New Board Designs

New design is a timing board, which extracts all the signals needed for genlock but does not generate any video. There will also be a fast video switch on a separate board using a MAX4313 which is both a buffer and a switch. Switching time is around 40ns, much slower than the TI switch I was planning to use. But this may still be OK and does not require a negative supply. I will still make the negative supply board though and if the MAX4313 is not suitable I can make another board with the TI switch.

This is a modular approach. I intend to try generating the CVBS from the uC, but if this does not work out I can make another board with an AD724 as originally planned.

Circuit schematics are complete, currently working on the routing.

Assuming 52uS of analog video, 320*240 resolution means each pixel lasts 162.5nS. This means the MAX4313 which takes 40nS to switch, will take around 1/4 of a pixel when switching.

Video timing board design is uploaded to Github:

Testing the Clamp Output

Connecting my video monitor to the clamp’s output results in a distorted signal (as shown on the scope) and nothing visible on the monitor. But without the monitor, the signal waveform looks normal.

With a 0.1uF capacitor, the black level and porches look “tilted”, with the left end at a slightly lower voltage than the right.

With a 4.7uF capacitor, the tilt is no longer visible. Same with a 220uF cap. But still no steady video on the monitor.

Putting a 75 ohm terminating resistor in the circuit as I would have in a real circuit, halves the amplitude of the video signal. I’m guessing 0.1uF / 75 ohms as the input stage to a 2* op amp as I plan to use would be OK.

Seems a CVBS waveform can pass through a capacitor OK, but not with enough power to drive a monitor. It needs buffering.

Clamping and Measuring

With the AD724, I measured 0mV at black level, -200mV at sync tip (using a Uzebox I have lying around). With the gameloader screen active, I got -100mV at sync tip and still 0V black. Noticed some movement up and down.

With my small PAL FPV camera, I measured around 180mV at black level, 50mV at sync tip.

With my Runcam action camera, I measured 20mv at sync tip, 180mv at black level.

In conclusion, everything I have, generates CVBS with a different DC offset. TV monitors do not care. They adapt to whatever the offset is. But if I am going to mix signals, they will both need to be clamped to a common DC offset. 0V sync tip should be fine. So I will try to make a clamp circuit using just a diode and capacitor.

Made up a clamp circuit on a breadboard using a BAT48 schottky diode and a 0.1uF ceramic cap. Observed that it did bring the sync tips to the same voltage for my 2 cameras, and that unclamped each camera’s signal had a different DC offset. However the sync tips were not at exactly 0 volts as expected but appeared to be 20-40mV below zero. This must be due to the diode voltage drop.

Now I am not sure what value capacitor to use in a real system. Many video circuits just use 0.1uF. But according to what I read, at least 220uF is needed to pass the full video bandwidth.

Revisiting the AD724

Made a design for an OSD board using a genlocked AD724. When I attempted this previously, one issue seemed to be mismatched DC levels for the input video and the OSD video. To get them matched, both signals will need to be clamped. Normally video signals that are AC coupled are clamped to 0V for black level, which means sync tips are -0.3V. So I need a circuit to do that.

Design Note DN327 from Linear Technology has a clamp circuit using a diode and a capacitor-resistor network. Not sure what DC level it actually clamps to, it is probably optimized for the amplifier in the circuit which would have sync tips at >= 0V.
Wondering if this actually matters? Assuming the next stage down from the OSD (the monitor, or the vtx) is going to be AC-coupled, the DC level is going to be stripped out anyway. As long as the black level of both signals is equal it is probably OK.

If I do clamp to a DC offset with no negative excursion, maybe I won’t need a negative supply for the amplifier and pixel switch either.

My plan for the time being is to experiment with clamp circuits before continuing with the OSD board design.