I2S, S/PDIF, Toslink connections

In searching for a new DAC the input choices are many. There are several ways in which inputs can be connected to a DAC and this can be compounded when an USB/SPDIF converter is inserted between computer and DAC. I wish to learn more regarding the SQ as afforded by these inputs.

Specific questions:
I am of the understanding that coaxial is superior to toslink when connecting CDP to DAC. Is this true?

What is an I2S connection and what are advantages to this option? Uses an HDMI cable i believe.

Thanks in advance for helping with my understanding.
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
I2S is also available on North Star as RJ-45, which I am using.
It certainly is the best connection.
I wish to thank all for their response. Are there no USB/SPDIF converters that output I2S to any of the aforementioned DACS?
Dunno but it is doable. I wrote a pair of articles many years back (1995-7) about how to do this. They appeared in Audio Amateur which has morphed into AudioXpress. The articles were called "Get on the Bus" parts 1 and 2. While the specific devices referred to are a bit dated, the general principles are the same.
Mesch- the Empirical Audio Off-Ramp 5 outputs both differential HDMI I2S and single-ended RJ-45 I2S. it works with W4S DAC2, PS Audio PWD, Northstar, Stello, Empirical Audio, Perpetual Technologies, Buffalo and others.

Steve N.
Empirical Audio
Steve, please consider including a coax input on the Offramp 5 so that I can buy one to put between my Squeezebox Touch and my W4S DAC2. I would really like to try feeding the DAC2 via I2S.
Cymbop - It has Coax. The OR5 has BNC coax (with RCA adapter), AES/EBU, RJ-45 I2S and HDMI differential I2S. All active at once.

The OR5 USB interface does not currently support Linux without a special driver. OR6 will be XMOS, so it will work. OR5 is upgradable.

Steve N.
Empirical Audio
did some extensive testing of the I2S connection when i got my PS Audio PWT/PWD back in 2009. all three listeners easily preferred the I2S connection....and i stress the "easily" part as it was obvious to us. we went on to compare the set-up to higher priced gear and the I2S connection held its own...besting transport/dac combo's that cost considerably more. the same was *not* true when we used rca or xlr with the PWT/PWD.

i walked away thinking I2S was the difference maker when properly implemented/executed. i still believe that today.

just my 2 cents though. your mileage could vary.
Steve, the only _input_ on the OR5 seems to be USB. And there's no USB Out from a Squeezebox Touch.
Again, I wish to thank all for their comments. When I upgrade I wish to take advantage of the I2S interface.

Steve, I must add that I very much appreciate the Empirical Audio Web page information.
Steve, Cymbob is asking for coax INPUT, not output.
Cymbop - Actually you can stream USB output from SB Touch, but it requires a USB converter with Linux driver compatibility. The HiFace2 will work.

If you wanted to reduce jitter from S/PDIF coax or Toslink, this can be done with a reclocker like the Synchro-Mesh. Unfortunately, there are none with I2S outputs .... yet.

Steve N.
Empirical Audio
Quick update on this. Reassuringly I2s is really catching on. These manufacturers (at least) offer I2S inputs or converters:

  • Pink Faun
  • Sonore
  • Wyred4Sound
  • PS Audio
  • K&K Audio
  • Audiobyte / Rockna
  • Wadia
  • Love Harmony
  • MSB Technology
  • Empirical Audio
  • Northstar Design
  • LKS Audio
  • Ayon
  • Stello
  • Audio Alchemy
  • Perpetual Audio
  • Camolot
  • Aquasound
  • Audio-GD

Way, way back I used I2S cables on Audio Alchemy gear.
AES/EBU is the best short of IP audio.
Interesting thread. I am currently connecting my Apple TV to my DAC (Cambridge Audio Azur 851 D) via toslink. I was advised that this was a better route than coax. 

I'm thinking of replacing the Apple TV with a Bluesound Node 2. Based on the opinions on this thread, it seems coax is a better alternative if ground loop noise is not a factor.