SPDIF is the short form of Sony Philips Digital Interconnect Format or Interface which is a data link layer protocol as well as a set of physical layer specifications for carrying digital audio signals between components as well as devices over either optical or electrical cable. The Sony Corporation and Philips are the primary designers of this interface and it is standardized in IEC 60958 as type II. The protocol has two main purpose namely to transfer compressed digital audio and carry the signal between the output of a computer or DVD player to a home theater system that is designed for Dolby Digital or DTS surround sound and also to interconnect commercial audio equipment.
This protocol is a minor modification of the original AES3 standard for consumer use that provides small differences in the protocol and also requires less expensive hardware. The physical connectors in this are usually either fiber optic or coaxial cable. This interface is a digital audio format as it is used for transporting stereo digital audio signals on CD players, DVD players, PC audio cards, car audio systems as well as other systems that transmit as well as receive stereo digital audio. Sony Philips Digital Interconnection Format two 192 bit blocks left and right channel data is divided into 12 words of 16 bits each with the first 6 bits of the first word being a control code.
This protocol does not specify a given data rate or resolution as this will be determined by the actual equipment containing it which will be accepted by both pieces of audio hardware. This protocol also utilizes the Bi-phase mark code that has either 1 or 2 transitions for each bit and this will allow the initial word clock to be directly extracted from the base signal itself. The most common data rates for this protocol are 44.1khz for the stereo CD audio and 48khz for the Digital Audio Tape.
The typical transmissions are limited to 16 bit audio and this is due to audio CD limitations with the actual supported transmission being 20 bit audio. The protocol can however be after-market adapted in order to support 24 bit audio which will be an extra 4 bits which is not directly supported by the protocol. Transmission of these protocol signals especially the ones that have bit rates of less than 20 bits will require you to sample accuracy and the unnecessary bits will be automatically be "shaved" to zero.
The data transmitted by this protocol is done in a stream of 32 bit data words and each data frame consists of 384 words in total whereby 192 data words are transmitted for the A stereo channel as well as the 192 data words transmitted for the B stereo channel. The physical connectors of these protocols has changed over the year from XLR which was for commercial and professional audio equipment to either TOSLINK which is an optical fiber and also referred to as EIAJ Optical or RCA jacks that have electrical coaxial cables. These changes have made this protocol to be far more cost efficient as well as simpler to use for most consumer applications.
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This article touched the basics of the topic. I have 2 more resources related to the above. They are spdif and spdif cable . They are worth a read.
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