[opendtv] Re: White paper from CEA
- From: Richard Hollandsworth <holl_ands@xxxxxxxxx>
- To: opendtv@xxxxxxxxxxxxx
- Date: Wed, 2 Nov 2005 16:11:06 -0800 (PST)
The CRC test results for single signals (multiple signals not tested):
Samsung/ALPS Tuner (8/05): Overload of +1.5 dBm, Sensitivity of -84.7 dBm
Zenith/LG PROTOTYPE (9/03): Overload of ?2.3 dBm, Sensitivity of -78.4 dBm.
LINX not tested.
So the Samung had over 3 dB higher overload point for single signal input.
Unfortunately, no data was taken for multiple signals. [Big oversight!!!!]
However, Mark said that for many tests they inserted RF attenuation in order to
potentially reduce intermods, with no significant advantage:
//www.freelists.org/archives/opendtv/08-2005/msg00116.html
So that would seem to generally rule out simple overload arguments.
--------------------------------
So what could a manufacturer do to make a higher overload tuner without
degrading sensitivity?
If it were easy, wouldn't we have seen something on the aftermarket by now,
like there has always been for AM, FM and HAM bands???
Someone should test the Funai/Sylvania 6900DTE with DX DTA-5000 Smart
Antenna/Variable AGC Preamp in a high signal level environment. The ATI patent
disclosure described a procedure whereby "bad" antenna combining directions
were discarded. It also looks possible to "hack" the DTA-5000 in order to use
your own choice of external antennas.
[My test was in the 'burbs, although now I've learned that there is 50 KW COFDM
MediaFlo "jammer" only a few miles away. Unfortunately Funai has so far been
unable to fix the DD5.1 outputs.]
http://www.avsforum.com/avs-vb/showthread.php?p=5979741&highlight=6900dte+4bay#post5979741
At least one OTA STB (I don't recall which one) had an internal Preamp
enable/disable switch to select for either maximum sensitivity or strong signal
environment.
Maybe add an external variable RF attenuator to supplement the internal
varactor? [But that didn't work...]
Maybe add an external tuned, passive RF filter to supplement the internal tuned
RF front end? [Did you try THAT yet???]
[Sounds like a feature for that High-End OTA STB I mentioned earlier....]
--------------------------------
One theory that I have not seen expressed in earlier discussion is possible RF
leakage into the tuner module from the high ambient electronic field in NYC.
This could be more of a problem for PROTOTYPES, which are likely to have the
tin can popped on and off dozens of times.
<holl_ands>
//////////////////////////////////////////
"Manfredi, Albert E" <albert.e.manfredi@xxxxxxxxxx> wrote:
Mark Schubin wrote:
> I, too, eagerly await the commercial introduction of
> the one prototype that worked so well in my apartment
> 15 months ago, but not everyone lives in a
> problematic-multipath environment.
I remember you telling me years ago that people from
several organizations had come to your apartment to take
measurements, but that you could not divulge what they
had found. Would have been instructive to see.
Since your rabbit ear OTA reception is 100 percent from
reflected signal, I have to believe that one big problem
with receivers through 3rd generation was their inability
to cope with pre-echo. If the receiver can't handle
pre-echo, it needs a stronger direct path than anything
reflected. Ergo, no reception.
When you reported on the new Samsung receiver initially,
I had not seen the CRC measurements, so I couldn't put
that report in perspective. Now that we see that the
Samsung measures better than the LG (caveat -- the 2003
measurements of the LG, which might not apply), I have to
conclude that in your specific case, further improvements
in echo tolerance might not help. If antenna placement
away from the window doesn't work for the Samsung, when
it did work for LG prototype, then my guess is that IM
distortion products in the tuner are eating into the C/N
margin. So while the C/N margin is exceeded with a window
placement of the antenna, it's not exceeded when the
signal (including multipath) becomes a little weaker away
from the window.
If my guess is right, what I don't comprehend is why
receiver makers don't use the very best front ends in
their initial attempts. And then attempt cutting corners
only after they have done the obvious as well as possible.
Bert
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