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

Dan Lanciani ddl5 at danlan.com
Wed Oct 27 20:42:03 UTC 2004

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

|> 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.

That's why you have to feed it "broadcast quality" to begin with...  But it may
sync beyond an extremely narrow range; it still gives the locked relationships
which can be useful.

|Even still, at 227.5*15.75kHz = 3.583125MHz.  A 
|far cry from the intended +-10Hz subcarrier tolerance.

Don't get hung up on the 15.75 figure.  It's just trade talk for NTSC.  Think
of it the way you think about nominal 1/2" pipe that has neither an ID nor an
OD of 1/2".

| 	So they did run it through a frequency-multiplier based on the 
|horizonal sync then?

Yes, as far as I can tell from tracing the circuit.

|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. :)

|> (*) 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).

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...

|> | 	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.
| 	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.

|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.

|> | 	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 

I pick the relationships...

|> | 	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...
| 	Again... very few people need to connect their mythtv box to 
|studio-editing hardware.

My XBR100 isn't studio-quality and there is consumer editing hardware (though
I really don't care about it).

|If you need that, it's pretty tough to expect a 
|$35 vid card to pull it off.

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.

|> 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.

| 	The AD724 seems to address all these pretty well from what I can 

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...

| 	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

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