[mythtv-users] VGA to component cables

Cory Papenfuss papenfuss at me.vt.edu
Sun Jan 7 22:46:53 UTC 2007

> Hi there,
> I'm confused by this. VGA _by definition_ carries RGB signals, no?
 	Yes.... but that doesn't mean that other things couldn't be sent 
out the DB-15 connector of a "VGA" card.

> I think you're saying that some ATi cards can, with the right
> drivers, output  YPbPr through their VGA-output connector. Is this
> correct?
 	It's slightly more than drivers, but not much.  There are a few 
different flavors of "RGB" video.  There's RGBHV with a discrete Red, 
Green, Blue, Horiz, and Vert sync signal.... that's "normal" VGA RGB and 
requires 5 separate signals.  Once upon a time, it was relatively common 
to combine the Horiz and Vert sync signal into a "composite" signal 
requiring only 4.  It was even relatively common to add this composite 
signal onto one of the video signals... typically the Green, for 
"Sync-on-green" video.  That requires 3 wires and a number of workstations 
did this.

 	This "Sync-on-green" is more or less the same thing as YPbPr, 
except that the D/A on each of the video channels puts out a 
color-transformed set of data.  If you take a plain VGA card that can 
output both:
- Sync-on-green
- Interlaced output
- 60Hz/15.7kHz sync rates
... and connect the green channel to the Y-channel of a component TV, 
you'll see a nice, crisp B&W image of the green channel of the RGB video. 
If you connect the other two, you'll see a nice rendition of an acid-trip 
color video, since the color transformation hasn't been done.

 	These "breakout" cables simply do that... and the driver tells the 
card to do the color transformations.

> If my understanding is correct, then the cable above would be better
> referred to as a "15-pin D-sub to component video breakout cable"; as
> you surmise it will not breakout YPbPr from a _standard vga_ (or SVGA
> or XGA &c) output.
 	Correct... but since when have consumer products been named aptly 
to prevent confusion?

> The fact that we're having this discussion, and the OP's questions
> about its suitability, suggest that the website's labelling of this
> cable is misleading - one would reasonably expect to plug a "VGA-to-
> component video breakout cable" into a VGA port at one end and a
> telly's component inputs and have at least the right colours come out.
 	See above.



* Cory Papenfuss, Ph.D., PPSEL-IA                                       *
* Electrical Engineering                                                *
* Virginia Polytechnic Institute and State University                   *

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