AV1 can accept composite sync, luma, or composite video as sync, but must be 75Ohm-terminated–sending AV1 TTL sync will likely blow something.
AV3 requires TTL CSync or HV sync–it will not accept composite or luma, nor 75Ohm-terminated signaling.
…and yet it will accept standard YPbPr and RGsB without TTL sync? I’ll make a note of that. Thanks again. Luckily, I still have AV1 generally unused for anything I encounter that happens to need RGBS, sync on luma, or composite video sync.