There has been a lot of interest recently in combining multiple NTSC and DTV signals onto one antenna. The reasons for this are simple – tower real estate is expensive and, in some cases, existing towers aren’t going to be able to support additional DTV antennas. If the signals can be combined at the base of the tower, one antenna and one transmission line can support multiple stations.
Recent experience shows adjacent channel combining is practical and does not effect DTV or NTSC performance. A new approach to DTV combining, the sharp-tuned filter, makes it possible to combine two adjacent channel DTV signals into one antenna. Let’s look at the current state of high-power adjacent channel combining and the impact of combining on transmitter correction circuits.
PRACTICAL EXPERIENCE
Telemundo has had an N+1 facility, WSNS Channel 44 NTSC and WSNS-DT Channel 45 DTV, operating in Chicago for the last few months. While we originally intended to do more measurements related to DTV reception, both indoors and outdoors, we found it necessary to be sure the adjacent channel combiner worked as promised and that the N+1 DTV signal did not cause interference at higher DTV power levels. We also installed an N+1 combiner at KSTS, Channel 48, in mid-1999 for use when the FCC grants our construction permit for KSTS-DT, Channel 49.
As of December 2000, we had not received a construction permit for KSTS-DT. Consequently, on-air testing was limited to the NTSC signal. We were able, however, to investigate DTV performance into a dummy load.
DIPLEXER DESIGN
The MicroCommunications Inc. combiner uses a notch diplexer design. Operation is similar to the aural notch diplexer most analog broadcasters are familiar with. In the case of the N+1 combiner, the NTSC signal is applied to a hybrid, which is connected to the antenna and the output of the DTV mask filter.
The NTSC signal reflected off the DTV mask filter, through the hybrid, into the antenna. One arm of the diplexer contains a notch filter tuned around the aural carrier. This allows the DTV mask filter to be somewhat broader than it would have to be if it had to reject both the visual and aural portions of the NTSC signal, minimizing distortion of the DTV signal. It also allows the aural notch to be symmetrical around the aural carrier, allowing the use of SAP and Pro-channel aural subcarriers. The combined DTV response of the system has a notch in the DTV filter passband where the NTSC aural carrier is located.
As you can imagine, if the aural notch drifts, the isolation between the aural carrier and the DTV signal would drop significantly. Losses on the NTSC side increase, because some of the aural carrier is not reflected back to the hybrid and antenna. The cavities dissipate more heat due to the increased loss.
AURAL CAVITIES
We had problems with arcing and overheating in the aural cavities with the first-generation combiner at WSNS. Paul Smith, director of Product Development at MicroCommunications, developed a thermal compensator system to solve the problem. He reported on this and the N+1 combiner in a paper delivered at NAB last year.
I’m pleased to report that we have not experienced any instability or problems with the MicroCommunications N+1 combiner during on-air testing at effective radiated powers of 5,000 kW NTSC and 467 kW DTV into an Andrew antenna system since installing the thermal compensators.
Except for field modification to the NTSC exciters’ receiver equalizer correction circuitry to bring the group delay above 3.9 MHz within the limits specified in the FCC rules, we had no problems meeting all FCC specifications for the NTSC signal at both KSTS and WSNS. In addition, stereo aural performance also met all BTSC specifications.
SHARP-TUNED FILTER COMBINERS
Robert Plonka, principal engineer at the Harris Communications Broadcast Communications Division, presented papers on another method of combining adjacent channels at the IEEE Broadcast Technical Symposium in Washington D.C., in September last year and the Iowa DTV Symposium 2000 later in the year. Plonka’s paper described the use of sharp-tuned filters in combining systems.
As in the notch diplexer combining method described above, the combiner depends on the NTSC signal reflecting off the DTV filter, through a hybrid into the antenna. Unlike the notch diplexer system, there is no separate notch for the aural carrier. The bandpass of the DTV filter is narrow enough and the filter skirts steep enough that a separate notch filter to reflect the aural carrier is not required.
