01.23.2003 12:00 AM
Hammett and Edison Support Minimum DTV Tuner Standards
In comments filed with the FCC, Engineering Consultants Hammett and Edison strongly supported Sinclair Broadcast Group's petition to require minimum performance standards in DTV tuners.
Hammett and Edison's comments include examples of poor TV tuner performance. When conducting field measurements of KNTV-DT channel 12, Hammett and Edison engineers found that in some locations with adequate DTV signal strength, an RCA Model DTC-100 consumer grade DTV receiver was not able to achieve signal local. Later, it was determined that this was because the tuner was unable to reject the fundamental frequencies of several nearby San Francisco area FM broadcast stations. When a tunable band pass filter was inserted in the antenna line, the receiver was able to obtain lock and "display virtually perfect picture and audio quality from KNTV-DT, more than 80 kilometers distant." This shows the problem was receiver overload by the FM stations' fundamental frequencies, not inadequately suppressed second harmonic energy from the FM stations.
The comments point out that required field strengths for DTV reception are much least than those for analog reception. At UHF, the difference between 64 dBu Grade B (F (50,50) and 41 dBu DTV threshold (F50, 90) -- equivalent to 48.1 dBu F (50,50), is 16 dB. While DTV signals are 13 to 16 dB weaker, the undesired signals from FM stations, NTSC TV stations, other services and man-made noise remain the same. In other words, DTV tuners have to deal with interference ratios 13 to 16 dB worse than those analog tuners traditionally had to.
Brute force overload is the only problem DTV tuners face. In the DTV channel allocation process, the analog image interference taboos were removed. This was based on the assumption DTV tuners would use double conversion to eliminate this interference. If DTV tuners use single conversion and the same 45 MHz intermediate frequency, they will be susceptible to image interference. Hammett and Edison found such a case in California when KTVU-DT, channel 56, received a DTV viewer complaint of interference from KKPX-DT channel 41 in San Jose, Calif. Reception of KTVU-DT was impossible when KKPX-DT was on the air.
The comments paint a poor picture of the future of terrestrial DTV if minimum DTV tuner standards are not adopted: "We fear that, if the Commission requires just 8-VSB tuners in television receivers, and not also minimum RF performance standards for such tuners, soon there will be a widespread universe of 8-VSB tuners that can handle only the approximately equal amplitude carriers typical of cable TV or downconverted DBS feeds, and not the more challenging desired-to-undesired signal ratios common for direct over-the-air reception of terrestrial DTV signals," Hammett and Edison said in its comments. "This would likely ensure the failure of DTV receivers with minimum-performing 8-VSB tuners to provide adequate reception in many of these situations. We cannot think of a worse 'poison pill' for the rollout of DTV penetration than to encourage the marketing of TV receivers with DTV tuners that work only in benign RF environments."