The Switch is Early

There is a delay of several hundred nanoseconds (I estimate 250-300, it is probably a multiple of the 4FSC period) between the RGB inputs and the output of the AD724. But the pixel switch activates almost immediately, before the overlay pixels are rendered. This is visible as a black bar to the left of the patterns below. The left pattern should be white-black stripes, and the right pattern should be white stripes only.

SwitchOverlay_NoRC — Notice the black bar to the left of the white bar. This is because the pixel switch is faster than the pixels.

I tried to eliminate this by putting an RC delay in front of the pixel switch. It does delay the switch, but the result is disappointing.

SwitchOverlay_47pF4KR — Delaying the switch with an RC network of 47pF and 4K. The delay is gone but there is a blue “halo” in its place, and the switch cannot toggle faster than the delay.

The worst problem is that the switch can’t toggle faster than the delay, which means overlays 1 or 2 pixels wide don’t even appear. I’m not sure what is causing the blue halo, it could be because the envelope for the switch signal is no longer a sharp square edge. Compare these 2 scope traces:

Switch signal, no delay and sharp edges

Switch signal, with RC delay

Note the sloping attack and decay. The last 2 peaks didn’t reach the threshold to activate the switch.

I need to find a way to delay the rise and fall of the switch signal equally, while preserving the sharp edges, even when the switch toggle time for 1 pixel (162ns) is less than the delay (250+ns).

The good folk at Stack Overflow have made some suggestions:

Use a hex Schmitt trigger with up to 6 small RC networks, in series. One can be tunable with a variable cap.
Simply use a clock delay IC such as the DS1110.
Use one or more flip flops, clocked from the MC44144.

To which I added an idea of my own:

Delay the signal in the microcontroller. This could be done by setting up a second DMA transfer to a different GPIO port. Of that, only enable 1 pin which will be the switch. Drive the DMA from a timer with the same period as the pixel clock, but started an arbitrary number of ticks later.

Of these, I like 2 and 3 for simplicity. 3 has the advantage that the switching will be in the same clock domain as the pixels. This may turn out to be important, otherwise the switching may still be in and out of sync with the overlay image.

My own idea would be a great one (if I do say so myself), if not for the fact that I will now have 2 DMA transfers contending with the CPU for the bus. From my reading, there are is an issue with the DMA2 controller on the STM32F4 series when concurrently accessing peripherals. So adding an extra stream willy nilly is something to be avoided. The finished product will need at least 1 other DMA transfer (to receive data from a UART), or most likely more if I end up integrating it with a flight controller.

I think I will try the flip-flop approach, and it will be an added incentive to clock the microcontroller from the pixel clock, if this is possible on a Discovery board. I will also experiment with 2 DMA transfers.

Edit: André in Portugal has suggested another way: use a comparator to monitor the overlay video output. If it rises above black level, the comparator activates the switch. Thanks André! This would be a bit like the “blue screen” chroma keying used to show the weatherman in front of a computer-generated weather map back in the old days, except this would be luma keying. It would free up an extra bit (along with the other spare bit I’m not currently using) allowing for more colours. But it would mean sacrificing the ability to draw black in the overlay. Everything is a tradeoff…..

Overlay, Take 2

Over the weekend, I tracked down the problem with poor saturation. This was due to an incorrect connection to ground in my clamp circuit. After doing this I had the opposite problem: the strong overlay signal was causing the monitor’s AGC to dim the rest of the picture. I compensated for this by increasing the series resistance after the AD724 from 75 ohms to 150 ohms. This didn’t alter the brightness of the overlay but the source video is now less dim.

I also used an RC circuit to shift the phase of the subcarrier clock, in an attempt to correct the colours. I experimented with different capacitor values and discovered that 1nF and above caused the clock to disappear completely. 20pF caused a barely perceptible shift in colour, but a 10K resistor with a 470pF capacitor gave correct red, green and blue colour bars. Subjectively they appear exactly as they should, however I will experiment further to see if there is any more scope for improvement. The colours now appear solid where before there was banding, I’m not sure why this has disappeared unless it was clock jitter that the RC network somehow mitigated.

Here is the result.

RGB overlay – no fake, and full sats

Overlay trace. Note the switching artefact.

I have to say it’s looking a lot better than last week. The only issue still remaining is a switch artefact. Notice that there is a black vertical line before the red bar appears. This shows on the scope as a small bright spot just above sync level. Since there is no artefact when switching from overlay back to source video, I suspect it is the switch responding more quickly than the AD724, switching in a blank image before the colour signal has been generated. If so, I should be able to mitigate it by introducing a delay to the switch.

Overlay

Well, sort of.

In fact the real image looks worse. The colours look more saturated in the photo than they do in real life.

Sync tip, colour burst, active video, overlay. The horizontal trace looks thick due to noise.

My test setup.

It’s a colour test pattern overlay, but there are a bunch of issues:

The colours are wrong. I had to change the order of the red, green and blue inputs to get even this result, because the phase delay from the MC44144 is 60 degrees. The colours are also poorly saturated, and look worse in real life than in the picture.
The image flickers, due to noise. This is most likely due to the use of a breadboard.
The source video appears washed out. I believe this may be partly the result of clamping which seems to compress the waveform by 200mV.

On the positive side, I did solve some problems:

I’m using an SN74LVC2G53 analog switch. This one is unbuffered. Since both the AD724 and the camera are both outputting at full-drive 2V p/p, there should be no need for buffering. The switch is very fast (<10ns) and seems to be performing as advertised.
Originally the 2 video signals were mismatched by around 200mV. This was enough to cause an unstable picture, as the overlay signal dropped below sync level. Worse, it even appeared to contaminate the source video in the LM1881, causing it to lose sync.

In order to continue with this approach, I will need to solve the subcarrier phase problem, and also find out why the colours look so terrible.

Another problem that has been bugging me is that the MC44144 often starts up without generating a subcarrier. It seems worse when using the USB power supply which is very noisy, although the filter on the board reduces it to < 5mV p/p. Both the chip and the crystal came from AliExpress, could it be a quality issue?