[opendtv] Re: 5th generation Test
- From: Tom Barry <trbarry@xxxxxxxxxxx>
- To: opendtv@xxxxxxxxxxxxx
- Date: Thu, 17 Mar 2005 06:34:41 -0500
Ron Economos wrote:
> To answer Tom's question directly, yes you can add
> a bandpass filter to the antenna input of your receiver.
> For HF receivers, these devices are called pre-selectors
> and (at least in the olden days) were manually tuned.
> I don't know of any out-board tunable bandpass filters
> for television frequencies, but manual tuning is a non-
> starter for consumers electronics. The correct solution
> is a tracking (electronically tuned) bandpass filter in the
> receiver.
Ron -
Of course all that maybe belongs in the box. But the boxes don't have
them. I was more considering the possibility of outboard modules
because I still have not given up on convincing someone to invent my
smart set top antenna I've mentioned here before. It could maybe fix
existing boxes for existing customers, including those with
semi-functional integrated HDTV's. And it maybe would not be hung up in
8vsb patents.
And it might also work with computer control to make the PCHD cards in
MythTV etc. really shine. But I have no idea how to build one.
- Tom
> At my work location in Milpitas, CA 95035, turning
> on the FM trap on the distribution amplifier makes a
> world of difference with the Samsung T165. Antenna
> is a small VHF/UHF log-yagi on the roof of a one story
> building that's pointed in no particular direction (rotor
> controller is in a lab that I don't have access to).
> KQED-DT on RF channel 30 goes from massive
> break-ups to perfect copy.
>
> KBHK-DT on RF channel 45 goes from occasional
> break-ups to perfect copy.
>
> KNTV-DT on RF channel 12 (8.9 kW ERP) goes from
> *not recoverable* to perfect copy.
>
> Of course, the strength of FM stations in the Bay Area
> is due to their location on mountain tops. A paper clip
> on the input of a spectrum analyzer will show these FM
> carriers way above the noise.
>
> The point is, a little front-end filtering can go a long way.
> To answer Tom's question directly, yes you can add
> a bandpass filter to the antenna input of your receiver.
> For HF receivers, these devices are called pre-selectors
> and (at least in the olden days) were manually tuned.
> I don't know of any out-board tunable bandpass filters
> for television frequencies, but manual tuning is a non-
> starter for consumers electronics. The correct solution
> is a tracking (electronically tuned) bandpass filter in the
> receiver.
>
> Ron
>
> Tom Barry wrote:
>
>
>>Manfredi, Albert E wrote:
>>
>>>>How much extra are we talking about here?
>>>
>>>
>>>Well, since everyone was marveling at the low cost of
>>>the first gen Freeview boxes, and they were dual
>>>conversion designs, the cost cannot be prohibitive.
>>>However, it won't follow Moore's law, so the cost
>>>will not erode over time quite so fast.
>>
>>Is this RF front end something that could be sold separately and stuck
>>on the antenna input of existing receivers?
>>
>>- Tom
>>
>>
>>
>>
>>>Tom Barry wrote:
>>>
>>>
>>>
>>>
>>>
>>>>It sounds like "IM3 products" (whatever they are) and
>>>>good RF frontends are supposed to be known technology.
>>>>
>>>>
>>>
>>>IM3 is third order intermodulation distortion products.
>>>Created in receivers when multiple incoming carriers
>>>and the local oscillator beat together. Ideally, any
>>>such IM products would fall way outside the IF passband
>>>and be attenuated nicely. But with single conversion
>>>tuners, they are not all outside the passband.
>>>
>>>It appears that the low cost of single conversion
>>>tuners is enough of an incentive that manufacturers
>>>seem to jump at the chance of going that route. As we
>>>were informed a couple of weeks ago, even COFDM did
>>>this. As Al Limbert and others explained, if your
>>>RF amp up front is tuned to the channel you want and
>>>can reduce its gain in the presence of strong signals,
>>>the IM distortion can be greatly reduced, so single
>>>conversion tuners can work okay.
>>>
>>>With co-located transmitters, the problem might not be
>>>so acute. Because you will never see unwanted signals
>>>that are louder than the desired signal. But if the
>>>unwanted signal is louder than the wanted signal, then
>>>this can be a problem. The numbers we saw showed that
>>>a 3 dB increased level to the receiver, from unwanted
>>>signals, can cause up to 9 dB increase in this in-band
>>>IM3 distortion (IIRC). So, big deal. And a 3 dB
>>>increase into a receiver is not so hard to imagine if
>>>there are loud transmitters close by, even if your RF
>>>amp is tuned. The shoulders of the RF amp's passband
>>>won't be vertical. Dual conversion, i.e. where the
>>>receiver goes through two IF conversions, takes care
>>>of this nicely.
>>>
>>>
>>>
>>>
>>>
>>>>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?
>>>>
>>>>
>>>
>>>Well, since everyone was marveling at the low cost of
>>>the first gen Freeview boxes, and they were dual
>>>conversion designs, the cost cannot be prohibitive.
>>>However, it won't follow Moore's law, so the cost
>>>will not erode over time quite so fast.
>>>
>>>Bert
>>>
>>>
>
>
>
>
>
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