[mythtv-users] Re: TV-Out: AITech Scan Converter vs CX2587x

[John Morris]

On Fri, 26 Sep 2003, Fred wrote:

> > Don't have that exact unit, but scan converters in general frame buffer 
> > the input and then output their signal from that buffer.  If it accepts an 
> > interlaced signal at all from the VGA end it will probably wait until both 
> > fields show up and pass the whole frame to the output.  That is why I'm 
> > dumping my scan converter and have an Audio Authority VGA -> Component 
> > converter on the way.  The darned things are just too smart.  For most PC 
> > to TV work modern scan converters are great, accepting a wide variety 
> > of vodeo modes and making a good effort to get a usable picture onto a 
> > TV. but not for video.
> 
> Yeah, I was concerned about that so I emailed the manufacturer first and
> they assured me it won't fubar the interlaced frames as they pass
> through.  Of course, I don't believe a claim like that until I see it
> for myself or one of you guys confirms it. :)  I took a look at the
> Audio Authority first but it appears to be HDTV only so that won't work
> for me.

You don't need HDTV, but you do need component inputs on your TV.  My 32"  
TV only accepts bare minimal 480i and the Audio Authority is putting out a
beautiful picture, no noise from the extra A/D-D/A conversion and
rescaling, perfectly overscanned.  I looked and looked for a similar
product that would go from RGB to S-Video and decided the animal no longer
exists.

> So since you care about this issue enough to dump your scan converter,
> does that mean you've got working interlaced output from Linux?  How?

Well I ALMOST have perfect video now.  It is interlaced but my ATI RagePro 
isn't accepting the 11.958 exactly, rounding it to 12.00 and printing out 
60.1Hz for the vertical time.  So tickers along the bottom of the screen 
look perfect for a couple of seconds and then bad for a few.  Switching on 
the deinterlacer causes it to look good then have a slight twitch for a 
few, etc.

Here is the mods to XF86Config to get 480i:

Section "Monitor"
        Identifier      "TV"
        HorizSync       15-33
        VertRefresh     43-72
#       Option          "DPMS"
#       ModeLine "ATSC-480-59.94i" 11.958 640 664 736 760 480 484 492 525 Interlace
        ModeLine "ATSC-480-59.94i" 11.958 640 674 746 760 480 484 492 525 Interlace
EndSection

The commented out line is the modeline I got by Google but it was a touch
off center so I slid the sync pulse over a bit, probably related to the
card rounding off the base clock.

Then add to the Screen section:

	DefaultDepth    24
        SubSection "Display"
                Depth           24
                Modes           "ATSC-480-59.94i"
        EndSubSection

In the end, even if the field sync problems are solvable I doubt truly
perfect video playback is possible from a VGA card because I suspect the
playback software really needs to be able to not only detect vertical
retrace, but be able to control it. (Or twiddle the audio rate to
perfectly line up with the video, but I have NEVER seen that capability on
a consumer audio device.  Once they add that it is a Pro card.) Typical PC
apps work because a computer monitor's refresh rate is so insanely high
that it isn't as visable, but when the display is running at around the
same framerate as the source material, unless they are EXACTLY the same
you must have dropped/duplicated frames, and a single glitched frame is
going to be noticable.

Longterm I'm putting my hopes on the PVR-350's output for video playback
and will use the VGA for things like MAME/MESS.  After looking at the
specs on the chips it looks like the video encoder gets clocked from the
mpeg decoder so sync should be perfect.

The PVR-350 only provides S-Video and analog audio but that isn't as bad
as it sounds.  A/B testing with a DVD between S-Video and Component
doesn't look very different at 480i and anything recorded from TV isn't
going to really benefit from the digital soundpath.  Guess I'll keep the
standalone DVD player for now.  :)

-- 
John M.      http://www.beau.org/~jmorris        This post is 100% M$ Free!
Geekcode 3.1:GCS C+++ UL++++$ P++ L+++ W++ w--- Y++ b++ 5+++ R tv- e* r
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