[opendtv] Re: 5th generation Test

  • From: Bob Miller <bob@xxxxxxxxxx>
  • To: opendtv@xxxxxxxxxxxxx
  • Date: Tue, 15 Mar 2005 19:32:39 -0500

Mark Schubin wrote:

>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.
I disagree with Mark on this one point. I think that you could literally 
count cars as they passed by counting drop outs. Maybe I paid more 
attention to this. Minor point however.

>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.
>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).

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