Video Overlay? Possible?

Evan in L.A.

Would it be possible to overlay graphics over an incoming NTSC signal with the Propeller?

chrisl

I have the same question...
Evan, have you answered this, or any advice from people?
Thanks a lot

Mike Green

It ought to be possible to do this, but noone yet has done it. It would have to be done in assembly and you probably would need an external sync separator to make it easier to find the sync edges to do the timing. I don't know whether you could directly overlay color text, probably not, but black or white text ought to be doable using a modified version of the existing text output drivers.

Phil Pilgrim (PhiPi)

This question has come up before, but I couldn't find a link to it, so here's the brief answer: A black and white overlay on an incoming video signal should be easy. Just use an LM1881 to extract the sync from the incoming stream and trigger the Propeller. The Propeller can then output pulses to lighten or darken the video, thus producing the overlay graphics. For best results, you'll probably want to DC-restore the incoming video before working on it.

Adding color to the overlay graphics is much more difficult, because then you have to lock onto the incoming video's colorburst and force the Propeller's colorburst to remain in sync with it. I believe — as an article of faith — that this is possible, but I'm not really sure yet how to achieve it.

-Phil

Update: Ah, Mike, you beat me to it!

chrisl

Thanks for the replies.
What about not locking onto the sync of the tv, but making your own (like the normal tv objects) and maybe buffering a line of tv voltages then outputing modified voltages? or something of a more dsp nature, thanks

Mike Green

To buffer the voltages from the tv, you'd need a high speed A/D converter and enough memory to hold the bits. The Propeller is hard pressed to store and do output on a 512 x 384 bitmap with limited colors (or greyscale values). With the Propeller's resources, it really needs to be done on the fly.