One pitfall not already mentioned is that simply combining the R-G-B and
sync signals coming out of a VGA card won't produce a US standard NTSC
signal. A US television signal (NTSC compatible) uses a 15.750 kHz
horizontal sweep rate. The lowest VGA resolution (640 X 480) uses a 31.5
kHz sweep. Higher resolutions use higher sweep rates. This scan rate
incompatibility may complicate your project. Of course, there are some
specialty video cards around that have an NTSC video output, in addition to
VGA. Those cards have an on-board scan converter.

Good luck,

Roger K6XQ

Roger,

Wouldn't this be be exactly double of NTSC, in which case the TV would
see every other frame of the composited VGA? Or, because I am
intending to run the card in Linux, could I create a resolution below
640X480? But you may be right, a surplus video card with S-video out
would sidestep this problem.

The Eternal Squire

Roger,

Wouldn't this be be exactly double of NTSC, in which case the TV would
see every other frame of the composited VGA? Or, because I am
intending to run the card in Linux, could I create a resolution below
640X480? But you may be right, a surplus video card with S-video out
would sidestep this problem.

The Eternal Squire

Wouldn't this be be exactly double of NTSC, in which case the TV would
see every other frame of the composited VGA?

If the TV would ignore the sync pulses that arrived too soon, you might get
two images, side by side. But that ignores the other complications that I
didn't mention: VGA and NTSC don't use the same vertical sweep frequency.
And NTSC is an interlaced vertical scan, meaning you get alternating lines
of display, rather than a complete image scanned top down. NTSC requires
two complete top down scans to get all lines of an image. VGA is
non-interlaced.

Or, because I am intending to run the card in Linux, could I create a
resolution below
640X480? But you may be right, a surplus video card with S-video out
would sidestep this problem.

The video card has more to do with the VGA scan rates than the operating
system. You often see crystal controlled oscillators on video cards so you
are locked into what the card manufacturer provides. I can't rule out
some flexibility here, but I think a video card with NTSC video output would
be the surest way to get this idea to work.

Roger

wrote:

Hi all,

It's me again, attempting yet another mad science project, and I have
practically googled the subject to death before thinking of asking this
group for help:

I am wanting to put video (preferably color but will settle for
grayscale) onto a UHF TV channel for inhouse use only. I am
considering doing it as follows:

1) Convert output of a VGA card to composite video using resistive
combiner
(found a few on net).

2) Place composite video on VHF channel 3 using a game modulator.

3) Upconvert the video signal on VHF to UHF by with a local oscillator
whose frequence is the difference between the channels. I'd use an
active mixer circuit from the VHF/UHF part of the Handbook to do this.

4) To test functionality, attempt to tune in using either an analog TV
receiver or a TV tuner card.

Question is this:

Is point 3, upconverting video from VHF to UHF, a generally workable
technique?

Any other gotchas or pitfalls to my intended approach?

Thanks in advance,

The Eternal Squire


Motorola/Freescale has several ICs designed for video modulators that
use PLL tuning. One of them may do just what you want, rather than
building an upconverter.

http://www.elecdesign.com/Articles/I...ArticleID=4890

--
Former professional electron wrangler.

Michael A. Terrell
Central Florida

I looked at the article. Nice try, but I don't have the equipment or
eyesight to work with TSSOP-24 package,
nor the money to play with a $300 evaluation board :)

The Eternal Squire

On Sun, 12 Jun 2005 13:41:29 GMT, Doug Smith W9WI
wrote:

[snip]

http://www.73.com/a/0019.shtml offers a "block converter". (among
dozens of other items! It's about 3/4 down the page on the left-hand
side, stock #AE047) It's a broadband frequency converter that will
convert a US channel 3 RF signal up to US channel 37.

I called these bozos to inquire about buying one of these. You get an
answering machine but not the courtesy of a call back. The items have
also disappeared from their web site inventory.

Any other sources?

Wes N7WS

In article . com,
says...

Hi all,

It's me again, attempting yet another mad science project, and I have
practically googled the subject to death before thinking of asking this
group for help:

I am wanting to put video (preferably color but will settle for
grayscale) onto a UHF TV channel for inhouse use only. I am
considering doing it as follows:

1) Convert output of a VGA card to composite video using resistive
combiner
(found a few on net).


And what magic resistive combiner makes VGA into NTSC??? There is
no such thing. There are active models which do this, but if your
VGA output is 640X480 or more, then NTSC will make the video suck.
NTSC does not have the bandwidth. Yes you can take a properly modulated
channel 3 signal, run it into a balanced mixer (well almost any mixer)
and add your favorite local oscillator signal and get 2 signals in
the uhf band. LO + ch3 and LO - ch3.

Good luck. Here's looking at ya or at least the unviewable signal.







2) Place composite video on VHF channel 3 using a game modulator.

3) Upconvert the video signal on VHF to UHF by with a local oscillator
whose frequence is the difference between the channels. I'd use an
active mixer circuit from the VHF/UHF part of the Handbook to do this.

4) To test functionality, attempt to tune in using either an analog TV
receiver or a TV tuner card.

Question is this:

Is point 3, upconverting video from VHF to UHF, a generally workable
technique?

Any other gotchas or pitfalls to my intended approach?

Thanks in advance,

The Eternal Squire

Gudmundur wrote:

In article . com,
says...

Hi all,

It's me again, attempting yet another mad science project, and I have
practically googled the subject to death before thinking of asking this
group for help:

I am wanting to put video (preferably color but will settle for
grayscale) onto a UHF TV channel for inhouse use only. I am
considering doing it as follows:

1) Convert output of a VGA card to composite video using resistive
combiner
(found a few on net).


And what magic resistive combiner makes VGA into NTSC??? There is
no such thing. There are active models which do this, but if your
VGA output is 640X480 or more, then NTSC will make the video suck.
NTSC does not have the bandwidth. Yes you can take a properly modulated
channel 3 signal, run it into a balanced mixer (well almost any mixer)
and add your favorite local oscillator signal and get 2 signals in
the uhf band. LO + ch3 and LO - ch3.

Good luck. Here's looking at ya or at least the unviewable signal.


There are a number of video cards with a NTSC video output in the
S-video format. I still have a couple I used in old PCs to drive a
Commodore 1701 monitor to trick people. My Commodore 128 was connected
to an IBM PC Junior monitor in CGA mode. The look on people's faces was
worth the effort.

--
Former professional electron wrangler.

Michael A. Terrell
Central Florida