I didn’t do much on this project lately. The pico_synth project attracks most of my spare time. However I managed to put few lines into a micro-controller to get some visual on the tv screen
You can clearly observe a sync issue, certainly has to do with the 625 lines of data.
However, I found an interesting post from LZX industries specialised in eurorack video modules regarding the encoder.
Here’s what happens, exactly…
For each channel (R, G & B)…
1) Input is buffered
2) Input is processed by attenuverter circuit
3) Input is summed with +/-1V bias control output
4) Negative-going voltages below 0V are clipped
5) Positive-going voltages above 0.9V are clipped
6) Signal is blanked to 0V during blanking interval (using CD4053 analog switch and Blanking signal from the 14-pin sync header, after being buffered by series 74HC14 CMOS inverters)
7) Signal is buffered
8) Signal is attenuated from 0.9V scale to precisely 0.714V.
9) All channels are AC-coupled going into the RGB encoder chip.
So, as you can see, most of what happens is before step 9, the actual encoder. On the top board of the CVE, there are test-points you could attach wires to, to get clipped, blanked, and buffered RGB signals to an external device.
The reason why clipping occurs at 0.9V is so that there’s 0.1V left for gain loss throughout a system, to make sure you can adjust for full scale. So effectively the RGB knobs at full CW position are at ~110% gain.
As for the clipping circuits, doing white and black level clipping without bandwidth reduction is difficult. We tried many different solutions, and end the end used two half-wave rectifiers in series with offsets on each input.
You could definitely tap off the CVE between points 8 and 9 above, and try sending them to another encoder device.
So, yes I’m being lazy and not really concerned about this project but allow me a question: why is there some many interesting stuff and so little time ? No, the answer isn’t 42. Pretty sure.
However, I think once the sync generator and encoder working the rest will come quickly.
There’re also lot of useful informations on LZindustries about standards, PAL, NTSC, refresh rate and all that (interesting) jazz: