DTV Scorecard

As of May 8th, 2002, the FCC reported 1506 stations, 89%, had been granted a DTV licence or construction permit. 256 Television stations are now on the air with licensed DTV facilities, plus another 201 stations are operating under special or temporary authority, for a total of 457 on-air. Where are the remaining 11%? Re: http://www.fcc.gov/mb/video/files/dtvonairsum.html

Z Technology, Inc. co-sponsors DTV Seminars in Phoenix and Atlanta

We are pleased to co-sponsor the ATSC Digital VSB Transmission Seminar's in Albuquerque, NM this past week, and Atlanta June 6th. Zenith Staff Consulting Engineer, Gary Sgrignoli, presents these seminars. See you there. http://www.zenith.com/digitalbroadcast/tradeshows.html

Television Signal Coverage Measurements

Until recently, station owners and engineers had few reasons to actually measure signals from television broadcast transmitters. The world of over-the-air broadcasting was relatively stable, and if a coverage problem did occur it was usually caused by a major event such as loss of the transmitter or damage to the antenna system. Such events were quickly resolved and coverage was back to the same footprint. A broadcaster seldom needed to consider measuring actual signal coverage.

This is changing with the rollout of terrestrial digital broadcasting. Signals from new digital transmitters are being overlaid onto existing analog footprints. Chief Engineers find themselves faced with the challenge of turning on a new DTV transmitter on a new channel, with a new antenna, even at a temporary antenna position. To make matters more unsettled; old channel taboos are no longer valid. An engineer may be required to bring up a UHF DTV signal with the added difficulty of replicating the footprint of his existing analog VHF signal.

The task will be even more challenging for the many broadcasters faced with a stepping stone build-out process. For these Chief Engineers the next few years will consist of sequentially stepping through several interim stages before finally getting to their permanent assigned DTV channel with full power on the proper tower using the final antenna array.

There are other factors adding complexity to the situation. The broadcaster who wants protection of the new DTV signal over the same footprint he now enjoys must demonstrate that his new signal pattern is the same as the old NTSC one. According to the FCC, sufficient and proper DTV coverage is required before the end of 2003 for commercial stations and by December 2004 for public stations.

It is important to understand your DTV signal coverage during each of the rollout steps for terrestrial digital television. This is a dynamic and changing situation and will continue to be so for the next several years. Perhaps the best way to state the requirements for the new DTV signal is to boil it all down to one real-world question: "Are you actually delivering a usable (decodable) 8VSB signal to people with digital television sets, DTV set-top boxes and PCs equipped with wireless data receivers?"

It is reasonable and responsible to take immediate steps to answer this question. It is also apparent that it is insufficient to simply rely on past signal coverage records or even to use computer predictions of theoretical but unproven DTV signal coverage. Now is the time for old-fashioned in-the-field measurement work. However, some care and a few new measurement techniques are required in order to properly carry out this important task.

DTV Signal Coverage Measurements

Z Technology has created a unique tool, the DSS5800 Signal Coverage DriveTest™ Measuring and Mapping System, especially designed to answer the signal coverage question. This portable system collects and documents DTV signal parameters that directly correlate with the ability to receive, decode and utilize the DTV data being transmitted.

DTV coverage metrics can be divided into RF Spectrum parameters and Decoded Baseband parameters.

RF Spectrum parameters are those collected from the RF spectrum of the off-air DTV signal. The most obvious is received channel power or "Integrated Power." The FCC specifies this value and the broadcaster is required to meet minimum Field Intensity levels within the coverage area.

Additional RF parameters give the user clues as to the "recieveability" a DTV signal. A received signal can be quite strong, for example, but impossible to decode because of one or more large notches in its spectrum. Therefore, in addition to measuring and recording RF Field Intensity Levels, it is useful to track and record notches in the 6MHz received spectrum, tilt across the channel and peak power within the bandpass. All these parameters are useful in characterizing the recieveability of transmitted DTV signals. Knowing these values provides the confidence you are delivering a receivable signal. A Table of these useful RF parameters is shown below.

