Digital Interconnects

S/PDIF specifies a Data Link Layer protocol and choice of Physical Layer specifications for carrying digital audio signals between devices and stereo components. more...

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The name stands for Sony/Philips Digital Interconnect Format, the two companies being the primary designers of the S/PDIF format. It is more recently part of a larger collection of standards IEC 60958 (often referred to as AES/EBU), where it is known as IEC 958 type II. S/PDIF is essentially a minor modification of the original AES/EBU standard for consumer use, providing small differences in the protocol and requiring less expensive hardware.

Applications

A common use for the S/PDIF interface is to carry compressed digital audio as defined by the standard IEC 61937. This mode is used to connect the output of a DVD player to a home theater receiver that supports Dolby Digital or DTS surround sound.

Hardware specifications

S/PDIF was developed from a standard used in the professional audio field, known as AES/EBU which is commonly used to interconnect professional audio equipment. S/PDIF remained identical at the protocol level, but changed the physical connectors from XLR to either electrical coaxial cable (with RCA jacks) or optical fiber (TOSLINK, i.e., EIAJ Optical), both of which cost less and are easier to use. The cable was also changed from 110 Ω balanced twisted pair to the already far more common (and therefore compatible and inexpensive) 75 Ω coaxial cable, using RCA jacks instead of the BNC connector which is common in commercial applications. S/PDIF is, for all intents and purposes, a consumer version of the AES/EBU format.

Note that there are no differences in the signals transmitted over optical or coaxial S/PDIF connectors—both carry exactly the same information. Selection of one over the other rests mainly on the availability of appropriate connectors on the chosen equipment and the preference and convenience of the user. Connections longer than 6 meters or so, or those requiring tight bends, should use coaxial cable, since the high light signal attenuation of TOSLINK cables limits its effective range. On the other hand, TOSLINK cables are not susceptible to ground loops and RF interference like coaxial cables. One deciding factor for many is cost—any standard 75 Ω A/V cable can be used for coaxial connectivity, while TOSLINK requires a specific cable which until recently was not very affordable.

Protocol specifications

S/PDIF is used to transmit digital signals of a number of formats, the most common being the 48 kHz sample rate format used in DAT, and the 44.1 kHz format used in CD audio. In order to support both systems, as well as others that might be needed, the format has no defined data rate. Instead the data is sent using Biphase mark code, which has either one or two transitions for every bit, allowing the original word clock to be extracted from the signal itself.

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