S/PDIF
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S/PDIF or S/P-DIF, for Sony/Philips Digital Interconnect Format (also IEC 958 type II, part of IEC-60958), is a system for carrying stereo digital audio signals between various devices and stereo components. S/PDIF is primarily used with CD players (and DVD players playing CDs), and it is becoming common on other audio components like MiniDiscs and modern computer audio cards. It is also popular in car audio, where the former mess of wiring can be replaced with a single fibre optic cable, which is immune to electrical noise.
S/PDIF was developed from a standard used in the professional audio field, known as AES/EBU which is commonly used in Digital Audio Tape (DAT) systems. S/PDIF remained identical at the protocol level, but changed the physical connectors from XLR to either electrical RCA jacks or optical TOSLINK, both of which cost less and are easier to use. The cable was also changed from 110Ω (ohms) impedance balanced twisted pair to the already far more common (and therefore compatible and inexpensive) 75Ω coaxial cable. S/PDIF is, for all intents, a consumer version of the AES/EBU format.
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.
The data format consists of a stream of 32-bit data words sent in blocks of 192 words for each channel of audio data. For normal stereo audio, a series of 384 words would be sent, a series of words for channel A (left), then B (right), then A again. A set of words for one sample for every channel is known as a data frame, in this case 64-bits for a set of words for A and B. A word for any particular channel in a frame is a subframe.
Within each frame multiple types of data can be sent, identified by a code in the first four bits of each word. Although this allows for up to 16 different codes, only three are defined:
- B - Start of block, data is always for channel A (left)
- M - additional data for channel A
- W - data for channel B (right)
Bits 4 to 7 are used for additional codes, also left undefined. In theory they could be used in a W word in order to specify additional channels, but this is not used in practice.
Bits 8 to 27 carry 24 bits of audio signal. If the signal source uses less than 24 bits per sample, the lower-order bits are filled with zeros. For instance, CDs use 16-bit samples, so when carrying a CD signal bits 4 to 12 are set to zero and the sample is sent in the rest of the bits.
The last four bits are various status and error checking codes. Bit 28 is the "validity" bit, if this is not set that particular word is not played. This is used by CD players to indicate that they had problems reading that sample. Bits 29 and 30 are used to send additional data at a low rate, the subcode and channel-status info. Bit 31 is a parity of the word, excluding bits 0 through 3.
The channel-status bits of any particular frame are extracted into a single 192-bit control word. Bit 0 indicates 2 or 4 channel audio, bit 2, if set, stops copying of the signal, and bits 9 to 15 are used to indicate the data type. The rest are currently unused.
Subcode is used to carry data for the SCMS system, indicating if the signal is an original or a copy. This is used by systems to control copying.
S/PDIF is defined by the international standard IEC 60958-3.
Another 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.
S/PDIF is covered by the following patent: EP000000811295B1. It can be downloaded here: http://depatisnet.dpma.de
See also: word clock