HDMI Frame Sync signal?

[lutzray]

Is the VSYNC signal (out of a VGA adapter) an OK way to measure and analyze the frame rate and clock output of a prosumer video camera spitting out HDMI?

 

My goal is to determine if the TC OUT of cameras is generated by the same circuitry that reads out the image sensor... if not, an easily detected drift will occur between the two clocks (and that would be a shame: I know #timecode is not genlock, but sapristi, c'est supposé être un signal de synchro).

 

Surely the DACs inside this adapter introduce some lag, but it should be constant and therefore won't produce any phase roll between VSYNC and TC.

[lutzray]

Short answer from my mastodon cross post: NO😒 

 

There is no direct match between VGA VSYNC frequencies and frame rates used in videography 😕

 

I dug a little and learned:

A- there's a lot of handshake when HDMI connections are established and

B- the transmission standard used in HDMI is asynchronous and it's the sink device job to temporize its received buffered frames.

 

Nonetheless, I think an average (but exact) framerate can be calculated "simply" by counting frames over a long period (at least so counts N and N-1 are indistinguishable) and divide by a precisely timed interval (obtained from orbiting atomic clocks 😏).