Choosing Between Consumer Digital Audio Interfaces

If you must use consumer S/PDIF or AES/EBU, for example between a transport or player and a dejitter box or DAC, here's what I and many other observers consider a hierarchy in order of decreasing sound quality:
  1. S/PDIF on true 75 Ohm BNC connectors and Coaxial cable.
  2. S/PDIF on RCA connectors.
  3. AES/EBU on XLR connectors and true 110 Ohm cables.
  4. S/PDIF over Toslink plastic or glass fiber optical.
Whenever possible, use true 75 ohm BNC and coaxial cable instead of 110 ohm AES on XLR. XLRs were never intended for the kind of frequencies the digital data runs at, so BNCs will preserve the digital waveform much better. I'm using the Canare BCP-C53 BNCs with DH Labs Silver Sonic D-75 cable. It's RG6 sized and has a good foamed teflon dielectric. Avoid the fairly common 50 ohm coax and connectors as they'll cause reflections and a degraded signal compared to 75 ohm. As an aside I strongly prefer XLRs for analog audio where they are awesome. Gold pin Neutrik XLRs are nearly universal for balanced audio connections and for many good reasons.

BNCs are objectively much better than RCA connectors at the Radio Frequencies (RF) that digital signals run at. As another aside I find the RCA connector wholly unsatisfying for analog audio also. RCAs were a poor audio connector design and an even worse choice for RF connections. BNCs were designed for high quality, high frequency instrumentation and are vastly better suited to digital signals. You can pretty easily replace any RCA connectors used for digital inputs or outputs with BNCs. Be sure to use true 75 Ohm BNCs with teflon insulators and keep the leads as short as possible.

Given a choice, avoid carrying S/PDIF over Toslink. Most Toslink transceivers have much more inherent jitter than the wired Coaxial interfaces on BNC or RCA. I would not recommend using a player with only Toslink outputs as a transport. If you have a service manual or a good eye for circuits you may be able to tap off the S/PDIF before it gets to the Toslink transceiver and wire up a bulkhead mount BNC connector for S/PDIF output instead. Keep the wire short and as close as possible to the board and tap off after coupling capacitors or transformers but before output opamps.

For a comparison of interface sound quality, my Pioneer DV-414 has both coaxial and Toslink digital outputs. Sending both to an Assemblage D2D-1 outboard jitter reducer/upsampler/interpolater and DAC-3, the Toslink still had a lot worse sound quality. It was fuzzy, hashy, grainy, etc. This is with the dual PLL jitter reduction, low-jitter I2Se interface, sample rate upconversion, etc. applied to both signals. The coax is much better: clearer, smoother, more continuous and solid. Again this is even after the pretty extensive processing to clean up the signal that this gear provides. So the Toslink was doing a lot of damage to the same signal sent simultaneously from the same transport, and even a lot of digital signal cleanup could not undo the damage. The obvious conclusions are that the DAC is important, the transport also important, but the interface type is very important. A high jitter interface like Toslink can definitely harm the signal, and should be avoided if you want to get the best sound.

One advantage of Toslink or other optical fibers that one might suppose is the isolation of electrical grounds, which would prevent ground loops between equipment. Ground loops can be a carrier for noise. However since most good audio equipment uses isolation transformers on digital inputs and outputs, optical fibers are often not an advantage in preventing ground loops.

Please see also my comments on Digital Interface Formats.