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Tivo analog out + PVR-350 in = grubby signal
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Author:  Human [ Wed Oct 20, 2004 4:41 pm ]
Post subject:  Tivo analog out + PVR-350 in = grubby signal
Has anyone recorded from their Tivo's analog out (coaxial cable) to their PVR-350's input? Did it look ok?

When I tune a channel off an antenna using the PVR-350, the overall picture quality looks right, albeit laced with static. Likewise, when I take my Tivo's analog output via coaxial cable and plug it directly into my TV, I get a good-quality picture.

However, when I send the Tivo's analog output to the PVR-350's input, I get a dark picture with reds stronger than other colors. NOTE: This is not the same cause as the "dark red" problem recently fixed for bttv users, as I use the ivtv drivers.

Playing the MPEG2 that I capture from my Tivo on a gamma-corrected monitor on another machine shows that the picture is in need of some contrast, brightness, and saturation tweaking in order to look ok. It's as if there's some signal loss going on.

Is it possible there are different impedances with the coaxial cable and the antenna? They're both supposed to be 75 Ohms, I thought. And if they were different, why would the TV get a good picture regardless of whether or not it's set to expect broadcast or cable as its coaxial input?

Has anyone seen anything like this? Is there a fix?
Author:  ceenvee703 [ Wed Oct 20, 2004 4:59 pm ]
Post subject: 
Why would you not use the Tivo's S-video out and the PVR-350's S-video in? That will definitely give you the best picture quality.

If by "coaxial" you mean you are using the RF modulator (in other words, you have to change your TV or your PVR's tuner to Channel 3 or 4 to get the picture), that is the worst possible picture quality. Even composite video (the yellow RCA connector) is better than that.
Author:  Human [ Wed Oct 20, 2004 7:10 pm ]
Post subject: 
ceenvee703 wrote:
Why would you not use the Tivo's S-video out and the PVR-350's S-video in? That will definitely give you the best picture quality.


You're certainly right. The reason I haven't been doing that is because I've been trying to make my box as general as possible, so it'll take a signal from analog cable, a TV antenna, or the analog out of a Tivo or digital cable, without having to reconfigure it. I have a fairly portable box, so I can demo it to friends and family and try to lure them away from their VCRs and/or Tivos :)

It does, however, give me something new to try in my debugging. I can try using the svideo inputs and see if I get the same bad coloration that I see with coaxial input.

ceenvee703 wrote:
If by "coaxial" you mean you are using the RF modulator (in other words, you have to change your TV or your PVR's tuner to Channel 3 or 4 to get the picture),


That's right.

ceenvee703 wrote:
that is the worst possible picture quality. Even composite video (the yellow RCA connector) is better than that.


Still, shouldn't the coaxial input look the same to the PVR-350 as it does to my TV? Of course, there's the extra connection at play (S-Video between PVR-350 and TV) when I watch the Tivo's analog output.
Author:  ceenvee703 [ Wed Oct 20, 2004 7:28 pm ]
Post subject: 
Human wrote:
Still, shouldn't the coaxial input look the same to the PVR-350 as it does to my TV? Of course, there's the extra connection at play (S-Video between PVR-350 and TV) when I watch the Tivo's analog output.


Can't say for sure, but it could have to do with the PVR's tuner being different than your TV's tuner, or just the different and considerably longer path being taken, as you suggest, although it's even a worse situation than it sounds:

Tivo connected directly to TV:
----------------------------------
cable signal->A/D convert->RTivo compressed to MPEG2->D/A convert->RF modulated->TV

Tivo to PVR-350 to TV:
--------------------------
cable signal->A/D convert->Tivo compressed to MPEG2->D/A convert->RF modulated->A/D convert->PVR compressed AGAIN to MPEG2->D/A convert->composite/S-video TV-out->TV

Beyond that, tough to say.
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