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VGA to NTSC line-for-line ?

P

Peter

Jan 1, 1970
0
Hi All,

I am after a converter which does VGA 640x480 to NTSC video.

There are loads of boxes which do this e.g. this Ebay reference:
190280908219

The catch is that I am looking for something which does not replicate
(or eliminate) any pixels *vertically*.

This is because this is for displaying TEXT on a product which has an
NTSC video input which displays on an LCD, and I think that the video
is mapped onto the LCD pixel for pixel, so if I keep this up in the
converter box also, I should get the best quality.

Obviously some lines will be lost because the displayed part of an
NTSC frame is less than 480 lines...

Any pointers would be appreciated.
 
N

Nobody

Jan 1, 1970
0
I am after a converter which does VGA 640x480 to NTSC video.

There are loads of boxes which do this e.g. this Ebay reference:
190280908219

The catch is that I am looking for something which does not replicate
(or eliminate) any pixels *vertically*.

This is because this is for displaying TEXT on a product which has an
NTSC video input which displays on an LCD, and I think that the video
is mapped onto the LCD pixel for pixel, so if I keep this up in the
converter box also, I should get the best quality.

Obviously some lines will be lost because the displayed part of an
NTSC frame is less than 480 lines...

It's actually 486 displayed lines (of 525 lines in total), although some
of those may be not actually be visible (esp. for CRTs).
Any pointers would be appreciated.

The main issue is that NTSC is interlaced, VGA isn't. So VGA will have
roughly twice the horizontal frequency of NTSC (e.g. 31.46875 kHz for
640x480@60Hz vs 15.734 kHz for NTSC).

A simple converter would digitise and store every other input line, then
display it at half speed. This doesn't actually eliminate lines; you would
use the even lines from one frame and the odd lines from the next, so each
line appears in one of the two fields.

OTOH, most PC video hardware is capable of generating NTSC timings
directly. The driver probably won't allow it if the card doesn't include
a "video-out" (composite, S-Video, etc) connector, as VGA monitors
typically can't handle it.
 
Hi All,

I am after a converter which does VGA 640x480 to NTSC video.

There are loads of boxes which do this e.g. this Ebay reference:
190280908219

The catch is that I am looking for something which does not replicate
(or eliminate) any pixels *vertically*.

This is because this is for displaying TEXT on a product which has an
NTSC video input which displays on an LCD, and I think that the video
is mapped onto the LCD pixel for pixel, so if I keep this up in the
converter box also, I should get the best quality.

Obviously some lines will be lost because the displayed part of an
NTSC frame is less than 480 lines...

Any pointers would be appreciated.

Oh wow, that is a problem that's more complicated than it first
appears.
Since there are no "pixels" as such in NTSC, the idea of a per-pixel
mapping in a LCD is ambiguous.
Even the lines themselves have no real meaning unless the LCD is
exactly 525 lines (give or take).
The best you can do is make the text large enough to be legible.
For example, in the '80s, home computers used TVs as their displays.
None of them had 80 columns on a TV though.
The computers with 80 columns either used monochrome monitors with
NTSC-like syncs but no color information in the video,
or had dedicated RGBI/CGA-style graphics.
(I don't include "80 column" software display modes with tiny 4 pixel
wide fonts on s-video monitors, but that's one way of doing it)
If you look at the fonts used back then, a low-res computer like a
VIC-20 used single pixel wide text and it was legible on TVs.
However, machines like the C64 with 320 pixels horizontally had to use
fonts with lines two pixels wide.
You'll have to try the same idea.
Now modern TVs, even CRT-based TV from less than 10 years ago, use all
kinds of processing to improve the video.
Lots of sharpening and DSP magic lets my clunky old 27 inch Wega
display re-scaled 800x600 (from my PC) almost as clearly as 800x600 on
a monitor.
But this won't happen with a small LCD or less-evolved tube TV.

Now as the other poster mentioned, since VGA is progressive, you'll
lose pixels in time. No way around that.
Unless you're scrolling fine text, it shouldn't be a problem.

Maybe your problem could be solved easier with a LCD display with a
dedicated input with true per-pixel graphics.
But that's a whole 'nother kettle of fish.
 
P

Peter

Jan 1, 1970
0
[email protected] wrote
Even the lines themselves have no real meaning unless the LCD is
exactly 525 lines (give or take).

I am quite happy to lose lines to make this work 1:1 in the vertical
direction.

Certainly I am not going to be building anything, because for a start
the line frequencies are different. This job needs a proper frame
buffer - just like the $50 boxes which are all over Ebay. They are all
seemingly based on the same chip - in the same way that all the RGB to
USB converters use a particular chip made by Philips (which has 3
9-bit A-D converters, etc).

I just wondered if any of these VGA-NTSC converters have the
capability to do a vertically-direct pixel translation. This should be
easy.
 
J

JosephKK

Jan 1, 1970
0
[email protected] wrote


I am quite happy to lose lines to make this work 1:1 in the vertical
direction.

Certainly I am not going to be building anything, because for a start
the line frequencies are different. This job needs a proper frame
buffer - just like the $50 boxes which are all over Ebay. They are all
seemingly based on the same chip - in the same way that all the RGB to
USB converters use a particular chip made by Philips (which has 3
9-bit A-D converters, etc).

I just wondered if any of these VGA-NTSC converters have the
capability to do a vertically-direct pixel translation. This should be
easy.

I have an old box from avermedia (an averkey imicro, KOC3) that
converts VGA, SVGA, and XGA to NTSC. I can't vouch for line for line,
but it works.
 
J

Jasen Betts

Jan 1, 1970
0
Hi All,

I am after a converter which does VGA 640x480 to NTSC video.

There are loads of boxes which do this e.g. this Ebay reference:
190280908219

The catch is that I am looking for something which does not replicate
(or eliminate) any pixels *vertically*.

set your VGA to run at the NTSC frame rate, and then use an RGB to
NTSC converter, or get a VGA with a composite output.
 
G

Greegor

Jan 1, 1970
0
Peter, I am curious as to why they made the LCD
to use NTSC input? What were they thinking?

What did they intend it to be connected to?
Have you tried feeding Composite NTSC
from an old VCR into it?
Does it just throw away the parts of the NTSC signal
it doesn't use?

Are these LCD's available cheap on some surplus market?
Got a lead?
 
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