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

Dan Lanciani ddl5 at danlan.com
Tue Oct 26 02:54:38 UTC 2004

Cory Papenfuss <papenfuss at juneau.me.vt.edu> wrote:

|On Mon, 25 Oct 2004, Dan Lanciani wrote:
|> Cory Papenfuss <papenfuss at juneau.me.vt.edu> wrote:
|> | 	It cost me about $10 in parts to build one.
|> I should have added "off the shelf" to the cost.
| 	Fair enough... I don't think that such a thing exists.

Harmonic Research makes some of the less expensive (though not cheap) units.
I have their component model, but not the RGB one.

|> |No rate conversion,
|> |just VGA->NTSC encoding (color subcarrier with UV modulation, etc).  Aside
|> |from having to set the VGA card up for 480i timings, (read: no console,
|> |only X), it works great.
|> How did you phase lock the color sub-carrier to the existing sync?
| 	Easy... I didn't... :)

Ok, but then it's not "real" NTSC.  You can get an off-the-shelf version
like that for ~$200, still with a reasonably good color encoder matrix.
Harmonic makes those too.

|I *did* adjust the crystal's trimmer cap 
|to be as close as I could to the 3.58MHz standard frequency.  I don't know 
|how temp stable it is, but it looks excellent on the TV.  I suppose it's 
|possible that on a suitably advanced TV (adaptive comb filters, etc) that 
|the frequency interleaving would be off a bit with a slightly non-standard 
|color subcarrier.  I suspect that the VCO's in most TV's will lock onto it 
|within 1000ppm or so anyway... should be well within the tolerance of a 
|tuned crystal circuit.

But it isn't just a matter of frequency.  In a real NTSC signal all the
components are locked to the same clock, being derived from (probably a
multiple of) the color subcarrier.  It's much harder to reproduce this
locked relationship after the fact since the highest frequency you have to
work with is the horizontal sync.

|> | 	Check out the AD724 chip.  A few passive components, one chip, and
|> |a crystal is all you need.
|> I did look at that chip, but it wasn't clear that it solves the hard problem
|> of locking a free-running color sub-carrier to the sync in the incoming RGB
|> signal.  The documentation seems silent on the issue, but if you look at the
|> block diagram there is no obvious path from the sync inputs back into the PLL.
|> It will lock to an external carrier clock, of course, but if you have that then
|> you have already solved the problem. :)
| 	Again... is it strictly necessary to do this?

Depends on what you are trying to achieve.  There are two issues.  In the
abstract you can mess up (technical term :) the distribution of power across
the frequency spectrum of the signal.  I don't remember the exact consequences
of even failing to flip the sub-carrier phase on each scan line, but it can
at least slightly degrade the picture quality.  On the practical side, various
gadgets depend on the exact timing relationships in various ways and to
varying degrees.  The more sophisticated the device, the more it may depend.
My old Zenith tube set really didn't care.  My XBR100 is pretty finicky.  The
problems can be subtle.  Nobody documents their exact dependence on the NTSC
timing because, after all, it is supposed to be correct if it is NTSC...
So the choice is between doing it right once or checking compatibility with
each target device (and hoping that you aren't cheating yourself by missing
a subtle problem).  Now if you have only the one target device, it may
not matter much.

In any case, I stand by my statement that generating real NTSC (especially
after the fact) is not trivial.

				Dan Lanciani
				ddl at danlan.*com

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