Originally Posted by Dpaws
PS Audio use an i2s feed over an HDMI cable. I am sure there are many other examples but none have been adopted universally.
Agree with everything you wrote. Sony, Tag Mclaren and Arcam also had proprietary digital audio interfaces that got it right, but none were standardised. There are a few other interfaces that achieve something similar though.

Sony i-link used flow control on a firewire connection to achieve what amounts to a DAC-master config, though its not synchronous (not sure if that really matters).

HDMI has a similar cabability confusingly called ARC (audio rate control: not audio return channel!) but only Ayre has implemented that properly (DX-5), while Sony & Pioneer have introduced proprietary implementations (idiots).

USB has an asynchronous mode that is becoming increasingly popular (google Gordon Rankin streamlength - what a hero).

Nick