I2S, S/PDIF, Toslink connections

The only company that uses I2S over HDMI is PS Audio.

Most I2S is not over HDMI (except as Kr4 pointed out, for PS Audio), but there are several different I2S interfaces. The difference between the I2S and the common s/pdif is that the I2S includes the word clock communication, allowing a DAC to sync with a transport, minimizing jitter.

Coax (s/pdif) is superior to toslink (optical) in almost all applications (due to lower jitter amongst other things). AT&T is superior to s/pdif in most applications...

USB... can be a good interface, but many times, not very well executed. HDMI, potentially a very good interface, but contains all kinds of copy right protection which can screw up the sound...

I2S is also used in the Wyred4sound dac2.

That's 2. Not as many as the original I2S and not standard HDMI either so that's a consideration.

Given that the system does not have significant ground-loop noise, the coax interface will outperform the Toslink, If you use a decent coax cable that is. The Optical conversions add jitter. If the system has a lot of ground-loop noise, the Toslink will be better simply because it breaks the ground loop.

I2S is the native interface to most D/A chips. Because there is no need to encode the clock and data into a single signal, this makes it superior technically to S/PDIF coax. Termination and handling of the transmission-lines of I2S is non-trivial however and many of the early implementations were not very good. I2S consists of 4 signal lines: MCLK, SDATA, L/RCLK and SCLK (aka BITCLK). The HDMI connector used with the differential version of I2S was chosen by PSAudio and adopted by Wyred 4 Sound and Empirical Audio, who also uses RJ-45. I2S has been used in single ended version with DIN and RJ-45 connectors for about 10 years. The earliest was Audio Alchemy. Differential version has 8 signal lines and ground. Single ended has 4 signal lines and 4 return lines (grounds).

Steve N.
Empirical Audio