For one I would like to have an overlay board.
I searched the 'net a found some others that have simple designs. They don't use the LM1881 or LMH1980 sync separator.
This one uses a microchip PIC.
(Scroll down to the bottom to see the You tube video for a quick demo)
http://www.micro-examples.com/public/microex-navig/doc/081-pic-osd-superimposer.html
PAL version with no inputs (demo code only)
Quote from the article
To superimpose text to a PAL video signal, we need to control timing with precision to get a stable picture.
We have to deal with vertical synchronization, which tell us when a new frame starts, and with horizontal synchronization, which tell us when a new line starts.
Usually, and external circuit is used to extract both vertical and horizontal synchronization pulses from the PAL video signal, the LM1881 integrated circuit does it very well for example.
Since I wanted to have a very simple circuit, I had to find a way to make the PIC do this job.
First, we must be able to know when a video line starts : we will use the PIC internal comparator module to do it. The internal voltage reference module will be programmed with a voltage clip level, the comparator will then trigger an interrupt each time the input voltage will become lower or higher than the clip level. This will be our horizontal sync separator.
Second, we must be able to know when a frame starts do get vertical sync : PAL signal uses special sync pulses to announce a new frame. We have to detect a 28 µs low level pulse, there are five of them in the vertical sync and none elsewhere. The internal timer module of the PIC will be used to count time of low level pulses.
This done, we must be able to know what to superimpose to the video signal. A bitmap representation of the text to be displayed is built in RAM from a 5x7 fonts table. On each new line interrupt, we check if we are in display time window for adding pixels or not.
To add a pixel to the video signal, we change output pin from high Z state to output, the output then imposes +Vcc or 0V to display either a white or a black pixel. The result is a superimposed text on transparent background .
NTSC version using the Mega8 and a couple of parts
http://www.viennawireless.org/balloon/hardware/overlay/index.php
Note: NMEA is just a serial standard used on boats for communication between GPS, Radar, Chartplotter etc.
Non overlay PAL PIC library:
So I have given you
- PAL design
- NTSC design on a Mega8
- PAL library with serial input option.
Note these use the same connector for video in and out. So the existing board could be used with the cable split in two for two leads. (Of couse the extra components would need to be fitted, but this could be done with stripboard or breadband to start)
So I challenge someone to use this to make an arduino version of a text overlay unit.
If you do let me know.
I'll be expecting the next version of telemate to have a jumper J7 for text overlay on/off!
Dan