S/PDIF

S/PDIF (Sony/Philips Digital Interface)[1][2] is a type of digital audio interconnect used in consumer audio equipment to output audio over reasonably short distances. The signal is transmitted over either a coaxial cable with RCA connectors or a fiber optic cable with TOSLINK connectors. S/PDIF interconnects components in home theaters and other digital high-fidelity systems.

TOSLINK connector (JIS F05)

S/PDIF is based on the AES3 interconnect standard.[3] S/PDIF can carry two channels of uncompressed PCM audio or compressed 5.1/7.1 surround sound (such as DTS audio codec); it cannot support lossless surround formats that require greater bandwidth.[4]

S/PDIF is a data link layer protocol as well as a set of physical layer specifications for carrying digital audio signals between devices and components over either optical or electrical cable. The name stands for Sony/Philips Digital Interconnect Format but is also known as Sony/Philips Digital Interface. Sony and Philips were the primary designers of S/PDIF. S/PDIF is standardized in IEC 60958 as IEC 60958 type II (IEC 958 before 1998).[5]

Applications

A common use for the S/PDIF interface is to carry compressed digital audio for surround sound as defined by the standard IEC 61937. This mode is used to connect the output of a DVD player or computer, via optical or coax, to a home theatre amplifying receiver that supports Dolby Digital or DTS. Another common use is to carry two channels of uncompressed digital audio from a CD player to an amplifying receiver.

Hardware specifications
Composite Video RCA connector (yellow)
Digital Audio Coaxial RCA connector (orange)

S/PDIF was developed at the same time as the main standard, AES3, used to interconnect professional audio equipment in the professional audio field. This resulted from the desire of the various standards committees to have at least sufficient similarities between the two interfaces to allow the use of the same, or very similar, designs for interfacing ICs.[6] S/PDIF remained nearly identical at the protocol level,[lower-alpha 1] but changed the physical connectors from XLR to either electrical coaxial cable (with RCA connectors) or optical fibre (TOSLINK; i.e., F05 or EIAJ optical), both of which cost less than the XLR connection. The RCA connectors are typically colour-coded orange to differentiate from other RCA connector uses such as composite video. The cable was also changed from 110 Ω balanced twisted pair to 75 Ω coaxial cable, using RCA jacks.

Signals transmitted over consumer-grade TOSLINK connections are identical in content to those transmitted over coaxial connectors, though TOSLINK S/PDIF commonly exhibits higher jitter.[7]

Comparison of AES3 and S/PDIF[8]
AES3 balanced AES3 unbalanced S/PDIF
Cabling 110 Ω STP 75 Ω coaxial 75 Ω coaxial or optical fibre
Connector 3-pin XLR BNC RCA or TOSLINK
Output level 2–7 V peak to peak 1.0–1.2 V peak to peak 0.5–0.6 V peak to peak
Min. input level 0.2 V 0.32 V 0.2 V
Max. distance 100 m 1000 m 10 m
Modulation Biphase mark code Biphase mark code Biphase mark code
Subcode information ASCII id. text ASCII id. text SCMS copy protection info.
Audio bit depth 24 bits 24 bits 20 bits (24 bits 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; however, not all equipment supports this, and these extra bits may be ignored.

S/PDIF protocol is identical to AES3 with one exception: the channel status bit differs in S/PDIF. Both protocols group 192 samples into an audio block, and transmit one channel status bit per sample, providing one 192-bit channel status word per channel per audio block. The meaning of the channel status word is completely different between AES3 and S/PDIF. For S/PDIF, the 192-bit status word is identical between the two channels and is divided into 12 words of 16 bits each, with the first 16 bits being a control code.

