S/PDIF
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S/PDIF is a collection of hardware and low-level protocol specifications for carrying digital audio signals between devices and stereo components. The name stands for Sony/Philips Digital Interface Format, the two companies being the primary designers of the S/PDIFSignaling 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.
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[edit] Applications
S/PDIF is primarily used with CD players (and DVD players playing CDs), but has become common on other audio components like MiniDiscs, modern computer audio cards and especially European Digital Video Broadcast receivers (both satellite and terrestrial). It is also popular in car audio, where a large amount of wiring can be replaced with a single fiber optic cable, which is immune to electrical noise.
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.
[edit] Hardware specifications
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 and for transmissions in professional studio recording. 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 aka 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, a consumer version of the AES/EBU format.
| AES/EBU | S/PDIF | |
|---|---|---|
| Cabling | 110 ohm shielded TP or 75 ohm coaxial | 75 ohm coaxial or fiber |
| Connector | 3-pin XLR, 25-Pin D-subminiature, BNC | RCA, BNC, or Toslink |
| Signal level | 3 to 10 V | 0.5 to 1 V |
| Modulation | Biphase Mark Code | Biphase Mark Code |
| Subcode information | ASCII ID text | SCMS copy protection info |
| Max. Resolution | 24 bits | 20 bits (24 bit optional) |
[edit] 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.
S/PDIF is meant to be used for transmitting 20 bit audio data streams plus other related information. To transmit sources with less than 20 bits of sample accuracy, the superfluous bits will be set to zero. S/PDIF can also transport 24 bit samples by way of four extra bits, but not all equipment supports this, and might ignore these extra bits.
Since the low-level protocol is almost the same, it is described in the AES/EBU article. The only difference is in the "Channel status bit".
[edit] The channel status bit in the S/PDIF
| bit num. | if not set means: | if set means: |
|---|---|---|
| 0 | consumer | professional |
| 1 | normal | compressed data |
| 2 | copy prohibit | copy permit |
| 3 | 2 channels | 4 channels |
| 4 | - | - |
| 5 | no pre-emphasis | pre-emphasis |
There is one channel status bit in each subframe, making 192 bits in each audio block. This means that there are 192/8 = 24 bytes available in each audio block. The meaning of the channel status bits are completely different between AES/EBU and the S/PDIF.
For S/PDIF, the 192 bit block is divided into 12 words of 16 bits each. The first 6 bits of the first word are a control code. The meaning of its bits are shown in the accompanying table.
[edit] Other information
S/PDIF is defined by the international standard IEC 60958-3.
S/PDIF is covered by the following patent: EP000000811295B1. The patent information can be downloaded here: http://depatisnet.dpma.de
[edit] See also
[edit] External links
[edit] External pictures
The previous two pictures were originally viewed at PCMag's encyclopedia
