Genlock Analog Video

Question

I am currently converting an TMDS signal into composite. Please see block diagram. The video encoder outputs the H-Sync and V-sync. I am also getting an external sync source that I need to use to Genlock the composite video that I am outputting. My questions are:

1. Should I just feed in the external H-Sync and V-Sync into the DAC ADV7393 and ignore the sync coming out of the ADV7610? If not why?
2. Is there an IC that can help me do what I want to do? If not do I need to change the encoder or DAC ICs that have genlock capabilities?

Answer 1

Should I just feed in the external H-Sync and V-Sync into the DAC ADV7393 and ignore the sync coming out of the ADV7610? If not why?

In the current circuit: no. The sync signal would have to be somehow supplied to the TMDS source, so that it would generate genlocked TMDS. Only then you could discard the sync provided by the TMDS-to-composite converter, assuming the converter has a minimal group delay.

Is there an IC that can help me do what I want to do? If not do I need to change the encoder or DAC ICs that have genlock capabilities?

By the time it reaches the DAC, it's too late more or less.

If the TMDS source cannot be genlocked - as would be the right way to do it - then the encoder would need to have a frame buffer.

And then it would just be the output that's genlocked, not the digital source. The encoder would be outputting the most recent frame, so sometimes a TMDS frame would get dropped, and sometimes it would be duplicated. That would depend on the relative phase of the TMDS vsync and the genlock vsync.

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So, my strategy would be:

1. Look into genlocking the TMDS source; or
2. Get an encoder that has a framebuffer and can re-time the signals to an external sync; or
3. Add a two-port FIFO between the output from the encoder and the DAC. Read out the FIFO based on the pixel clock recovered from the genlock source. To protect against underruns, the FIFO would need a loopback from output to input that's enabled when the genlock VSYNC is very early in the source frame (say the first 1-2% of the frame period). That way the frame would be duplicated.

Answer 2

1. No of course not, it will not work because the two video sources have different clocks that have arbitrary frequency and phase relation to each other so if you use pixel clock and data from TMDS you also need H and V syncs that are related to TMDS data.

2. If you can't generate the pixel clock for the TMDS source from the external syncs you want to use, then the only thing left to mix two arbitrary video signals together is a video processor with a frame buffer. You feed in the TMDS video with the clocks and syncs it has into frame buffer, and then you use the external syncs to clock the video out from the frame buffer. You do need the pixel clock for the output too and that may be difficult to achieve stably with a PLL.