[mythtv-users] Hardware for Best TV-Out?

Cory Papenfuss papenfuss at juneau.me.vt.edu
Thu Oct 28 11:45:51 UTC 2004

> |(say, by 910 for a 14.318 MHz clock) should be a 1:1 operation.  I suppose
> |there would be jitter, etc on that, though.
> I just don't know enough about what's going on in the VGA card to guess one
> way or the other whether it would be a problem.
 	Here's an NTSC-spec modeline:
ModeLine "coryntsci" 14.318 720 760 824 910 480 484 492 525 interlace
 	So the card sets the dotclock to 14.318MHz, and then draws 910 of 
those dots between horizontal sync pulses.  That makes the appropriate 
frequencies.  What you'd really like would be to get at that dotclock 
directly, but it doesn't come out on the VGA port, nor consistently on the 
"VGA Feature" connector.  If you re-multiply the HSYNC by 910 for the 
subcarrier (*4, actually... but that's what the AD724 wants anyway) you 
should get a line-phase-locked carrier as accurate as the VGA card's 
onboard oscillator.

> Nope, no progressive scan conversion.  If you don't buy the "transcoding"
> option then only the output type corresponding to the input you are using
> is live.  If you buy the option then all the outputs are always live.  I
> think the option actually removes some interlock logic...

 	I stand by my statement of a horrible rip-off.

> It is (stereo) but I'm not exactly sure what it is doing.  I don't like
> the quality of the MTS encoding at all.  This was a medium expensive (~$600)
> unit but it was one of the first ChannelPlus offerings in the range and
> even they admit that it isn't great.  (They didn't build the audio encoder
> module; they had to buy it to comply with the license.)  This is another
> thing on my list to replace, maybe with a Blonder Tongue "professional" unit.
> Because the good ones require a channel-specific filter module I'm waiting
> for the whole digital/analog debacle to resolve itself.  If all VHF OTA TV
> really goes away then I take down my VHF antenna and use that space for my
> in-house channels.  But I think that sounds just a bit too easy. :)

 	I was briefly looking for a stereo RF modulator before I decided I 
didn't really need to go that route.  Fortunately, it looks like the new 
chip MC44C401 will provide a relatively inexpensive RF modulator products 
soon.  I thought about making one, but again... little incentive and I 
probably couldn't get the chip.  From the specs it looks like the  CE LABS 
2000SD uses it, for instance.
> I guess that's the price we pay for flexibility.  Back on the ATI EGA Wonder
> if you wanted to use the composite out you programmed the timing for NTSC
> rates.  There was no scaling or resampling performed by the card.  Now when
> people talk about getting a TV to sync to 800x600 modes of an nVidia card
> it isn't even clear what they mean.  (Not that the EGA Wonder produced good
> looking TV output, mind you.)

 	Especially when they drone on about "getting an exact NTSC 
modeline"  If it's 800x600, it isn't exact.  Even if you try to make one 
that's standard (e.g. 720x480 or 640x480), it's not exact because of stuff 
that the card is doing you don't have control over.  The guy at NVIDIA I 
emailed said something about certain modelines on certain cards disabling 
the scaler, so maybe there's hope.  I have no doubt that the EGA Wonder 
produced good looking TV output if you had to code it up for NTSC rates. 
Since it's (almost) exactly 1/2 the speed of bone-head 640x480 at 60Hz 
standard VGA, the DACs are capable of excellent RGB signals at that rate. 
If they weren't you'd send the card back because it was broken.  :)

> | 	It would appear that $200 is the magic number for something
> |"professional enough" to put out good, standards-compliant signal.
> Just to be clear: you aren't saying that there exists something I could
> buy for $200 that will do what I want, right?  Because I'd be happy to
> spend that money.  Or even twice that money...
 	I thought you said that the Harmonic Research does exactly 
that.  From their site, it looks like model CV121A is it, and can be found 
online for $250ish (quick check).  You said you already have the component 
version of it?  You could always buy/build an RGB-Component transcoder (an 
analog generation loss, though) to run through the one you've got.


* Cory Papenfuss							*
* Electrical Engineering candidate Ph.D. graduate student               *
* Virginia Polytechnic Institute and State University 			*

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