Going low cost

I'm planning to design the system so it can be used with consumer grade cameras. Those don't have audio input for an external mic so that precludes a direct electrical connection for the audio sync signal my gizmo will output. Solution: acoustic coupling! Here's a small speaker that can be taped facing the mic and transmit syncing signals without too much environmental noise.

But some degradation will occur and the modulation scheme (encoding the 1PPS and the time of  day) should be immune to this. I did a test with (audio by definition) LTC timecode playing through a speaker into a computer mic and below you can see the catastrophic degradation the Manchester encoding incurs (bottom is the original signal, two SMPTE timecode frames are displayed: you can see the start bits at the left and in the middle):

So I'll rather use frequency shift keying, aka FSK modulation. Since the signal is an audio frequency, it's called AFSK. And, yes, that technique was used by old modems (Bell202 standard, 1980), listen!