Video Overlay

MrTrick

I want to build a controller for a video-based stream. The output could be through a 2x16 LCD display, but with the Propellor's video features, would it be possible to have it 'overlay' the interface over an existing video signal?

cgracey

Yes. To drive the luminance level up on your "pixels" is easy. To lock to the colorburst and replace portions of the original signal with color data is much more complex. The first approach is only a few passive components and an object. The second might require more active components. Anything's possible, you just have to figure out how to do it.

MrTrick said...
I want to build a controller for a video-based stream. The output could be through a 2x16 LCD display, but with the Propellor's video features, would it be possible to have it 'overlay' the interface over an existing video signal?

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Chip Gracey
Parallax, Inc.

MrTrick

Hmm....

TBH I don't understand that well how the video signal works, but could a dedicated COG selectively pass-through or replace the incoming video signal depending on what part of the screen it's dealing with?

- If there's a few millisecond delay, that wouldn't be a problem.
- It'd be a text-based overlay, and rectangular.
- It'd have to work at multiple resolutions.

Paul Baker

Read this reference. Chip is saying that changing the Y-value (luminance or brightness) is relatively easy, ie if the video is red, you can change it to pink or brick red easily, this is done by locking on the sync pulse, then at the appropriate delay shifting the Y-value higher or lower. To change the color altogether would require locking onto the sync pulse and also locking on the color burst signal then setting the color value at the appropriate time. This is done by outputing the appropriate phase shifted color burst to generate the color. The zoomed in region of figure 1 is this color information (labeled as the Cb and Cr).

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Bean

My video overlay board simply does a level shift to raise the signal level making it lighter/whiter where the overlaid text pixels need to be. You could also shift the level down to make a darker pixel.

A sync seperater IC (like a LM1881) would be useful to get the timing.

Bean.

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