[mythtv-users] Hardware for Best TV-Out?
papenfuss at juneau.me.vt.edu
Wed Oct 27 12:06:11 UTC 2004
> 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.
Since you say it uses a PLL like I was thinking of doing, then it
really does boil down to how accurate your HSYNC is. I'm sure that the
lock-in range of the PLL on their circuit won't sync beyond a certain
(hopefully narrow) range. Even still, at 227.5*15.75kHz = 3.583125MHz. A
far cry from the intended +-10Hz subcarrier tolerance.
> 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
Market-speak at its best.
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.
So they did run it through a frequency-multiplier based on the
horizonal sync then? Trouble with that is now your HSYNC's tolerance is
227.5 times more important to get right.
> (*) 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...
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). The former is
easy to do without much smarts, and a frame or two every few seconds won't
be noticed. A difference of 5/6 framerate at least *should* be given some
> | OK... true enough. It isn't "real" NTSC as in broadcast-quality
> "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.
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. 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).
> 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.
... except for the discrepancy that we're talking about.
Without either storage or phase-locking two devices together, it's not
possible to get truly standards-compliant NTSC from a setup like we're
talking about. With COTS VGA card, you're choices are:
1. AD724 async (like I'm currently doing)
3.58MHz subcarrier accurate in frequency, but not phase-locked to
2. AD724 sync with PLL (like Harmonic does, apparently)
3.58MHz subcarrier will be off by the ratio of error in HSYNC to
the standard of 3.58E6/227.5=15.734kHz. Subcarrier will be phase-locked
to HSYNC, however
> 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.
Right... it would be nice, but it appears that the "Dot Clock"
signal on the "VGA feature connect" isn't always available. I don't know
how stable it's guaranteed to be (as far as ppm drift, etc), but it *is*
programmable via the modeline.
> That's what the Harmonic product does. I don't think it is entirely trivial
> to do well...
What I was thinking would be to run a PLL off the HSYNC, but could
be programmed via DIP switches or something. Then for a modeline like:
ModeLine "coryntsci" 14.318 720 760 824 910 480 484 492 525 interlace
it could be set to re-muliply the HSYNC by 910, thus reproducing the
14.318MHz clock. That's 4*fsc and could be fed right into the AD724. Of
course it depends on X setting the clock *exactly* to 14.318Mhz to get a
truly accurate subcarrier frequency. Now that I think about it, however,
the 910 shouldn't have to change... even if you doubled the modeline to
28.6MHz and 1440x480 resolution. It's the *time* that matters, and
that'll stay constant. What the DIP switches would allow you to do is fix
the signal if the card wouldn't do exactly the right dotclock. Hrm...
910/4 = 227.5. Coincidence? I think not... :)
> | Much harder... actually almost impossible without resampling.
> 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
> | 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.
Gotcha... so it's a 180 degree flip as per the 227.5 multiple,
rather than 90 for PAL.
> | 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
Again... very few people need to connect their mythtv box to
studio-editing hardware. If you need that, it's pretty tough to expect a
$35 vid card to pull it off.
> |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...
Me too... that's why I built mine. I think that most of the
quality is from the scaling and temporal interpolation that needs to be
done to rectify the scanrate problems. Laying down a standards-compliant
NTSC raster should be easy, since they've got their own clocks and
multipliers to work with. Use a master clock at Fsc (or a multiple) and
count everything else from that.
> 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
The AD724 seems to address all these pretty well from what I can
see. The biggest drawback is also it's feature to allow for a
free-running oscillator. Throw a multiply-by-910 PLL on it to generate
the subcarrier from HSYNC and I think we're both happy.
How the *HELL* did we got off-topic so badly on the mythtv-users
list anyway? Sorry everybody, but maybe this thread enlightened some
folks as to why most TVOUT doodads suck so horribly. Even if you are
willing to work with 1/2 VGA speeds, it's *STILL* difficult!
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