Z Technology, Inc.

1815 NW 169th Place, Suite 3070
Beaverton, Or. 97006-7368 USA
Ph: 503/614-9800  Fax: 503/614-9898
www.ztechnology.com

APPLICATION NOTE 27

Maintaining the DTV Signal


DM1010 Measurement Receiver shown with AV1010 Audio/Video Decoder

Transmitter Performance Measurements with the DTV 1010 Measurement Demodulator

By: Guy Lewis
Z Technology, Inc.

Maintaining the DTV signal:

Congratulations. Your new DTV transmitter is installed and operating. What do you do now?

You have a big investment, and a modern transmission system that may be running in parallel to your old NTSC friend. This new plant is your future, and will become your favorite as the old system slinks into retirement.

This new DTV friend is a lot like your NTSC companion; brave and strong, and ready to send your station’s programming off to your customers. And it does this in a familiar analog way! In fact, it is analog that works even harder, and must be a more faithful than any analog transmitter before it.

The DTV signal shares many characteristics of the NTSC signal. Both formats travel over-the-air as analog signals in 6 MHz television channels. Both are vestigial sideband signals. The DTV signal is an amplitude-modulated vestigial-sideband suppressed-carrier signal, carefully processed and filtered to efficiently fit within the 6 MHz television channel with a minimum of interference to adjacent channels and other services. This implies a very high performance RF chain, and that is what you are required to maintain.

This application note describes a hypothetical transmitter to help the reader understand the principles of DTV as broadcast in the United States. Please consult your transmitter’s manual to learn how your transmitter implements DTV in compliance with transmission standards prescribed by the Federal Communications Commission.

The modulating data:

The input to your transmitter is an MPEG-2 transport stream in SMPTE 310M format. It may contain multiple channels of compressed video, audio, and other content, and various tables to sort the data when it is finally decoded at the destination. Your transmitter handles this data in a relatively straightforward manner, first processing the digital data for robust transmission, then modulating and band limiting that data to fit into the 6 MHz broadcast channel.

Our hypothetical transmitter processes the MPEG-2 program data digitally into an ATSC transmission format, and then broadcasts that data as a digitally modulated high-power analog signal. Figure 1 describes the signal as it transitions from program data input to transmitter RF output. Low-level stages of the transmitter, from program input through the IF DTV output, are generally referred to as the ‘exciter’. The up-converter processes the signal to the channel frequency, and may also be a physical part of the ‘exciter’. The on-channel DTV signal is then amplified by one or more high power linear amplifiers and finally filtered to meet stringent FCC spectral requirements.


Figure 1, Conceptual block diagram of a DTV transmitter

Digital Processing:

The input signal to our DTV transmitter is a synchronous serial interface (SSI) 19.39265846 Mbits/Sec binary data stream in a SMPTE 310M format. The exciter removes the data sync byte, pseudo-randomizes the data stream and adds 20 bytes of Reed-Salomon (RS) forward error correction (FEC) to each allowing correction at the receiver for up to 10 data byte-errors per packet. Data is then interleaved over a 4.2 ms interval to protect against burst noise and organized into trellis-coded 3-bit symbols, at a symbol rate of 10.762 MHz. Segment and field syncs are then multiplexed with the randomized payload data, creating a binary string of 3-bit symbols, noise-like for 2484 bits with a coherent 12 bits of data segment sync.


Figure 2, The 8 demodulated VSB 3-bit data levels

These coherent syncs will provide a robust time/rate pattern for multipath compensation in the receiver demodulator and synchronize the data decoder in the receiver.

Analog Processing:

The I/Q modulator or its equivalent accepts the 8-level data and modulates it onto an IF carrier. Our conceptual phasing-method I/Q modulator suppresses the lower sideband and carrier. A DC component creates a phase imbalance in the modulator to produce a 0.3 dB pilot carrier leakage (about 7%), which aids in recovery of the modulation at the receiver.The resulting frequency spectrum, usually centered on an intermediate frequency of 44 MHz, is heavily bandpass filtered to limit most of the energy to a six MHz channel bandwidth, which by Nyquist theorem can convey the 10.76 symbol/sec signal with a modest overhead. This IF signal is then up-converted to the desired television channel for amplification and final clean-up bandpass (FCC mask) filtering as it leaves the building for the antenna.


Figure 3, RF Spectrum of the on-air DTV signal

The generic name for this modulation scheme is 8VSB; 8 for the eight amplitude levels modulated on the RF carrier, and VSB for the presence of the vestigial carrier and rapid roll off of frequencies below the carrier frequency. DTV power can be measured by integrating energy across the 6 MHz channel using the FFT Spectrum approach shown in Figure 3.