One megahertz from the channel edge, the STF attenuation is 20 dB greater than that of standard mask filters. Close to the channel edge, within a few 100 kHz, the STF filter has a notch in its response that drops over 30 dB.
In his papers, Robert Plonka described the performance characteristics of the sharp-tuned filter combiner. Near the edge of the STF passband, group delay and frequency response changes rapidly for both NTSC and DTV signals.
Across a 5.38 MHz DTV signal, group delay changes by almost 370 nanoseconds, increasing symmetrically above and below the center of the channel. In an N+1 combiner, on the NTSC channel below the DTV channel, the group delay increases to approximately 125 nanoseconds at the 3.58 MHz color subcarrier and to almost 300 nanoseconds at 4.2 MHz. Across a 200 kHz spectrum centered on the aural carrier, group delay increases by more than 450 nanoseconds.
The change, however, is smooth and monotonic. For comparison, when testing the notch diplexer N+1 combiner used at KSTS, at 4.10 MHz the group delay was 610 nanoseconds.
GROUP DELAY
If the NTSC channel is above the DTV signal, the effect is shifted. The STF causes the 3.58 MHz color subcarrier to have about 100 nanoseconds less delay than at the visual carrier. Group delay below the visual carrier increases to more than 350 nanoseconds.
For the DTV signal, the frequency response of the STF varies less than half a dB over the 5.38 MHz above the pilot carrier. Across 5.9 MHz, the response drops approximately 2 dB as it approaches the channel edges. For an NTSC signal on the channel below the DTV passband, at 4.2 MHz the response drops about half a dB. At the 4.5 MHz aural carrier, the roll-off is still less than 1 dB, with less than 0.2 dB variation +/- 100 kHz from the aural carrier.
These characteristics allow the STF to combine adjacent channel DTV signals as well. Of course, the rapid group delay and frequency response changes for both DTV and adjacent signals place extra demands on the transmitter exciter.
TRANSMITTER REQUIREMENTS FOR COMBINING
Robert Plonka showed that the STF reduces the signal-to-noise ratio (SNR) of an unequalized DTV signal to only 16.1 dB, corresponding to an error vector magnitude (EVM) of 12.8 percent. However, with equalization the SNR is improved to 30.2 dB and the EVM to 2.7 percent. As long as equalization is used, an SFT should have no effect on the receivability of the DTV signal, and Robert Plonka’s tests found the STF had no measurable effect on DTV receivers.
The frequency response and group delay distortions on the adjacent analog channel can also be removed using correction available in the exciter. If two DTV signals are combined, Robert Plonka’s tests showed that the SNR could be equalized to better than 32 dB and the EVM to approximately 2 percent, even when the DTV power levels differ by 10 dB.
ONE COMPLICATION
There is one complication when equalizing a combined DTV signal. Many modern DTV exciters use a form of adaptive precorrection to compensate for changes in the system. By now, it should be apparent that the combiner introduces distortions in the RF path that have to be corrected. However, if the signal is sampled at the output of the DTV combiner, the sample will also include the channel combined with the desired channel.
During testing of the MicroCommunications N+1 combiner and the Comark/Thomcast DTV exciter, we found the adaptive precorrection added a vestigial carrier above the DTV signal, apparently in an attempt to eliminate the aural carrier it detected below the DTV passband. Thomcast says it now has a way around this problem.
Robert Plonka explained that the Harris DTV exciter uses a special digital filter in the exciter to remove interference from the other signal in the combiner, regardless of whether the interfering signal is analog or "digital." If you are planning to use a combined system with a DTV transmitter that has adaptive precorrection, make sure it will still function when another signal is added in the combiner.
Although Robert’s papers are based on Harris products, I commend him for focusing the papers on the technology and its value to the broadcast industry instead of his company’s products. This approach reflects favorably on Harris and, I believe, enhances its reputation as a technology leader, even when the component focused on in the paper isn’t made by Harris.
The sharp-tuned filter and combiner was designed and manufactured by Andrew Corporation’s Passive Power Products division.
Visit Doug Lung's Web site atwww.transmitter.com. E-mail:dlung@transmitter.com.
