Which Digital Audio Cable, Coaxial Or Toslink?
SPDIF Interconnects can be Coaxial Electrical Cable or Optical Fibre
May 18, 2009
Digital connections minimise cables and make wiring A/V sound simpler and easier. Instead of the two cables (for stereo) or even five cables (for surround) that would be needed for analogue audio connections there is just one cable, with no risk of getting channels swapped.
Choosing the Optical or Coaxial Digital Audio Interface
Many digital audio sources offer electrical and optical connections. An optical output cannot be directly connected to a coaxial (electrical) input or vice versa. The choice of which one to use is often made by what the audio destination accepts. If it takes only a coaxial electrical connection then coaxial is the only option, likewise if it only takes a Toslink optical input use an optical cable.
If an optical output has to drive an electrical input or vice versa, then optical/electrical and electrical/optical converter boxes are available, however they add complication to an A/V setup and need ther own mains power supply.
Do Optical or Electrical Connections Sound different?
Each type of connection can reliably transfer the digital data from source to receiver, so there is no good reason to expect a sonic difference. However, optical connections tend to have a lower bandwidth, which can smear data transitions, making it a little bit harder for the receiving device to recover the digital clock from the signal.
This can result in more digital clock jitter in the receiver, which can theoretically impair the accuracy of the output of a digital to analogue converter. Whether or not this has a sonic impact depends on the implementation of the receiving device. It shouldn't make any difference to the sound, but some audiophiles assert that there are slight sonic differences between optical and electrical connections. Those who feel this is an issue will do their own listening tests - the cost of the cables is not great!
Consider Optical Toslink When Interfacing to PCs
Toslink was designed to electrically isolate sender and receiver and eliminate ground loops. Computers are notorious for generating a lot of electrical noise, so there is something to be said for preferring Toslink to interface to a PC. The distance over which Toslink can operate is quite restricted, one reason is the shocking loss in the cheap all-plastic fibre (APF) commonly used. Toshiba cite a maximal length of 40m for their Toslink 190 modules using APF. This can be pushed out to 1000m with plastic coated fibre (PCF) which has a silica core with lower losses than APF. However, it is more delicate and is not common in the consumer market, so a maximum unbroken APF distance of 40m is the practical limit on Toslink connections. The issue of clock jitter does not always affect PCs - for instance if the signal is being used to burn a CD then the clock jitter on the SPDIF digital receiver is irrelevant as long as the signal is correctly received which is almost always the case.
Optical and Electrical Digital Interface Connectors
Electrical digital interfaces are usually presented on phono (RCA jack) connectors, or rarely as a BNC connector. Cable with a 75 ohm impedance should be used for best results on longer runs. In practice for the typical 1.5m cable length a regular phono analogue cable usually works fine.
Optical connectors are usually a square connector moulded onto the cable. Unlike electrical connectors which can be resoldered and changed on a cable at home, optical connectors and cables need to be purchased ready-made.
There is an alternative type of optical connector which goes into a 3.5mm jack socket for portable audio devices like minidisc players. Adaptors from this to regular Toslink connectors are available, though it is better to use an optical cable with Toslink plug one end and the 3.5mm optical plug at the other. Optical cable is less flexible than coaxial cable and should not be kinked or bent tightly round corners.
In general, where there is a free choice, coaxial digital interconnects are cheaper and easier to use within an A/V stack, but optical connections have their place for eliminating ground loops to PCs and in more extended systems like home studios where ground loops are harder to avoid.
References
Fiber Optic Devices Toslink Product Guide, 2002, Toshiba Japan
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