RF Parameter & Abbreviation

Parameter Definition

Ideal Value & (units)

Comments

Integrated Power

(Int Pwr)

Integration of total power of signal within 6 MHz channel bandpass

>41dBuV/m (dBuV/m)

This parameter is the total amount of all the power being received within the 6 MHz Channel

Peak Power

(Peak)

Peak Power within

Bandpass

Large (dBuV/m)

The max. signal strength being received at the highest point within the Channel

Spectrum Tilt

(Tilt)

Tilt across channel bandpass measured at the two inner markers

0 (dB)

For a perfect signal, power will not trend up or down across the channel bandpass

High-Low Differential

(High-Low Diff)

The difference between the spectrum high & low points within the inner bandpass markers

0 (dB)

A measure of notches within the spectrum over the channel bandpass

Standard Deviation

(Std Dev)

A mathematical definition which specifies how signal strength within the inner bandpass markers deviates from average

0(units of sigma, in dB)

For an ideal, perfectly flat bandpass, Std Dev would be 0 dB. A deep, wide notch would be a high Std Dev value

RF SPECTRUM PARAMETERS USED TO CHARACTERIZE DTV SIGNALS

Decoded Baseband parameters can provide important information about the recieveability of the DTV signal. These parameters are obtained by decoding the 8VSB transmitted signal and can be used to determine what "margin" a signal has before reception fails. Key parameters include signal to noise ratio (SNR), segment error rate (SER), sync lock and equalizer 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 parameters are good quality indicators, well worth tracking over time and across the coverage area. A summary table of decoded parameters is shown below.

Decoded Parameter

& Abbreviation

Parameter Definition

Ideal Value & (units)

Comments

Synchronizer Lock

(Sync Lock)

Receiver synchronized with incoming data signal

Locked

(Locked/Unlocked)

Fundamental to successful decoding of the data signal.

Equalizer Lock

(Eq Lock)

Equalizer locked on signal & producing decoded data

Locked

(Locked/Unlocked)

Next step required to successfully decode data.

Tap Energy

Energy received via different transmission paths, not including the main signal path

a large negative number

(dB)

The sum of energy received at the decoder due to reflections vs. the main signal.

Main Tap

Energy in the Main Tap

 

reference

This value is a reference number used to normalize the Tap Energy value

Signal-to-Noise Ratio

(SNR)

Ratio of signal power to noise power

15 (dB) or better

For current receivers, the SNR must be at least 15 dB to decode any part of the signal.

Mean Squared Error

(MSE)

Mean Squared Error

0 (dB)

This is a value used to calculate SNR. The smaller it is the better.

Segment Error Rate

(SER)

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

< 3

(segment errors per sec)

The digital signal is encoded into segments. The SER count keeps track of segment errors per second. The industry has defined 3 as the threshold of visibility of segment errors.

DECODED PARAMETERS USED TO CHARACTERIZE DTV SIGNALS

How do we gather this information on our DTV signal?

If we have good readings for all RF and Decoded signal parameters, we have a signal that should provide acceptable reception to our viewers. It would be impractical to measure and record all of these parameters at every location, and be impossible to monitor every location at all times. By gathering information periodically, however, we can have good confidence of providing a commercial level of service. Here are three steps we can take to determine our DTV signal service level:

First, make a quick survey, or DriveTest™ of the area, gathering enough information to determine what to study in greater detail. This can be done in a modern, rapid and automatic way. With a little understanding of some useful measurement parameters, a broadcaster can gain understanding of the quality and effectiveness of DTV signals delivered to the market.

DriveTest™ is a survey of signal coverage that samples the DTV signal from a moving vehicle. The DSS5800 can sample and record all of the above RF and 8VSB decoded parameters, along with GPS location information, and plot selected data in color on street maps.

