DTV interference, doppler, and reception

06 Nov, 2008 | Eric Adler
I took a trip out to some transmitter sites recently with my DTV Pal, a few different antennas, a monitor, a power inverter, and a consumer set with built-in DTV tuner - also, I brought my 2.5" rear-projection Sony Watchman NTSC TV (that runs on 4 AAs which I haven't changed in a few months).
Unfortunately, I forgot my remote control, so the DTV Pal was left on the last virtual channel it was tuned to, luckily this was one of the channels at the transmitter sites. First test, right outside the main transmitter of a local station, 2-bay bow-tie antenna. The DTV Pal showed a beautiful picture (as was expected), as did the NTSC set.

Next test, driving away from the transmitter, with the antenna facing towards it. The picture very quickly went from perfect to blocky and soon thereafter to nonexistent (followed by a note from the box about no signal). Stopping at a stop sign and waiting a bit brought the picture back, briefly, as we began to move again. The 2" B&W running on 4 AA batteries picked up the signal and decoded it into a viewable picture as we moved; when we got into an area where the picture was too unbearable to watch, I could still receive clean audio - and the set even has a switch to turn off the CRT and just listen.
Next test, on top of a hill that houses some FM transmitters about 50 miles away. Reception was beautiful, on the DTV Pal, the NTSC set, and in both digital and analog modes on the set with the built-in DTV tuner.
Last test, about 15 miles further away, on top of a hill that houses a retransmission system that receives the original channel, hooked to an omni directional antenna on the tower, the DTV Pal showed flawless picture, three high-end receivers were failing to lock (with the same antenna, however they did lock on occasion for brief moments), the set with the built-in DTV tuner failed to find the digital channel in its search but showed the NTSC signal flawlessly (even better than at the previous test site) as did my NTSC set (just held the center conductor of the antenna line to the built-in antenna, collapsed fully).
Later, it was discovered that on an adjacent tower at the same site as that last test was a transmission facility that had recently started operating on an adjacent frequency to the original ATSC signal, showing that the DTV Pal works in places that others won't due to either interference or overloaded signal. All said, I am highly disappointed at the commission's decision to go with a system such as this that does not respond well to motion and the Doppler effect. My old Watchman worked perfectly in transit, runs on only 4 AAs, and can give me usable signal in situations where the digital signal is not usable (I cannot say for sure that it will give usable picture in locations where my DTV Pal will not show picture, when stationary, as I have yet to encounter this situation). The lack of mobile reception of ATSC is disturbing and undermines a great deal of history, including EBS/EAS and ancillary spectrum used to stream data to mobile units (both ENG and outside services such as police and fire department uses).
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