re. BACKSCATTER signal path......and ATSC Diversity Receiver references.
A few dozen posts ago, I mentioned BACKSCATTER as being one way that a signal can bounce off nearby objects,
back into the beam of your antenna, even though the source of the transmissions might be on the back side of the antenna gain pattern.
For the same reason a mirror reflects the most light back towards the light source,
the worst Backscatter sources originate from the back side of the antenna pattern,
and will be scattered back towards the highest gain portion of the antenna pattern.
Depending on geometry, sidescatter can also be a problem, esp for those directions where the antenna has significant sidelobe response.
A measurement of the backscatter and sidescatter levels can be found by looking at the relative strengths
of the received multipath signals for a given DTV channel.
[Unfortunately, STBs only display this information to the Design Engineers.]
Refs. [a] and [b] below give some insight on this process.
Our buddies at the Canadian Research Center, the Canadian government's technical experts on ATSC and COFDM,
conducted some signal level surveys in various locations, including the described survey around Ottawa.
The ATSC (and COFDM) waveforms transmit a known pseudorandom sequence of bits (PN-511) for 47.48 usec every 24.2 msec.
The ATSC Receiver uses this known data sequence to initialize the equalizer by minimizing
the mean square error between the received sequence and the known PN-511 sequence.
The equalizer tap settings can be observed to log the multipath strength vs delay.
And once the multipath delay is known, it can be used to find the path difference between the direct and reflected path,
using the time it takes light to travel one mile: 5.376 usec/mile.
They found that for a large number of suburban locations, the strongest multipath signals arrived within about 0.2 usec,
which is a path difference of only a couple hundred feet.
That means that your neighbors (and perhaps your interior/attic) are the strongest contributors to multipath.
(Especially your neighbor's two story house with the wire mesh stucco, Al. foil back insulation, Al. gutters, electrical wiring, etc.)
How strong? Roughly equal strength down to 10 dB below the direct path.
On the average, the multipath reflections were insignificant (20 dB below the direct path) for multipath delays
in excess of about 5 to 10 usec, which is a path differential of 1 to 2 miles.
Urban multipath delays were about twice those in surburbia.
So how much multipath should an STB accomodate?
About +/- 20 usec ought to do the job unless there is a tall skyscraper more than a few miles away from you.
So most people shouldn't expect the Zenith/LG 5th Gen chip to work miracles.
Indeed, the Linx/Micronas chip had much better performance for the 0 dB Echo test cases,
including Brazil ensembles, such as might occur for indoor and attic locations.
Of course, there were some infrequent excursions well beyond the "average".
ATSC A/74, "ATSC Recommended Practice: Receiver Performance Guidelines", 18 June 2004,
contains several dozen of these "exceptions" for test purposes.
(So why is it that this information is never collected and released from U.S firms???)
References re ATSC Diversity Receivers:
a. "UHF Wideband Mobile Channel Measurements and Characterization using ATSC Signals with Diversity Antennas", Semmar et. al., May 2004:http://lrts.gel.ulaval.ca/publicatio...ication_11.pdf
b. "UHF DTV Band Channel Characterization for Mobile Reception", Semmar et. al.,
Canadian Conference on Electrical and Computer Engineering, Vol.3, pp 1339-1342, 2-5 May 2004.
c. "ATSC Digital TV Receiver using Spatial Diversity Technique", Ju-Yeun Kim, et. al.,
IEEE Vehicle Technology Conference, Sep 2004.
d. "Antenna Diversity for Improved Indoor Reception of US Digital Terrestrial Television Receivers",
Meehan, IEEE Trans on Consumer Electronics, Vol. 48, No 4, Nov 2002.
e. "Joint VSB Terrestrial Digital TV Indoor Reception Project", Wetmore, Schnelle, Meehan, et. al.,
IEEE Trans on Broadcasting, Vol 48, No. 4, Dec 2000.