Luma encoder DIY?

for greyscale you can try combining h+v sync with the greyscale signal and blanking. you don’t need colorburst for b/w, but im not sure actually if not having colorburst will freak out some gear that may expect it.

1 Like

Most of what you need for a luma output is already in C2… three times over! :slight_smile:
You need black/white level clipping (C2), blanking (C2), 1V to 0.714mV gain translation (C2).

Then, instead of the AD724, you need a video opamp 2X gain buffer/ 75 ohms driver, with CSYNC insertion. These are the parts that are not part of the C2 circuit. If you post a schematic image I can verify if it’s correct or not, if you want to make a modified C2 circuit.

1 Like

Oh, thanks Lars. Sounds like this should be easier than expected… i Figure.
Not that i am anywhere near able to design this in the near future. But its good to have something to aim for… :slight_smile:

1 Like

but maybe I can help you :slight_smile:

3 Likes

A luma out module is a great idea. :slight_smile: Also, if you have this, you have the luma channel for a component output! Would just need RGB-to-YPbPr matrix circuit as a pre-conditioner (between black/white clippers+blanking and the output buffers) and you could do component out. Or you could use 3x of them to do RGB out (even if you have sync on red and blue, should be fine.)

1 Like

OK, I’m in the process of making this. I could use a svelte little B&W encoder as well.

I found this sync insertion circuit (below). It appears to be using an op-amp like a switch by powering it on and off with a sync pulse, allowing the sync signal to pass during the blanking interval. Then it occurred to me the signal is blanked already in the C2 circuit.

So far my circuit is one channel from the C2 schematic, up to the blanking switch. Is this Rube Goldberg circuit here what I want, or can I just use a 2x gain summing amp to sum CSYNC with the output of the blanking switch from C2?

26%20AM

1 Like

Something like this?

12%20AM

I think I’ll just go test it and report back. Also, is that cap just for coupling the chips? Do I still need it if I’m going out to an op-amp?

Report: the summing amp “works” but it’s noticeably darker than the luma output from the 724 and it’s also filthy.

Conclusion: I’m going to make a tidy little green-input-only version of C2, tie the other inputs to ground and use the onboard luma output from the 724.

This basically:

Or, I guess I could just tie all the inputs of the 724 to the single clipped signal.

– OK, fixed a couple small errors in the above schematic and threw together a prototype. @LauLindqvist – I just ordered a bunch of PCBs, so it might be a while until I send stuff out to fab again, but I’ll send you one of these eventually if it works.

5 Likes

How come you didn’t go with more SMT? Could have made the board even smaller I bet.

I have those thru-hole parts in stock. My priority is the shortest route to a functioning module. This is intended as a prototype, but if it works the way I want it to, it will also be the final version. I may do another rev only to move the banana jacks pads in a bit so that everything can mount to a panel evenly.

1 Like

Ah, I figured that was probably the case. Makes sense.

1 Like

Ay yi yi, I got some protos fabbed and there were a couple fatal errors I missed. Ordering some new things soon though, so the revisions of these will be in that batch. Luckily I caught the mistakes before wasting an encoder IC. :sweat_smile:

1 Like

@creatorlars, would you mind taking look at this? Before I start tracing the signal or troubleshooting my layout, I just want to verify that there aren’t any glaring errors in my circuit. I omitted the AD724 NTSC/PAL switch circuitry, and I think I have the correct circuit for NTSC?

The goal is to have a simplified, monochrome version of C2 with an attenuator and luma output. My C2 is a little deep for my travel case, I never use RGB anyway, and my wired-up banana and BNC jacks were starting to bother me.

I’m getting a composite signal out, but it’s unstable. Ramps display correctly, but get torn weirdly when they’re processed. Oscillators seem to display properly when they’re h-synced at high frequency, but they freeze and stutter when v-synced at audio rate. My working theory is that my clock circuit is wonky somehow.

  • The “14-pin header” is correct on my PCB even though it looks wrong – my way of avoiding 14-conductors is using 16-pin headers and installing the key backwards, but the cable/signals are connected where they need to go.

  • Power to the ICs is all correct/measured/verified.

  • No hot chips.

  • The crystal cap is a 1% 10pF MLCC.

  • The crystal reads “3.58 mHz” on the schematic, but it is in fact a [ABRACON +/-20ppm 3.579545MHZ].

Also, as this is basically a direct clone of the C2 – I have no intention to distribute it. Just for personal use, and will send one to the OP if I can get it working.


02%20PM

23%20PM
43%20PM
55%20PM

1 Like

duh. my clipping circuit output is on the wrong side of the last diode. kludged. fixed. working. @LauLindqvist – DM me if you want a board. I’ll gladly send you one for inspiring this thread. you’ll just have to cut a trace and put in a kludge wire. also, the format is banana/BNC, but it can be adapted for 3.5mm jacks with a few wires.

3 Likes

Sorry, just looking at this. You don’t need the AD724 at all!!

  1. Buffer your video after blanking switch.
  2. Sum CSYNC (attenuated to 300mVpp) with your clipped and blanked video.
  3. Drive it thru 75R resistor at 2X gain with a final driver opamp.
2 Likes

Luckily I had a 724 already, so I’m not out $10. The circuit works and the module is installed in my case already, so I probably won’t do another one, but I’m going to post a revised schematic just for posterity.

3 Likes

So I was linked to this thread recently and have been thinking of making a similar luma encoder for a preview monitor. I am wondering what the best way to replace the 14pin sync with RCA sync and pass through. I believe lm1881 would be the chip to use but just wondering what the best way to go about this is.

1 Like

Looking at the C2 schematic (see relevant bit below), the C2 only uses the csync and blanking signals from the 14-pin sync. Csync is definitely one of the outputs of a lm1881 (and I think it’s what an RCA sync signal is in general, right?). But blanking will need to be derived somehow. I’m not sure but I think it’s the kind of thing you can get by rigging up a one-shot of a particular length (tuned via trimmer?).

C2’s 14-pin sync input:

2 Likes

yeah I found this reference about using the cd4638 but I’m not exactly sure. I think a lot of the schematic I have drawn up is overkill.

1 Like

I’m somehow a little surprised there’s no video on YouTube about this IC.