Figure 4, Amplitude modulated 44 MHz DTV IF Carrier

If we were to look at the on-air DTV signal with a single-sweep oscilloscope, voltage vs. time, we would see an amplitude modulated RF envelope. Since the modulating data is pseudo-random, the display would fill to flat amplitude over successive sweeps of the oscilloscope. If we could sync the oscilloscope we would see the see periodic coherent syncs. By recovering a clock from this RF signal, using some clever circuitry, we can sample the signal to discover the closest of eight possible levels occurring at the time of each sample. Each level represents a three-bit binary word that contributes its pattern of three bits to the recovered ATSC baseband data stream.


Figure 5, Constellation Display of a DTV signal showing the
eight possible levels on the I axis.

The ability for the receiver to faithfully recover the correct 3-bit symbol is aided first by the transmitted pilot signal. It is then up to the demodulator to evaluate the instantaneous amplitude level of the signal to estimate which of eight possible modulation levels is the correct symbol to report as a 3-bit word. Once the signal is successfully sampled, additional locking and correction can take place to recover the Data Segment and Data Field Syncs. Then error correction and data decoding can begin.


Figure 6, DTV decoded eye pattern showing a minimum
of inter-symbol interference at the sampling time.

Inter-symbol interference resulting from the severe band limiting of the data is avoided by using level information gathered only during the sampling time. At the instant of sampling, ringing from previous and subsequent symbol levels will be at a zero crossing and only one symbol will contribute to the RF envelope. This is illustrated by the measurement demodulator eye diagram display, shown above.

Successful transmission of the DTV signal, then, requires a properly adjusted exciter/modulator, an RF amplifier chain that exhibits very linear performance, and proper filtering at the transmitter output to minimize radiation of out-of-band components.

Measuring RF performance:

Your encoder/modulator stuffs a massive amount of data into a data channel only 6 MHz wide. The data must get it to the receiver intact, and you have to protect neighboring spectrum users, maybe even your own adjacent NTSC channel. The analog transmission channel carrying this data, your RF channel to your customer, must handle this signal with as little distortion as possible to permit successful reception and recovery of the ATSC data stream.

Analog Distortions Affecting the DTV Signal:

Distortion at any point after modulation reduces the effective signal-to noise ratio of the digital signal and makes the data more difficult to recover at the receiver. Distortions include:

Poor frequency response (amplitude vs. frequency)

Any variation in amplitude response vs. frequency will cause symbols changing at different rates to achieve inconsistent levels. As a result, the data level will be more difficult to identify. The measurement indicator will be a tilt or distortion of the frequency spectrum, a spread on the I axis in the constellation display and an increase in the Error Vector Magnitude (EVM) reading. The Signal to Noise Ratio (SNR) will also be degraded. Frequency response error may be caused by:

DTV exciter misalignment

Initially, any pre-correction available in the exciter should be turned off. After higher power stages are adjusted, exciter correction can be engaged to optimize frequency response.

RF amplifier stage mistuning

A tube-type RF amplifier will require periodic tuning. The noise-like nature of the DTV signal provides a flat, noise-like energy spectrum that can assist in amplifier tuning. By viewing the RF spectrum at a directional coupler tap before the high power bandpass mask filter, amplifier tuning and loading can be adjusted for best flatness within the 6 MHz channel, consistent with a minimum response outside the channel. This adjustment will maximize amplifier efficiency, minimize heating in the high power mask filter, and conserve input power (and minimize the cost of AC power).

Mistuning of the mask filter at the output of the transmitter

The mask filter is adjusted at installation using a network analyzer. A cursory check across the 6 MHz active channel can be made by comparing frequency response at directional coupler taps before and after the mask filter. If damage or mistuning is suspected, the mask filter should be re-checked and any adjustment done using a network analyzer. This would be a good time to get the manufacturer involved since this tuning is done infrequently and the field engineer will have the proper equipment and experience.

Poor group delay response (phase vs. frequency):

Damage to the output mask filter.

Mask filter amplitude response changes very rapidly with frequency at the channel edge, with delay varying from center channel up to 120 – 160 ns. This is compensated by design in the exciter and accommodated by the receiver.

Mistuning of the IOT output cavity in an IOT transmitter

Phase response will be optimum when the frequency response flat across the channel. Additional adjustment or compensation may be required if there are notches or peaks in the frequency response just beyond the channel edge.

Incorrect compensation for the mask filter in the exciter

Compensation for both frequency response and phase response may be available in the exciter to minimize EVM error at the output of the mask filter.

The result of linear distortion error is a deviation of the demodulated RF signal amplitude from its intended position at the moment of sampling. The difference in the recovered level at each sample time and the theoretical level is measured in terms of error vector magnitude (EVM) and is considered noise (affects SNR). A target value for EVM is approximately 4.5%, or approximately 27dB digital SNR. A measurement demodulator will be optimized to read SNR correctly around 27dB SNR. These values are more indicative of transmitter performance when measured before receiver equalization.
Figure 7, WinDM-PRO measurement of a degraded on-air DTV signal.

