Transceiver passthrough in Cyclone V

Hi Sebastian,

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I'm afraid that will not work.

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You're right, but it also might work just fine.

If the input source is SDI video and the output destination is SDI video, or the output destinations are all SDI video, eg., multiple displays with the same images, then the SDI reference clock frequencies will all be similar within some parts-per-million tolerance.

I'm not familiar with SDI, but I suspect there are likely points in time where you can add or drop an entire frame without anyone really noticing. If that is the case, then your FIFO logic would simply need to be designed to store one or more frames. The FIFO flags can be used to determine when a full frame is present in the buffer, and when that is true, you start transmitting it over SDI. If once the frame is transmitted the FIFO does not have a full frame, a blank frame could be sent, or perhaps the duplicate of the last frame (in which case you'd need a save-the-last-frame FIFO).

Ultimately because you need to transfer complete frames between sources and destinations that have different clocks, you need to consider your options. In practice if you do the math for the ppm differences in clocks, you may find that the accumulated error takes many hours to accumulate a one frame difference. If that is the case, duplicating or dropping a frame would likely never be noticed. If your logic could detect the black frames between transitions, then you could simple send an extra black frame, or drop one of those frames.

I've not played with video for a long time. I'm sure there are common techniques similar to what I am suggesting.

The bottom-line is that unless you can control all the reference clocks to all the transceiver blocks and displays, then you cannot create a truely synchronous system.

Of course, if you are building your own mega-wall-display and you have the option to define the clocking, then go ahead and make the clocks synchronous! I think solving the asynchronous clocking scheme would make the system much more scaleable.

Cheers,

Dave