[MPlayer-users] Re: Display card recommendations

[D Richard Felker III]

On Tue, Sep 17, 2002 at 09:20:34PM +0200, Josef Wolf wrote:
> [Automatic answer: RTFM (read DOCS, FAQ), also read DOCS/bugreports.html]
> On Mon, Sep 16, 2002 at 09:30:54PM -0400, D Richard Felker III wrote:
> 
> > Hmm, my external vga-to-tv works rather well. I *did* spend a lot of
> > time tweaking with fbset to get it working though...and it doesn't
> > help that matroxfb rounds the values you give it without telling you
> > how it rounded them...
> 
> Is this a homebrew adaptor? If it is homebrew: would you like to share
> the circuit drawings and the software setup?

Basically it's the reference design from the Analog Devices AD724 IC
manual, using that chip for NTSC encoding. You can find the manual at
their website (www.analog.com).

> I am currently considering a VGA->SCART adapter. IMHO this should be
> not very compilcated, since SCART can eat RGB. AFAICS the most
> complicated thing would be the XFconfig to meet the TV freq's.

This is a bit complicated, but not too bad. Start out with the right
vertical resolution (including blanking and sync pulse lines, and
don't forget interlacing!), and whatever horizontal resolution you
want, adding about 25% extra for the horizontal total. Then compute
the necessary pixel clock, and play with the relative lengths of the
hsync pulse and left and right borders until the image looks right.

With SCART it should be rather easy as long as you don't make the
sync pulse too short. For NTSC it's much harder, as the television set
and/or the AD724 is sensitive to bad sync pulses when trying to pick
up the color carrier for composite video.

> Second problem would be to choose good cable so the signal quality
> would not be hurt on the 15 meters from PC to TV :)

Really it doesn't seem to be too much of an issue. The biggest problem
IMO is connecting the PC to the converter device, since all VGA cables
costing less than $75 or so are basically junk. My solution was to run
VGA over cat5 shielded twisted pair (STP). The following forum thread
describes the idea well:

http://www.avsforum.com/avs-vb/showthread.php?s=a997a3ad1059817faecb48e10679cb6b&threadid=23850&highlight=vga+cat5

Then I'm able to switch between monitor and TV output with simple RJ45
jacks. The biggest obstacle to this design is getting shielded RJ45
jacks and/or couplers. Right now I don't actually have enough STP for
the line to the NTSC encoder, so I'm just using UTP, and not getting
any terrible artifacts.

If you put the converter box right by the PC, the cable type shouldn't
matter much since it's a short run. Cat5 UTP makes things simple. Then
you can let the composite video cable be the long run.

Of course, if you're going to be doing SCART with RGB instead of
NTSC/PAL, the whole setup should be a lot easier. You might even be
able to just cut off one end of a SCART cable and solder it into a
HD15 head to connect to the VGA port. At most you'll need a few
capacitors and resistors to filter, fix DC bias, and/or tweak the
voltage ranges.

Rich