There are a number of common-sense qualifiers in taking digital signal measurements while in motion. With so many parameters being measured and processed for each data set, each parameter will be recorded for a slightly different location. In practice, complete sets of data are taken in about one second, and a number of data sets will be recorded over a very short distance.

It is easy to agree that swept signal measurements taken in motion, in the clear, and in the absence of multipath reflections, will correlate well with fixed position measurements. It is more difficult to contemplate a swept spectrum display, which plots signal strength versus frequency over a period of time, while the receiver is moving. One might then be concerned about the validity of gathering swept spectrum measurements from a moving vehicle in the presence of severe multipath. Experience has shown that, even in severe conditions, Integrated Power within the 6 MHz channel is a reliable indication of total received power weather the measurement is taken in motion or while stationary.

Multipath reflections will both subtract and add to signal power as it is measured at each point across the measured frequency spectrum. When the power in each of these points is combined to compute Integrated Power, the motion of the vehicle tends to provide an average of the integrated power across the distance traveled during the measurement sweep time. Taking a swept signal measurement while in motion provides an averaging effect on the Integrated Power reading not present in a random stationary measurement. When stopping to take a stationary measurement, in the presence of strong multipath, the operator may have to carefully move the antenna to find the proper position for best reception. Moving the antenna by only a few inches may significantly affect the signal pattern.

In summary, DriveTest™ measurements provide a valuable map of potential problem areas and provide an ongoing confirmation your system is operating as designed.

Second, stop and gather detailed information where the signal is questionable. While in-motion drive testing provides a level of confidence and indicates problem reception locations, it is no substitute for calibrated testing when data reception appears to be marginal.

With the DriveTest vehicle parked, the operator can examine the spectrum and tap value displays, carefully adjust antenna positioning and even raise the antenna mast to a traditional 30 feet. NIST traceable measurements can be made for FCC filings, engineering records, and comparison with predicted data. These calibrated measurements can also be used to characterize in-motion DriveTest™ data. Both the moving and stationary data is actual information, taken with calibrated, traceable equipment, with variables of environmental setup. Stationary testing allows the normalization of the antenna position, and removes the uncertainty of motion, but it can be much more time-consuming if many locations are to be recorded.

Third, fix the measurement system on one signal path to study seasonal and equipment performance variations. When the DSS5800 Measurement System is not otherwise in use, it can be temporarily installed in a fixed location to track seasonal or equipment performance variables. Just as a DTV demodulator may be installed in a local closed-loop around a DTV transmitter, a fixed location DSS5800 DTV measurement set can be installed open-loop to independently monitor the transmitter, transmission line, antenna and propagation performance. It can be used to study any seasonal or atmospheric effects on the transmission path that may impact a viewer's ability to use the signal from your station. And the long-term record will prove invaluable when transmission problems are suspected or equipment changes made.

Summary

By measuring received signal strength over a service area and determining other RF and Decoded Parameter values, an operator can properly gauge signal receiveability. Automatic documentation of this data provides a more detailed record of actual signal coverage than could be manually entered. This allows the engineer to move from a guessing-game scenario to the reality of having useful measured data from a transmitted DTV signal for review and analysis.

This issue has described the DSS5800 http://www.ztechnology.com/pdf/ds_dss5800.pdf

and procedures for the 8VSB DTV signal to address current activity in the U.S. Z Technology, Inc. also manufactures the DVS5600A http://www.ztechnology.com/ds-dss5600a.html for DVB-T, COFDM digital television modulation standards. Please contact us for more information, sales@ztechnology.com or visit our website, www.ztechnology.com.

Guy Lewis
Director, Sales and Marketing
Z Technology, Inc.
email: guyl@ztechnology.com
Phone: 503-614-9800
FAX: 503-614-9898
1815 NW 169th Place, Suite 3070
Beaverton, OR 97006 USA
http://www.ztechnology.com