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

Cory Papenfuss papenfuss at juneau.me.vt.edu
Wed Oct 27 22:23:55 UTC 2004

> |Trouble with that is now your HSYNC's tolerance is
> |227.5 times more important to get right.
> That's why I said that this is hard to do after the fact. :)
 	Yes and no.  Aside from the fact that Modelining up a very precise 
frequency may or may not result in that frequency from card to card, you 
will at least know what it is from the modeline and X logs.  Aside from 
the drift in the dotclock, re-multiplying what was divided by the modeline 
(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 guess it depends on whether or not it's trying to account for a
> |single frame here and there being out of sync, or a truly repetivive 50/60
> |framerate (I'm assuming you mean PAL/NTSC transcoding).
> No, I'm talking about allowing component in and composite out (or the reverse)
> on an NTSC-only TBC that already supports both kinds of inputs and outputs and
> has a full frame buffer.  That's what they call a "transcoding" feature.  Makes
> you wonder...

 	What a rip off then!  They're basically the same (unless they'll 
do progressive, fancy deinterlacing, etc).  480i on component isn't any 
different temporally... just adding a bit, filtering a bit different, and 
demoding the chroma.

> | 	Strictly speaking, I agree that "real" is standards-quality NTSC
> |with all timing relationships you mentioned related and referenced to one
> |master clock.  The difference is how "real" you need your mythtv box to
> |be.  With a sufficiently dumb TV (or more likely anything but a
> |super-advanced TV), it'll work with a pretty sloppy signal.
> But I don't want to feed the signal to a single TV.  I want to feed it to
> my A/V switch which drives several other devices and an RF modulator so
> I can watch on any TV in the house.

 	OK... so you've got a more complicated setup, so I can imagine 
a problem with non-locked subcarrier.  I'm assuming that the RF modulator 
is trying to use the subcarrier for something then?  In fact, if it's a 
BTSC-stereo RF modulator, I think it's a requirement, no?

> |It's not
> |something you'll try to use studio-editing equipment with, but for a
> |one-off, isolated system it'll be fine.  From what I understand regular
> |consumer VCRs and DVD players often produce rather horribly
> |non-standards-compliant signals (Macrovision notwithstanding).
> That may or may not be true, but (Macrovision notwithstanding) I've never
> had any trouble connecting any combination of consumer quality VCRs, DVDs,
> DVRs, ATSC decoders, and TVs to produce an acceptable picture.  (And I have
> a wide variety of equipment including ED-Beta.)  In particular, I don't have
> to tweak the settings for any of these on a per-TV basis nor do I have to
> tweak the TV.  For TV-Out on the other hand we seem to have to start with a
> non-standard signal that looks bad and then tweak it to make it even more
> non-standard in hopes that it will look ok on a particular TV.
 	Like I said, I haven't tried the TVOUT on most vid cards because 
the digital end of it (scaling, resampling, etc) sucks so hard, that it's 
already irrlevant if the NTSC output isn't quite spec.  It's already crap.

> |> You don't need to resample to make the clock *relationships* correct.
> | 	Again, take your pick on which accuracy is more important... the
> |color subcarrier frequency, or the line relationships between it and
> I pick the relationships...

 	Now that I've been reminded of the difference, I think I do to. 
Of course the circuit's already build and working.  If I were to do it 
again, a PLL with adjustable multiplier would be nice... and allow for 
simpler "calibration" for the average Joe.  I didn't even have a spectrum 
analyzer to trim the 3.58MHz subcarrier... had to use an RF generator and 
try to get both traces to stop... DOH!

> I never expected a $35 card to work well enough.  That's why I was looking
> for other solutions. :) I still think DV-out to a Sony DVMC or Miglia codec
> could be the way go.  It might also solve the other problem I frequently
> see with TV-Out support: hum bars from some sort of ground differential.

 	It would appear that $200 is the magic number for something 
"professional enough" to put out good, standards-compliant signal.  I with 
the NVIDIA guy would return my email again about disabling the on-card 
scaler.  That has potential, but since nobody but NVIDIA knows (and they 
haven't put it in their docs), it's still an unknown.

> | 	The AD724 seems to address all these pretty well from what I can
> |see.
> I'll believe it when I see it. :)  Note that the last time I actually built
> something like this was in the days of the LM1889 and that was *really* bad.
> So I'm prepared to be pleasantly surprised, yet not too hopeful...

 	Not by itself.  It addresses the sync/async issue and allows for 
external subcarrier clocking.  With the addition of a PLL, it would do 
what we're looking for, without hacking the VGA card.  Maybe they'll make 
another version with a built-in PLL off the HSYNC?  :)

> | 	How the *HELL* did we got off-topic so badly on the mythtv-users
> |list anyway?
> I don't know.  I think getting at least a consumer-grade Svideo or composite
> out that is generally compatible with other consumer gear is a pretty important
> feature.  Any off-the-shelf DVR is going to have it.  It's taken me several
> rounds of messages to realize that I may be the only one who cares about such
> a thing...
> 				Dan Lanciani
> 				ddl at danlan.*com
 	... and I thought *I* was particular!  :)  I wan't control over 
absolutely everything (modelines, frequency, sync, interlace, no scaling, 
etc)... now I know it's not quite perfect.  Damn!  :)


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