[opendtv] Re: 5th generation Test
- From: Tom Barry <trbarry@xxxxxxxxxxx>
- To: opendtv@xxxxxxxxxxxxx
- Date: Tue, 15 Mar 2005 19:53:54 -0500
Manfredi, Albert E wrote:
It sounds like "IM3 products" (whatever they are) and good RF frontends
are supposed to be known technology. But if that's the case then maybe
we (collectively, not me) also know what it would cost to add them to a
box with an LG5 chip in it.
How much extra are we talking about here?
- Tom
> 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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