Pete Putnam (www.hdtvexpert.com) copied me on an e-mail to Dave Arland at Thomson Electronics commenting on a recent letter Arland, Thomson's Director of Public and Trade Relations, sent the FCC outlining Thomson's work in lowering the cost of DTV receivers and improving receiver performance. The Thomson letter also pointed out some DTV implementation problems of which broadcasters should be aware.
Thomson is planning to meet the FCC's digital tuner/decoder mandate. The letter states, "Thomson is deep in the development cycle for its 2004 line of HDTV consumer products and intends to fully meet the requirement that at least 50 percent of our unit sales of televisions 36-inch or larger include integrated terrestrial tuner/decoders by July 1, 2004. We are now developing products that will ensure that we meet the other requirements of the digital tuner/decoder mandate from 2004 through 2007." The letter, however, emphasized the need for "plug and play" cable TV functionality and approval of the HDTV "Plug & Play" agreement pending before the FCC. "The fact that the vast majority of Americans receive their television from cable makes 'HDTV-overcable' functionality a critical element to 'mainstreaming' the digital television transition for consumers, and thus speeding the transition toward a more rapid conclusion." Thomson added, however, that broadcasters would not be left out: "Every Thomson HDTV Set labeled as "cable-ready" for HDTV will have both QAM and VSB (ATSC) tuning/decoding capability."
Arland's letter said that Thomson's stand-alone and integrated DTV tuner/decoders are designed for an average noise figure of 7-dB on all channels. This is the noise figure specified in FCC OET Bulletin 69 for UHF channels and is 3 dB better than the noise figure specified for VHF channels. As reported in my latest RF Technology column in TV Technology, a study by ATTC found that adjacent channel performance of most DTV tuners was much worse than specified in the ATSC planning factors. Arland said "Thomson's latest design will mark a 7 to 10 dB improvement over our first generation products. At this level, adjacent channel rejection is limited by the spurious energy in the adjacent band radiated from the interfering transmitter. Similar results have been attained with input sensitivity, which improves by some 2.5 dB in Thomson's latest design."
The outstanding multipath performance of the LINX DTV tuner has been described in several TV Technology articles, including my recent RF Technology column. Broadcasters waiting for this technology to appear in consumer DTV receivers will be happy to see Dave Arland's comment on this in the FCC letter, "Thomson has designed, manufactured, and sold competitive products to handle multipath interference and we are investing even more resources to insure constant improvement. We believe that promising new multipath cancellation technology from LINX Electronics will minimize the majority of multipath problems where received signal strength is sufficient. Thomson is planning to utilize LINX technology in future generations of digital TV receivers."
After pointing out the work Thomson is doing to improve DTV reception, Arland criticized local broadcasters for not doing their part to help the transition. "Notwithstanding these advances in receiver capabilities and performance, accurate and robust reception is inevitably and directly related to the strength of the signal transmitted by the broadcaster." "Regrettably, most local broadcasters are NOT transmitting their digital TV signals at full-power. In fact, the Commission's most recent figures indicate that only 25 percent of commercial broadcast stations are 'on the air' with a digital television transmission signal that covers their analog station service areas."
As noted in the DTV Build Out Snapshot in this week's RF Report, the July 5 FCC CDBS shows 370 licensed DTV stations, which would be operating full power and 769 DTV stations operating under an STA. While the power level under the STA could be close to the station's authorized power, in most cases it is substantially less. The DTV Build Out Snapshot in the May 11 RF Report showed 66 percent of the UHF STAs were for power levels less than the 50 kW minimum UHF DTV power allocation and over 32 percent were for 5 kW, one tenth the minimum UHF DTV power allocation.
Arland pointed out, "The availability of only a low-power signal can significantly hamper the ability of any terrestrial receiver--regardless of input sensitivity--to properly receive, tune, and decode digital TV signals." He also noted stronger adjacent channels can interfere with low power DTV signals and said "spurious transmitter radiation, especially for digital television transmitters" exacerbated the problem. As noted in previous articles, Richard Citta found adjacent channel signals over 20 dB stronger than the DTV signal caused problems during his testing of the LINX receiver.
The Thomson letter put the blame for poor reception on broadcasters and suggested a remedy, saying "...the suggestion by many broadcasters that 'insensitive receivers' are somehow to blame for poor consumer reception of digital TV signals misses the real problem, which, Thomson respectfully suggests, lies not with receiver sensitivity but rather by a lack of commitment of the broadcasting community to transmit their digital TV signals at full power. To that end, as Thomson and others have suggested in comments in the Commission's Digital Television Periodic Review proceeding (MB Docket 03-15), the Commission, at a very minimum, should establish an interim deadline of July 1, 2004 (concurrent with the first digital tuner/decoder deadline), by which all broadcasters must transmit a digital signal of sufficient strength to serve their entire Grade A contour."
Pete Putnam, whose e-mail prompted this article, disagreed with Thomson's argument. He said he had done adjacent channel tests with older Panasonic set-top boxes and found no interference problems with adjacent channel 8-VSB carrier variations as great as 20 dB and noted that current generation set-top boxes "do a whole lot better in this regard." He also questioned the comment about transmitter spurious radiation.
In my experience, I have not seen a problem with excessive out of channel radiation from DTV transmitters. The mask filters present on all DTV transmitters and the adaptive pre-correction present in most of the high power DTV transmitters keep out of channel radiation within the FCC limits. I have seen problems, however, with out of channel radiation from older klystron transmitters using pulsers to increase efficiency. In these transmitters, sync pulse modulation is applied to the tube, rather than in the exciter, and out of channel energy is not attenuated by the VSB filter in the exciter. The only way to control is it through pre-correction in the exciter, which can be difficult to set up. Unless the pulser timing and exciter linearity, sync timing and ICPM correction are set properly, lower sideband radiation will increase.
Pete and Dave agreed on one problem with broadcast DTV--transmission of incorrect PSIP data. Pete said many of the e-mail messages he receives reporting DTV reception problems turn out to the PSIP related, not due to RF problems. Both felt the FCC needed to require broadcasters to send proper PSIP data.
Thomson's filing is available through the FCC's Electronic Comment Filing System (ECFS). Enter the world "Thomson" in box 5 and a list of all filings on Thomson's behalf will be displayed. Alternatively, this link should bring up David Arland's letter to the Chief of the FCC Media Bureau, Kenneth Ferree. Pete Putnam's Web site is www.hdtvexpert.com. It has a wealth of information on HDTV and DTV reception, including reviews of DTV equipment, indoor and outdoor antenna performance, and instructions on modifying the Channel Master 3022 UHF antenna for better DTV reception.
