TV-out (was Re: [Mplayer-users] Flickery OSD)

Stephen Davies steve at
Wed Jun 6 12:02:02 CEST 2001

On Wed, 6 Jun 2001, Dariusz Pietrzak wrote:

> > > Is there any good news around for G450 support on Linux/X?
> > No.
> Why would you like TVout when there are cheap 17, 19 and 21'' monitors
> around?
> You can't beat quality of the picture TV versus computer crt.
> (hm, anyone to comment? what is the real resolution of say 720x240 divx
> played via TVout?)
>  And you already got two monitor card.
> ;)

Well - first off a 28 inch monitor isn't cheap (or, if they are, point me
at 'em...).  And a 19'' or so is a bit small for living room viewing?

The issue of good quality output to a TV is something I've been
researching and experimenting with in some gory detail.

TV resolution is much lower than monitors, that's true.  However, TV
resolution is a good match for the characteristics of digital video.  Well
- this stands to reason, really, seeing TVs have always been the output
device for video.

A TV doesn't really have enough bandwidth to give good quality display of
"computer images" (text etc) - a "checkerboard" pattern (alternate pixels
white/black) over 768 pixels per line is a frequency of about 6MHz.  
That's right at the limit of the TV bandwidth I reckon (past it for
non s-video/rgb).  So you can forget about crisp pixels for that sort of

But video output isn't a "computer image" - doesn't tend to have those
sharp transitiions (= high frequencies) so it comes out well on the TV.  

The next issue I've had is that in practice it isn't easy to get good
PAL/NTSC rate video out of a PC.

The best way seems to be to use a TV with RGB input (better TVs with
SCART), and a VGA card that can directly generate interlaced video at
PAL/NTSC scan rates.  A simple interface can convert VGA's separate syncs
into a composite sync and fix the levels.

I have such a TV, and have built such an interface.  I've tested the
principle using a laptop and it works; but that laptop couldn't make
interlaced video so I could only drive 1/2 the vertical resolution.

In my "production" machine I used my Voodoo3 card on the assumption that
it can do TV-out so the VGA can presumably do interlaced video.  I was
going to use the RGB approach, but hoped to also use the TV out for things
like videotaping the output of the PC.  I've since discovered that the
Voodoo3 chip can't do interlaced directly; a separate bt869 takes the VGA
standard from the Voodoo3 and turns it into standard video.

Two other catches I've found with the Voodoo3 TV out are that:
  - I don't think that the bt869 makes very good quality video.
  - The bt869 does overscan compensation; I don't want that -
    it messes up the display quality, and I can't get my video full-screen
    on the TV.

So I will probably swap the Voodoo3 for the G450 and experiment with doing
interlaced RGB directly.

The catch in _that_ direction is that it would sometimes be useful to be
able to generate composite video - for example for recording to video
tape.  VCRs don't do RGB.  And the TV output on the G450 isn't supported
under Linux/X.  :-(  Still - it'll come along eventually, I suppose.

A'rpi seems a fan on the G400, and has his special mga driver.  Its always
a good idea to have similar hardware to the key developer, right?

It is possible to build an outboard conversion frmo RGB to
s-video; there's a Philips chip to do the job; I did see a design

My _next_ adventure is to do with interlacing - making sure that capture,
file formats and playback keeps the fields and plays them correctly.  
This is going to be a minefield, I know - because few people are actually
using interlaced output; most want to get rid of the interlacing as quick
as possible.  Still - that for another mail, and for "phase 2"...

I started this project for fun and learning, and boy am I learning!


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