RF amplitude linearity distortion:

The primary contributor to distortion is the imperfect transfer characteristic of the RF amplifier. In a perfectly linear amplifier, output varies in a direct relationship with the input. This is a more critical parameter in a DTV amplifier since the signal data is contained over a wider min/max power range than an NTSC format signal.

The DTV receiver must discover which of eight possible levels is correct. Any non-linearity moves the actual signal level closer to the edge of the expected amplitude-sampling window and makes the demodulation process more sensitive to noise.

Input amplitude vs. output amplitude:

As the modulation swings towards a minimum, the transfer characteristic may be non-linear, or compressed near signal cut-off. At modulation peaks, the signal may be compressed due to amplifier saturation. These characteristics are determined by the amplifier design and by aging of the amplifier devices. Pre-compensation may be available in the exciter. The affected measurement parameters are EVM and even spacing of the levels along the I axis in the constellation display.

Input amplitude vs. output phase:

Input amplitude vs. output phase distortion is most pronounced as the amplifier impedance match changes at minimum and peak modulation levels. Compensation may be available in the exciter. The affected measurement parameters are the EVM value and straightness of the vertical lines along the Q axis in the constellation display.

Z Technology manufactures several systems to help you set up, measure, document and maintain your DTV system. The key instrument for these tasks, installed at your transmitter or studio, operated locally or over your network, is the Z Technology DM1010 Measurement Demodulator.


Figure 8, DM1010 DTV Measurement Receiver

The DM1010 Measurement Receiver features a calibrated, high performance RF front end for NIST traceable RF signal level measurements and a comprehensive set of measurement functions for DTV transmitter plant setup and maintenance. The DM1010 may be operated locally, with or without the WinDM-PRO measurement graphic application, or over a network with WinDM-PRO running on a remote PC.

Transmitter Site Measurement Installation:

At the transmitter site, measurements are observed at several points in the transmission chain. The signal out of the DTV exciter would be monitored at the 44 MHz IF frequency, and later stages would be monitored on the channel frequency. By monitoring before and after the mask filter, you will see how hard the mask filter is working. By monitoring the feed to the antenna, you see the DTV signal being delivered to the community, and power reflected from the antenna reveals the location and amplitude of any reflections.


Figure 9, System performance measurement setup at transmitter

Connected to the Internet through a local area network, the DM1010 located at the transmitter site becomes a part of a nationwide monitoring and quality control network.

Studio Site Monitoring Installation:

The requirements are a little different at the studio site. Many parameters measured at the transmitter can also be measured off-air, with degradation expected due to lower signal level, environmental noise, reflections, and the presence of other signals. At the studio, the DM1010 provides a measurement quality frontend for an AV1010 Audio/Video Receiver. The DM1010 provides off-air measurements, and outputs a VHF DTV signal for the house monitoring system. It also outputs an SMPTE 310M transport stream for data analysis. As at the transmitter location, a DM1010 may be connected directly to a local PC or to the local area network for measurement and remote control using an internet-connected PC at any location.


Figure 10, System monitoring setup at studio site

At the studio, the AV1010 Audio/Video Receiver may be configured to receive its input either as an IF signal from the DM1010, or connected to an outside antenna. The AV1010 allows selection of the appropriate program and access to guide, caption, and other information available to a home viewer.

A DM1010 Measurement Receiver installed at your studio site will provide an NIST traceable measurement of signal power at its input. This establishes a reference monitoring point that can provide an indication of signal strength due to equipment or path variations. Z Technology measurement receivers such as the DM1010 include a Windows™ WinDM-Pro software application to display eye diagram, constellation, RF spectrum, echo profile, and measure and capture a clear-text data record of MER, EVM, SNR, signal strength, tap energy, SER, threshold of visibility, sync lock and equalizer lock.

Nationwide Monitoring of DTV performance:

Any internet-connected DM1010 Measurement Receiver can be accessed with a unique URL from any internet-connected PC running the Z Technology WinDM-PRO measurement application. All functions, controls, and displays available in the application are available nationwide. Alarm profiles and data recording routines can be set up to meet the interests of each monitoring location.



Figure 11, Alarms may be uniquely configured at each monitoring location

Real-time transmitter monitoring:

The constellation display should show eight evenly spaced vertical lines of random-phase data. Any deviation from a straight line on the Q axis, or any lines closer to another line on the I axis will reduce the receivers ability to separate that data from noise. A sampled constellation display and recording of error vector magnitude or signal-to-noise ratio is sufficient for remote monitoring since transmitter adjustments are not being made.


