Hi Rudi,
The reason that I²S is very popular amongst some audio products is that it is the standard protocol for pretty much all audio chips (off the shelf DAC chips, filters, asynchronous sample rate converters, SPDIF receivers, CD transports etc.). It has been around for a very long time, there’s only a few wires, and you essentially just wire everything together, making it very easy to implement.
The catch is that I²S has been designed to connect chips together on a board, meaning with PCB tracks, that are carefully routed so the delays and impedance are carefully matched. When that signal is sent over, let’s say, an HDMI connector and cable, the first thing to consider is the skew of the signals. Because there are multiple wires, with one of the wires providing the clock for another wire’s data, any mismatch will cause horrible crackling as the data is decoded incorrectly.
Then, there is an issue with impedance mismatches causing reflections in the cable – and because the clock is generally high speed, often just moving the cable can cause issues, ranging from increasing jitter to (again) bad crackling. This is why HDMI is typically used as the connector/cable of choice. The HDMI standard runs at very high rates, so the tolerances on HDMI cables are generally pretty tight. If you add to this the fact that it’s a raw audio interface (no other useful data like with S/PDIF), changes to the playback chain such as switching sample rates are going to cause bangs through the system.
In the real world, I²S over HDMI will probably have higher jitter than a correctly implemented S/PDIF connection - because the source is transmitting a raw clock over a long way, and because it is just a raw audio signal you can’t use a PLL to filter out any interface jitter.
The reason S/PDIF was invented was because I²S is unsuitable for transmitting audio over any distance – HDMI coming along has helped I²S work for some companies, but that doesn’t alter the fact it’s not a format that we deem as appropriate to use in this way, especially considering formats like S/PDIF exist specifically for this purpose.