S/PDIF control word components
BitUnset (0)Set (1)
0Consumer (S/PDIF)Professional (AES3)
(changes meaning to AES3 channel status word)
1NormalCompressed data
2Copy restrictCopy permit
32 channels4 channels
4——
5No pre-emphasisPre-emphasis
6–7 Mode, defines subsequent bytes, always zero
8–14 Audio source category (general, CD-DA, DVD, etc.)
15 L-bit, original or copy (see text)

Bits 8–14 of the control code are a 7-bit category code indicating the type of source equipment, and bit 15 is the "L-bit", which (for most category codes) indicates whether copy-restricted audio is original (may be copied once) or a copy (does not allow recording again). The L-bit is only used if bit 2 is zero, meaning copy-restricted audio. The L-bit polarity depends on the category, with recording allowed if it is 1 for DVD-R and DVR-RW, but 0 for CD-R, CD-RW, and DVD. For plain CD-DA (ordinary nonrecordable CDs), the L-bit is not defined, and recording is prevented by alternating bit 2 at a rate of 4–10 Hz.

Limitations

The receiver does not control the data rate, so it must avoid bit slip by synchronizing its reception with the source clock. Many S/PDIF implementations cannot fully decouple the final signal from influence of the source or the interconnect. Specifically the process of clock recovery used to synchronize reception may produce jitter.[9][10][11] If the DAC does not have a stable clock reference then noise will be introduced into the resulting analog signal. However, receivers can implement various strategies that limit this influence.[11][12]

TOSLINK optical fiber, unlike coaxial cables, are immune to ground loops and RF interference.[13] The fiber core of TOSLINK, however, may suffer permanent damage if tightly bent.

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See also

Notes
  1. Consumer S/PDIF provides for copy protection, whereas professional interfaces do not

References
  1. "S/PDIF Information". Intel. 21 July 2017. Retrieved 3 April 2018.
  2. "S/PDIF". Retrieved 3 April 2018.
  3. "SoundSystem SixPack 5.1+ True 6 Channel + Digital In & out – Stuff Worth Knowing" (PDF). TerraTec. 5 July 2001. p. 43. Retrieved 18 January 2011.
  4. Mark Johnson; Charles Crawford; Chris Armbrust (2007). High-Definition DVD Handbook : Producing for HD-DVD and Blu-Ray Disc: Producing for HD-DVD and Blu-Ray Disc. McGraw Hill Professional. pp. 4–10. ISBN 9780071485852. ...connections such as S/PDIF do not have the bandwidth necessary to deliver uncompressed surround sound...
  5. "Sound card". kioskea.net. Kioskea Network. Retrieved 4 August 2010. The components of a sound card are: [...] An SPDIF digital output (Sony Philips Digital Interface, also known as S/PDIF or S-PDIF or IEC 958 or IEC 60958 since 1998). This is an output line that sends digitised audio data to a digital amplifier using a coaxial cable with RCA connectors at the ends.
  6. Finger, Robert A. 1992 'AES3-1992: The RevisedTwo-ChannelDigital Audio Interface', J.AudioEng.Soc.,Vol.40,No.3, 1992 March, p108
  7. "Toslink or Coax". Retrieved 15 April 2015.
  8. Dennis Bohn (2001). "Interfacing AES3 & S/PDIF" (PDF). Rane Corporation. p. 2. Retrieved 18 January 2011.
  9. Giorgio Pozzoli. "DIGITabilis: crash course on digital audio interfaces" tnt-audio.com.
  10. Chris Dunn, Malcolm J. Hawksford. "Is the AES/EBU/SPDIF Digital Audio Interface Flawed?" AES Convention 93, paper 3360.
  11. Norman Tracy. "On Jitter, the S/PDIF Standard, and Audio DACs."
  12. Lesso, Paul (2006). "A High Performance S/PDIF Receiver" (PDF). Audio Engineering Society. Archived from the original (PDF) on 4 June 2014. Cite journal requires |journal= (help) AES Convention 121, paper 6948
  13. Joseph D. Cornwall (31 December 2004). "Understanding Digital Interconnects". Audioholics.com. Retrieved 12 July 2007.
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