Integrating tv.spin with a ntsc signal

Dave Goodrich

I'm using a chip to decode vertical sync, composite sync, odd/even line and burst/back porch signals. I would like to display the ntsc signal, from a video camera, over the top of the tv and put two text lines from the tv.spin program on the bottom of the screen.

I have been unable to get a solid sync between the ntsc signal and the tv.spin signal. The ntsc signal sets the vertical sync and the tv.spin signal drifts across the bottom of the screen and then jumps back.

Can anyone point me to a working example?

Phil Pilgrim (PhiPi)

TV.spin is not designed to be used with an external sync, which is necessary when doing a video overlay. For displaying just a couple lines of text, you could do that yourself in assembler by triggering on the VSYNC output from your LM1881 and counting HSYNCs until you get near the bottom of the screen. Then use the character font in ROM, along with WAITVID, to superimpose the characters. This is actually much easier to do than generating an entire NTSC signal from scratch. One thing you won't be able to do, though, is produce the characters in color, since that requires genlocking to the incoming video's color bursts — a feat which, for the Propeller, has so far proven elusive.

-Phil

Mike Green

Bean (Hitt Consulting) has developed an NTSC text overlay board using a Propeller and a sync separator. The software is "open source" and can be downloaded from his website. The schematic is also downloadable.