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

Dan Lanciani ddl5 at danlan.com
Wed Oct 27 04:15:26 UTC 2004


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

|> Harmonic Research makes some of the less expensive (though not cheap) units.
|> I have their component model, but not the RGB one.
| 	From their website:
|"If you can furnish broadcast-quality RGB or Component signals, then these 
|units can generate broadcast-quality video*. All sync and timing signals 
|are regenerated from the input reference and include a true phase locked 
|subcarrier."<snip>
|"The inputs must be analog, and they must be reasonbly close to the video 
|scan rates (15,750 Hz for NTSC, 15,625 Hz for PAL)."
|
| 	That still doesn't quite add up.  I'll buy that they can make a 
|spec-compliant raster, but without frame (and in this case, *line*) 
|locking to the original source, it would seem that you'd have to have 
|sampling/resampling to get around the slight frequency mismatch.  If you 
|didn't a vertical bar on the computer screen would look slightly diagonal 
|on the TV.

They do line-lock to the original source; they do not change the timing.  The
generated sub-carrier will be off by whatever percentage your sync is off.
(It is not a TBC at all.  If you want a quality transcoding TBC(*) it's more
like $4k off the shelf, but much cheaper on eBay.  I have one with component
in, but I don't have an RGB one to try.)  You are expected to provide the
Harmonic unit with correctly timed video to begin with (that's why they say
broadcast-quality in :).  If you are not "reasonably close" it will not work
at all.  If you are _only_ close then the output will also be only close, but
the frequency relationships among the components will be the correct.  That's
what you are paying for.

I think the comment about regenerating all sync and timing signals is slightly
misleading.  It is _true_ (I traced the circuit to see what they were doing;
to their credit they do not sand the part numbers off the ICs like some)
but it is more a matter of convenient use of the components they chose than
necessity.  They could have created the locked sub-carrier and used the sync
as provided, but by the time they had done the former they already had both
vertical and horizontal sync available as a side effect.  Using the original
sync at that point would have required a more complicated circuit, plus they
might have had to worry about propagation delays.  And of course, the input
sync pulses might not have quite the right widths or such.

(*) Pet-peeve: have you noticed that some TBC manufacturers make you pay
extra for the "transcoding" feature beyond what you pay for the input and
output options?  Given that they are basically rendering to a frame buffer
and generating video from that it's hard to see how a TBC could fail to
transcode unless they go out of their way to prevent such behavior when you
haven't paid for it...

|> |> 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.
|
| 	OK... true enough.  It isn't "real" NTSC as in broadcast-quality 
|NTSC.

"Real" NTSC will have the defined timing relationships.  Broadcast-quality
will be "real" and also get the color encoding matrix right, have a highly
stable clock, etc.

|I'm just reading through the datasheet one more time, and I do see 
|some references to "synchronous" video systems vs. "asynchronous" video 
|systems.  You're referring to the former, and I'm using the latter

I don't know exactly what terminology they are using.  Historically a
synchronous system was one where multiple sources were synchronized to
the same master time base.  Moreover, every source was not only at the
same point in the field but was in the same field of the four-field
sequence.  This used to be very important to broadcast studios and entire
networks so they could do seamless cuts between sources without any video
storage.  These days it is fairly easy to interpose a TBC whose output is
synchronous and external sources can be asynchronous.  But a single NTSC
signal unto itself is always synchronous.

| 	Now that you've successfully gotten me off on this tangent, I'm 
|looking into the feasibility of using the 14.318Mhz clock that's on the 
|VGA card.  The AD724 datasheet mentions using this for a synchronous 
|system, and I might be able to get ahold of it via the "VGA Feature 
|Connector"

You would be making the VGA your "studio master" clock and slaving the
AD724 to it.  That might work, but I'm not sure how stable that clock
is or whether it is even always available.

|That would greatly simplify the circuit 
|construction/components/tuning since it wouldn't even need a crystal then 
|(and would also be more accurate NTSC).  If that doesn't work, a PLL 
|programmed to reconstruct P/Q times the dotclock from the HSYNC signal 
|might be a very viable alternative.

That's what the Harmonic product does.  I don't think it is entirely trivial
to do well...

|> 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.
|
| 	Much harder... actually almost impossible without resampling.

You don't need to resample to make the clock *relationships* correct.

|> 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.
|
| 	That's a PAL-ism, no?  I thought NTSC didn't flip the SC phase.

No, it's an NTSC-ism.  (PAL uses a 90 degree shift.)  It's built into the
timing.  Each scan line comprises exactly 227.5 cycles of the color sub-
carrier.  With an odd number of half-cycles per scan line the initial sub-
carrier phase flips each line.  And it gets better.  With an odd number of
scan lines per frame the phase also flips per frame.  This is why there are
actually _four_ distinct NTSC field types rather than the two that you often
hear about.

|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.
|
| 	I must admit that I've really only tried my circuit on two TV's. 
|A more advanced TV might try to do fancy comb filtering to try to extract 
|*all* the Y/C info from the CBVS signal.

It really isn't just a question of comb filters.  I've seen some evidence to
suggest that my XBR100 uses the sub-carrier to adjust the fine tuning.  Time
code sensitive devices (editors, etc.) use the phase of the sub-carrier to
know where they are in the four-field sequence.  I don't know what VCRs might
do...

|I would find it interesting to know how good TVOUT's 
|NTSC is.

All the ones I've tried (and I've been trying since the ATI EGA Wonder) look
pretty bad... to me...

|Since it has the advantage of having some hardware clocks 
|available, I'd bet it's pretty good.

Ah, but there are so many other ways to screw up the signal.  The color
encoding itself isn't trivial, and you really need to limit the bandwidth
of the luma channel without distorting it too much.  There's only so much
left to spend on the TV-out function of a retail $35 card.

				Dan Lanciani
				ddl at danlan.*com


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