[mythtv-users] How do you set up a MythTV frontend to output YCbCr via HDMI

[Paul Gardiner]

 > Hi, a little late to the thread addressed to me but here
 > now.
 >
 > I'd be suspicious that studio levels were required too,
 > the nvidia-settings gui quite correctly disables the
 > Full/Limited selection box. Studio/Limited levels is an PC
 > RGB thing, in the colour difference world there is nothing
 > other than Studio levels.

That's what I thought, although I suppose it does make sense
if an RGB frame buffer is still being used, and in that case
you would expect to require studio values.

 > This suggests to me that there is something not quite
 > right in the colour space conversion in MythTV, I did
 > receive a response from a developer WRT this and some new
 > code better matching up with VDPAU's colour space
 > handling. I'm unable to find those emails just now and I
 > don't think they went to the list, this was somewhere
 > around 0.22-0.23 and I recall that after this revision the
 > difference between the graphics card outputting RGB and
 > YCbCr was much less.

I'd be interested if you can find them, especially if there
is a mention of which commit it was. I noticed a subjective
degradation in image quality somewhere during the period when
I was on 0.25. Possibly I just preferred the incorrect mapping.
I could experiment backing it out if I knew which commit.

 > I should find the time to look through that part of the
 > MythTV code and understand what's really going on. Most of
 > my work is around dealing with the problems that occur
 > when PC or IT background people get involved in Broadcast
 > Television or less frequently when the reverse happens.
 >
 > The things that were the most noticeable differences
 > between graphics card in RGB mode and in YCbCr were colour
 > casts in shadow that varied in colour over time and the
 > quantity of noise in shadows. Most domestic TVs may hide
 > this effect as they may have bigger issues in shadows or
 > other low light areas so not everyone will be able to see
 > this even if they know what to look for. If you don't see
 > it then I guess you don't need to worry about it...

I wondered if that might be a problem with the TV. Presumably
colour controls are implemented in YCbCr space, so a TV fed
with RGB has to convert to YCbCr and back to RGB. Some TVs
might not do the first conversion well. I had a TV that would
put a colour cast on any darkish area, but that seemed to be
independent of input colour space.

 > It's also possible that my display (Optoma HD80 DLP
 > Projector) is especially poor when driven as RGB and
 > therefore gives better results with YCbCr. I don't have
 > ready access to any broadcast monitors just at the moment
 > to check more thoroughly.

Ah right. You were thinking the same. Definitely a situation
where the YCbCr out feature is useful.

 > I have recently upgraded my VDPAU card to a GT640 (fanless
 > too  and noticed some small but welcome improvement in
 > video quality. I need to recalibrate the projector soon so
 > I'll re-run the tests I did initially to see if I can find
 > any difference with current Nvidia & MythTV code.

I'm going to buy an ION2 board as soon as the seeming
world shortage comes to an end.

 > > that. I was hoping that running this way would reduce
 > > the number of processing stages between codec and TV
 > > screen, but if that setting still has effect, I guess
 > > the decoded YCbCr is still processed before it gets to
 > > the screen buffer.
 > >
 > > Do you know how this works? Is the screen buffer YCbCr,
 > > or is it still RGB with the graphics chip turning it
 > > back to YCbCr just to output to HDMI?
 >
 > The internal buffers _should_ be YCbCr but in the TV
 > industry Nvidia is generally considered to play fast and
 > loose with little details like colour space, it's supposed
 > to be one of the differences between gaming and
 > professional cards & drivers but even in the pro drivers
 > on pro cards it's still considered a little off by some.
 > AMD always used to get this stuff right but that's no use
 > to MythTV.

Maybe in X based systems you can't avoid an RGB framebuffer.
If using a YCbCr frame buffer, every library used to write
to it would need to perform the translation, or in some
situations avoid translation. I'm thinking of the menu
system and the OSD. Sounds very difficult to get right, whereas
converting an RGB frame buffer to YCbCr on output sounds
a lot easier.

Cheers,
	Paul.