Hi guys!
I have been working on a http://www.laserdisc-replacement.com project for probably two years now and am really trying to use the Raspberry Pi as part of it (results thus far are very encouraging!).
Laserdiscs contained information such as current frame number on some of the hidden lines shown during the VBI (vertical blanking interval). For example, it would store data on lines 11, 16, 17, and 18.
I think it has been established pretty firmly in these forums that we cannot modify the way the Pi outputs these hidden lines (ie it will always make them just black).
However, I had a crazy idea a week or two ago that I could generate the required data on these lines using a moderately clocked AVR device and an LM1881 sync separator chip (so that the AVR could find the start of each line).
The problem is how to do this without harming the Pi!
And that is what I am looking for advice about from some hardware hackers.
Quick theory: The analog composite video signal is supposed to have a range from about 0V - 1V assuming the display device (ie TV) has a 75 ohm resistor (my tests show that the Pi doesn't go quite down to 0V and goes a little over 1V but that's ok).
The AVR I am thinking about using will probably be outputting 5V. I believe I already know how to drop that 5V down to 1V (using a voltage divider with two 225 ohm resistors should do it quite nicely if I've done my math right).
I basically need to output bright white lines (ie the full 1V) at 2 microsecond intervals during a horizontal line (for example line 17). An AVR at 8+ MHz should be able to handle this easily with a little hand-tuned assembly.
However, I am worried about sending this voltage back through the Pi's TV out port and having something bad happen.
Can someone give me some advice about how I can protect the Pi from harm while still being able to add voltage to the video signal from the AVR?
Thanks in advance!
--Matt