Figure 12, DM1010 rear panel connections

A real-time I/Q analog output is available for immediate feedback during transmitter adjustment using the Z Technology DM1010 Measurement Receiver’s rear panel I/Q outputs and an inexpensive X/Y oscilloscope.


Figure 13, X/Y oscilloscope display of demodulator I & Q outputs

Transmitter typical adjustment summary:

This discussion is provided for understanding and procedures may or may not apply to your particular transmitter. Please refer to your transmitter manufacturer’s information for specific instructions.

Here is how you might adjust a typical DTV transmitter:

Connect the DM1010 Measurement Receiver to the forward port of a directional coupler at the appropriate monitoring point. Connect a Windows™ PC to the DM1010 and run the WinDM-Pro™ measurement application to provide graphic displays at the transmitter adjustment location.

Turn off any exciter pre-compensation. Check to see that dc operating parameters (bias, plate voltage, etc.) are as recommended by the transmitter manufacturer.

Connect the DM1010 to the forward tap of a directional coupler at the output of the transmitter, before the mask filter. Adjust RF amplifier bias to operate in the most linear part of the power curve consistent with the desired dc power efficiency and limits suggested by the transmitter manufacturer. Operate the transmitter into the station dummy load to allow observation of any energy beyond the channel edge. Amplifier bandwidth and tuning should be adjusted for best flatness and minimum tilt as observed on the DM1010 spectrum display.

Viewed after the high power mask filter, into a resistive dummy load, the DTV spectrum on the DM1010 spectrum display should be flat, with a minimum of tilt and min/max deviation, with sidebands attenuated outside the 6 MHz channel. With transmission line and antenna connected, impedance discontinuities may distort the spectrum display. Into the station dummy load, the spectrum within the channel should not be significantly different input to output of the high power mask filter. A directional coupler can help separate forward energy from any antenna reflections for in-service monitoring.

A real-time I/Q constellation display from the DM1010 Measurement Demodulator connected to a wide-bandwidth X/Y oscilloscope provides the most immediate feedback during transmitter adjustment. With exciter pre-compensation turned off, adjust the RF amplifier chain bias for clean, straight vertical lines on the Q axis, with even horizontal spacing between lines on the I axis.

If exciter pre-compensation is available, turn it on and adjust linearity and phase compensation circuitry to pre-distort the transfer characteristic from the exciter for the best overall, exciter plus RF amplifier chain, linearity as indicated by evenly spaced straight lines on the I/Q constellation display. This setting will be consistent with a minimum EVM value and best digital SNR.

Monitoring your transmission line and antenna system:

The DM1010 Measurement Receiver provides two convenient methods you can use to monitor the integrity of your transmission line and antenna system. Forward and returned power can be monitored with a two-port directional coupler installed just before the gas barrier. The measurement receiver connected to the return port will provide an echo profile graph indicating the magnitude and phase of any power reflected from impedance discontinuities such as elbows, transmission line connectors, combining networks and the antenna input complex.


Figure 14, Short range echo profile with reflections at .836 uS, 1 uS, and 1.2 uS.

By setting the Echo Profile range to display 0.0 to 5 uS, you will see transmission line reflections to a resolution of about 43 ft. Any significant change in the phase or amplitude of a reflection would be cause for investigation.

A spectrum display can also be used to monitor for impedance mismatch, but the presence of more than one reflection will complicate the spectrum display.

Summary:

DTV is a refinement of the traditional television signal broadcast direct to the home. The ATSC, 8VSB signal format is extremely efficient in terms of occupied spectrum and AC power consumption. The stability and efficiency of modern transmitter design makes DTV possible, but the DTV signal is much less tolerant of transmitter drift, mistuning, and component aging. Modern transmitter design has advanced to the point where DTV can be efficiently transmitted with NTSC reliability. DTV transmission should be no more difficult with a new transmitter than NTSC has been with a modern NTSC transmitter.

As with NTSC, you do have to maintain the performance of your DTV transmitter, and modern tools exist to make that process easier.

References:

Gary Sgrignoli, “ATSC VSB Transmission System Tutorial”, Zenith Corporation, 2001

David Sparano, “ DTV Transmitter Installation and Proof of Performance” Harris Corporation Broadcast Division, 1999

David Sparano, “What Exactly is 8-VSB Anyway?”, Harris Corporation Broadcast Division, 1997

Jonathan Zane, Interview, DMT USA, Inc., 2005

Guy Lewis, “DTV RF Transmission Path Measurements”, Z Technology, Inc., 2003

Bruce Franca, NTA Presentation, Federal Communications Commission, Office of Engineering and Technology, 2005

Jerry C. Whitaker, “DTV: The Revolution in Digital Video”, McGraw-Hill, 1999