RF Reflections and Predictions
This month I'll look back to address comments received on my November RF Technology column and look forward to TV broadcast RF issues I believe will be important in 2004. CORRECTIONS AND CLARIFICATIONS
In my November column I investigated some cases of DTV into analog interference. I received several e-mail messages and phone calls about the column.
Responding to my statement that "ATTC studied DTV into analog interference and as a result of the testing was able to recommend desired-to-undesired ratios above which interference should not result," Charles Rhodes and Gary Sgrignoli both reminded me that the interference ratios were based on Level 3 on the CCIR five-step impairment scale, not "no interference."
According to CCIR Recommendation 500-1 (Kyoto, 1978, Vol.XI), Level 3 on this scale is defined at "slightly annoying." Level 5 is "imperceptible" impairment and Level 2 is defined as "annoying." Level 1 is "very annoying." I should have said "ratios above which annoying interference should not result." Charlie pointed out that Level 3 impairment has also been defined as "definitely perceptible but not objectionable to the average viewer."
The desired-to-undesired ratio of -12 dB for upper adjacent channel DTV into analog TV interference I used to analyze interference was taken from FCC Bulletin OET-69 dated July 1997. This is the version currently posted on the FCC's OET Web site, www.fcc.gov/oet. Gary pointed out that FCC Rule 73.623(c)2 states the ratio to be used is -14 dB, allowing 2 dB more DTV signal than the ratio specified in OET-69.
However, as I noted in the article and as Charlie emphasized in his e-mail, the FCC used D/U ratios determined under "weak" desired signal levels by ATTC. Both Charlie and I have written about how these ratios change under strong signal conditions. Refer to Charlie's Nov. 26, 2003 TV Technology column for more information on recent suggestions for the proper D/U ratios for strong signal environments. I have details on this in my December RF Technology column as well, which is available on-line at www.tvtechnology.com.
Wayne Bretl has done extensive testing of DTV-to-analog interference and said that in these tests he was never able to discern any pattern in the interference, although he agreed that the "noise" created by the DTV signal is not purely gaussian. The difference is more easily discerned by turning up the peak on the analog TV set. It is likely the pattern I saw in the co-channel interference case that was caused by nonlinear effects in the TV tuner or intermodulation from other TV transmitters near the receive antenna (-4 and +2 channels from the desired channel). Since this was in the early days of DTV, it is also possible that the co-channel DTV station was transmitting a nonstandard signal.
I received a number of e-mails from engineers at stations experiencing co-channel DTV interference. I also received a number of messages from stations concerned about interference from adjacent channel DTV interference, but no additional reports of adjacent channel interference.
In one case of co-channel interference at a cable company, the station used a bucket truck to try to find a "sweet spot" for the receive antenna on the cable company's property where the desired signal could be received. A spot was found about two feet off the ground on the side of a hill. Unfortunately, when the gate to the property was closed, the interference came back. Lowering the receive antenna was one way to reduce interference from distant stations. The same station was able to solve another interference problem by lowering the receive antenna and using a nearby building for shielding. Interference wasn't eliminated but was reduced enough that people stopped complaining!2004 PREDICTIONS
At this time of year, many columnists look back at the past year and forward to the coming year. I thought I'd take the opportunity to reflect on some of the things I'm looking for this year.
There is increased interest in using DTV to send video to portable devices. If you receive my weekly RF Report newsletter, you know a number of countries are planning to use a reserved datastream in the DVB-T standard to send low bit-rate video to cellphones, and multiple manufacturers have announced cellphones that can receive these DTV signals. In the U.S., a low bit-rate video service is available to Sprint PCS-Vision subscribers, but it depends on Sprint's data network, rather than broadcast signals. Mercedes is introducing DTV reception as an option in cars sold in Europe and there seems to be a lot of interest in other countries for TV reception in cars.
Although it appears unlikely that the ATSC 8-VSB standard will work well for mobile reception, mobile reception should be possible with the enhanced VSB standards. Linx has tested 2-VSB transmission, including a demonstration at NAB2003, and found it has great potential for mobile DTV reception. The superior carrier-to-noise and multipath performance of enhanced VSB should also allow it to work with portable receivers such as cellphones.
One of the problems with more robust DTV transmission methods, both DVB-T and enhanced DTV, is that the available data rate is significantly reduced as robustness increases. Fortunately, new advances in coding, including technologies such as MPEG-4 and Microsoft WMA, are allowing high-quality standard definition programming at data rates in the 1 Mbps range. Unfortunately, until standards are set that define acceptable coding methods, there will be a lot of compatibility issues. Cellphone manufacturers using the DVB-T standard appear to have standardized on an MPEG-4 coding method.
It will be interesting to see how enhanced VSB advances in 2004. Although it appears impossible for a standard to be adopted fast enough to allow actual products to appear this year, I'm looking forward to some interesting technology demonstrations at NAB2004.
One trend that started in 2003 that I expect to accelerate in 2004 is increased broadcaster interest in delivering a strong, reliable DTV signal. This interest is being driven by a larger number of viewers with DTV tuners. There is a core of viewers that appreciate that over-the-air TV reception with a properly designed antenna system can exceed the quality available from most analog cable, almost all digital cable and all digital satellite systems. These viewers are buying DTV set-top boxes and DTV sets. When they experience reception problems, they call the TV station. Although these viewers tend to be technically savvy and are willing to work with antennas to get the best reception, FCC rules requiring DTV tuners in new TV sets will increase the number of less tech-savvy viewers with DTV reception capability, and they too will be calling the station if they are having trouble with DTV reception. Stations won't be able to ignore their DTV signal.
Another factor driving DTV signal improvement is the need to deliver a quality signal to cable headends. Broadcasters using a DTV simulcast signal instead of their analog signal to feed cable companies will often look better on the cable system than stations using off-air analog feeds. An FCC decision last year allowed broadcasters to use DTV to provide an acceptable signal to cable companies under must-carry rules. As more cable companies switch to DTV reception, DTV system reliability will become much more important.
As part of this trend to improve DTV signals, interest will grow in distributed transmission networks, on-channel repeaters and DTV translators. The FCC should have rules in place for this in 2004. Several broadcasters are already experimenting with these techniques under Special Temporary Authority. Once FCC rules are in place, more broadcasters will start designing and building digital systems to replace or supplement their analog translator systems and improve coverage in problem reception areas whether in urban canyons or rural valleys.
Finally, I hope 2004 will bring us low-cost DTV sets (under $500 without HD displays) and inexpensive set-top converter boxes (under $200, ideally under $100). Terrestrial DTV set-top boxes have already reached this price-point in other countries and prices continue to drop. Why are ATSC-based DTV set-top boxes so expensive here?
In my RF Technology columns this year, I'll be looking at ways broadcasters can improve their DTV signals and make sure DTV receivers can properly decode their signals.Comments are always welcome. Drop me a note at firstname.lastname@example.org