S-PDIF

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Overview

The term "S-PDIF" is an acronym for "Sony-Phillips Digital Interconnect Format" and is sometimes written as "SPDIF." The S-PDIF format was superseded by the IEC 60958 type II standard. It describes both the format of the encoding of the digital audio, clocking, and non-audio information, and the physical format (unbalanced 75 Ohm coaxial electrical or optical). The most common form of electrical connection is a coaxial 75 Ohm cable with "RCA" (or IEC or "Cinch") connectors. The most common optical connection is "Toslink;" which was developed originally by Toshiba.

History

In order to address the need for a standardized connection to carry stereo linear PCM digital audio between pieces of equipment, SONY/Phillips adopted a version of the professional AES digital audio interface standard for consumer use. As technology progressed, the connection was adapted for alternative formats employing data compression for multi-channel audio formats for video (DTS and AC-3).

Basics

In a manner analogous to analog audio cables, digital interconnects are used to carry digital audio information from one piece of equipment to another. One example would be form a CD player to a stand-alone DA converter. Another example would be from an AD converter to a computer recording system. Because two digital devices must "speak the same language" to communicate and the devices must somehow be synchronized for the receiving device to properly interpret the incoming signal; the digital information must conform to a previously approved standard format.

The S-PDIF standard was developed as a consumer-oriented solution to this problem. In addition to the Linear PCM encoded audio information, the signal contains synchronization signals and an embedded clock. Additional non-audio information referred to as "status bits" and virtual "space" for user information is also included in the digital data stream.

The S-PDIF format adopted the basic format of the professional AES3 stereo digital audio standard with some differences: 1.) The status bits are different in many instances and in some cases the same status information appears at different locations in the data stream. For example- the consumer format includes SCMS data not present in the professional format. 2.) The physical format was intended for low cost interconnection of pieces of consumer digital audio equipment located in close proximity to each other. An unbalanced electrical connection with relatively low signal voltage (1V p-p unterminated) and an optical format based on low-cost plastic fiber optic interconnects were chosen for this purpose.

Due to problems associated with unbalanced connections; the optical version of the S-PDIF interface was developed to eliminate ground-noise issues. Fiber optic technology has advanced since the S-PDIF standard was developed; so in many instances a significantly longer optical interconnect can be used in place of a unbalanced coaxial connection.

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