Coaxial vs. optical

Your Squeezebox has two digital outputs - a coaxial one and an optical one. Which should you use?

Coaxial digital

Coaxial digital refers to converting the digital S/PDIF signal to a voltage level and transmitting it electrically. Coaxial digital connectors are coloured orange. The cables look like conventional analog left/right RCA cables. However these cables have to have an impedance of 75 ohms.

What does that mean? It means you can use a cable specifically designed as a coaxial digital cable as it meets this specification. Analog video cables will also meet this specification, the plugs are usually coloured yellow. Regular audio cables whose connectors are usually coloured white and red may or may not work - they do not have to meet the 75-ohm criteria, although some do.

Optical digital

Optical digital refers to converting the digital S/PDIF signal to light and transmitting it through a fibre optic cable that conducts light. Specifically, the S/PDIF signal is used to modulate (vary the intensity) of a red LED. A light receiver on the other end receives the light and decodes the signal from it based on the light modulation. As long as the Squeezebox is powered there should always be a red light illuminated in the optical digital output, even when it's not playing any music. You should be able to see this light coming through the edges of the door of the Squeezebox's optical digital output, and if you insert an optical digital cable into this output, red light should come out the opposite end of the cable.

Since Toshiba developed this technology, you may see it referred to as "Toslink".

Optical digital jacks are squarish inset connectors covered by a small hinged door. The optical digital cable has a connector that pushes the door out of the way and locks in place. The end of the optical digital cable has a tiny lens that contacts the LED. Take great care not to scratch this lens! Many optical cables come with covers to protect the lens when the cable is not connected. Use these covers and do not lose them. Some covers even come attached to the cable.

Optical digital cables can be made internally out of glass or plastic. Glass is known to transmit light better and some state that this will lead to better sound. However the needs of S/PDIF are well-met by plastic cables since the demands of S/PDIF are quite modest for fibre optic technology. You may not hear any difference in your equipment. This is a subject of considerable controversy and if you feel this is important, you should do further research, ask questions or conduct listening tests.

Which is better?

Both have their advantages and disadvantages. Optical digital is more common - some equipment has more optical digital than coaxial digital inputs, and other equipment does not have any coaxial digital inputs at all.

Advantages of coaxial

• sturdier cables and connectors
• cables can be cheaper

Disadvantages of coaxial

• electrical connection can lead to ground loop problems, a low-level hum or buzz, in some situations

Advantages of optical

• no electrical connection, so no ground loop

Disadvantages of optical

• cables and connectors more fragile. Since the cable contains a plastic or glass strand, it cannot be bent very sharply. Care must also be taken not to scratch the lens at the end.
• cables can be more expensive, especially glass types

OK, but which sounds better?

Again, a subject of great debate. Theoretically coaxial digital can lead to less jitter, a digital timing error that can degrade the sound. However it all depends on the quality of the components used to convert the PCM to S/PDIF and back again, so this can hardly be considered a rule.

It's best to experiment to see which sounds better to you, with your ears, your music, your room and your equipment. You may not hear any difference at all.