S/PDIF

S/PDIF or S/P-DIF stands for Sony/Philips Digital Interface Format, also IEC 958 type II, part of IEC-60958. It is a collection of hardware and low-level protocol specifications for carrying PCM stereo digital audio signals between devices and stereo components.

S/PDIF is a consumer version of the standard known as AES/EBU; it provides small differences in the protocol and requires less expensive hardware.

Applications

S/PDIF is primarily used with CD players (and DVD players playing CDs), but has become common on other audio components like MiniDiscs and modern computer audio cards. 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.

Hardware specifications

The "S/PDIF" specifications allow several different types of cables and connectors which must match the device being used. Key words for the electrical type are "coaxial" and "RCA jack." The other type is called "optical" with the word "TOSLINK" often used, or less-often, EIAJ Optical. Adaptors do exist to convert the coaxial RCA Jack S/PDIF to the optical TOSLINK S/PDIF and vice versa (though most individual adaptors are one-way only). All require some sort of external power supply. The "optical" version is advantageous for applications where electrical noise or many wires are an issue such as car audio.

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 RCA jacks or optical TOSLINK, both of which cost less and are easier to use. The cable was also changed from 110 Ω (ohms) impedance 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 found in broadcast television. S/PDIF is, for all intents, a consumer version of the AES/EBU format.

**Main differences between AES / EBU and S/PDIF**
	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)

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".

The channel status bit in the S/PDIF

There is one channel status bit in each subframe, making a 192 bits word each audio block. This means that there are 192/8 = 24 bytes available each audio block. The meaning of the channel status bit is completely different between AES/EBU and the S/PDIF.

For S/PDIF, the 192 bit word is divided in 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 detail in the following table:

**Meaning of the control word in 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

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

External links

Epanorama.net: S/PDIF

References

J. Watkinson, The Art of Digital Audio, Third Edition, Focal Press, 2001

Audio engineering Sound

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