[opendtv] Re: 5th generation Test
- From: Mark Schubin <tvmark@xxxxxxxxxxxxx>
- To: opendtv@xxxxxxxxxxxxx
- Date: Tue, 15 Mar 2005 18:50:59 -0500
I've been traveling and just got in. Let me add some two cents.
The earlier LG 5th-generation tests were truly plug-&-play. It didn't
matter what antenna I used or (within reason) where I positioned it or
how I oriented it. Even rabbit ears worked for the UHF DTT stations.
There was only one station -- far away -- that required any care in
antenna positioning.
With Bob's new box, no matter what we did, we couldn't get WABC-DT at
all. That station IS adjacent to WNYW-DT, and maybe it was off the air
at the time.
Of more concern was WCBS-DT. It's on channel 56. As best I know, the
nearest adjacent station is an NTSC WAY out in Riverhead (the center of
Long Island's "fish tail") on channel 55. That NTSC station has barely
registered on a spectrum analyzer in previous tests. WCBS-DT stands alone.
We know it was on the air at the time because it came and went. Bob
says we could count cars. Well, no. But the breakup DID seem
indicative of varying multipath. Certainly, reception was affected by
us walking around the room. That's the first time that has happened on
WCBS-DT since the second generation boxes.
We tried multiple antennas in multiple positions, including affixing
them to the window. Nothing allowed WCBS-DT to be received reliably.
Surprisingly, WNYW-DT, which does have adjacent and co-channel issues,
came in fine in a number of antenna positions and orientations.
TTFN,
Mark
Manfredi, Albert E wrote:
>Bob Miller wrote:
>
>Doug wrote:
>
>
>>>It's now time to try the el-cheapo 5th generation box
>>>with a really really good single-channel filter
>>>(Blonder-Tongue?) in front of it for the difficult
>>>channels. If it really is the front end, that should
>>>make it work. It could also be something else, like
>>>the chip allows different amounts of external memory,
>>>or uses aid from the main box CPU, and these were
>>>lacking in teh cheapo box.
>>>
>>>
>>Not being an engineer I don't know but at Mark Schubin's
>>you could count the cars going by by watching the
>>monitor for drop outs on CBS for example. That sounds
>>like multipath to me.
>>
>>
>
>It does sound like multipath problems, but Doug might
>still be on target. If you recall from the Rhodes
>discussions, IM3 products occur in the passband, and
>eat at your signal margin. So if you have a cheapo front
>end, the IM distortion created from loud stations at
>adjacent, or also bad news were the third over,
>either above or below your selected station, the "louder
>than one might think" IM product will erode the signal
>margin you have upstream of the equalizer.
>
>So, a cheapo front end that causes the signal margin to
>fall below ~15.2 C/N, even with excellent equalizers, will
>cause the thing to fail.
>
>A good front end is the best and quickest defense. More
>innovative FEC schemes will also play a part in the future,
>by lowering the required margin closer to the Shannon
>limit (10.47 dB).
>
>Bert
>
>
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