Regular S/PDIF COAX digital cable is actually not a bad system, though using BNC connectors is best. Only a true i2s digital connection is better because the DAC/source does not have to encode the stereo digital signals into a single wire signal. That is the S/PDIF method.Keep in mind that i2s connections will typically use an HDMI cable. This does not mean it’s an HDMI interface. It just uses the cable/connectors as the physical medium for transmitting the i2s signals (which are completely different from normal HDMI audio/video).
I2S is not necessarily a panacea either, although its a good start.
Most of my products have I2S interfaces, both SE and differential (HDMI connector), so I know how they behave.
Galvanic isolation of I2S is a non-starter because it adds too much jitter. Because S/PDIF is a zero-crossing signal, at least it’s easy to add a high-quality pulse transformer and get isolation.
I have compare with my own DAC the difference between S/PDIF input (on BNC) to I2S input (on RJ-45). The difference is actually really small, almost audibly undetectable. This is partly because I use a AK4114 receiver, which reduces S/PDIF jitter significantly.
As for differential I2S, the problem there IMO is the LVDS driver and receiver that add jitter. I think the SE I2S on RJ-45 is better.
The other advantage of S/PDIF over I2S is that a world-class cable for SPDIF is a lot less expensive than a world-class cable for I2S.
Steve N.
Empirical Audio