View source for S/PDIF

{{short description|Standardized digital audio interface}}
{{Use dmy dates|date=August 2020}}
{{Use Canadian English|date=December 2011}}

[[File:Digital-audio-interfaces-usb-spdif-toslink.jpg|thumb|S/PDIF and [[TOSLINK]] connectors on a piece of audio equipment]]

'''S/PDIF''' ('''Sony/Philips Digital Interface''')<ref>{{cite web|url=https://www.intel.in/content/www/in/en/support/articles/000006034/boards-and-kits/desktop-boards.html |publisher=[[Intel]] |title=S/PDIF Information  |access-date=3 April 2018|date=21 July 2017}}</ref><ref>{{cite web|url=https://www.pcmag.com/encyclopedia/term/50760/s-pdif |title=S/PDIF |access-date=3 April 2018}}</ref> is a type of [[digital audio interface]] used in consumer audio equipment to output audio over relatively short distances. The signal is transmitted over either a [[coaxial cable]] using [[RCA connector|RCA]] or [[BNC connector|BNC]] connectors, or a [[fibre-optic cable]] using [[TOSLINK]] connectors. S/PDIF interconnects components in [[home theater]]s and other digital [[high-fidelity]] systems.

S/PDIF is based on the [[AES3]] interconnect [[Technical standard|standard]].<ref>{{cite web|url=http://terratec.ultron.info/Audio/SiXPack5.1+/Manual/SiXPack5.1+_Manual_GB.pdf |publisher=[[TerraTec]]|title=SoundSystem SixPack 5.1+ True 6 Channel + Digital In & out&nbsp;– Stuff Worth Knowing|page=43|access-date=18 January 2011|date=5 July 2001}}</ref> S/PDIF can carry two channels of uncompressed [[PCM audio]] or [[Audio compression (data)|compressed]] [[5.1 surround sound]]; it cannot support lossless surround formats that require greater [[bandwidth (computing)|bandwidth]].<ref>{{cite book |title=High-Definition DVD Handbook : Producing for HD-DVD and Blu-Ray Disc: Producing for HD-DVD and Blu-Ray Disc |author1=Mark Johnson |author2=Charles Crawford |author3=Chris Armbrust |publisher=McGraw Hill Professional |date=2007 |isbn=9780071485852 |pages=[https://archive.org/details/highdefinitiondv0000john/page/4 4–10] |quote=...connections such as S/PDIF do not have the bandwidth necessary to deliver uncompressed surround sound... |url-access=registration |url=https://archive.org/details/highdefinitiondv0000john/page/4 }}</ref>

S/PDIF is a [[data link layer]] protocol as well as a set of [[physical layer]] specifications for carrying digital audio signals 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).<ref>
{{cite web
| url         = http://en.kioskea.net/contents/pc/carte-son.php3
| title       = Sound card
| work        = kioskea.net
| publisher   = Kioskea Network
| access-date  = 4 August 2010
| quote       = 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.
}}</ref>

==Applications==
A common use is to carry two channels of uncompressed digital audio from a CD player to an amplifying receiver.

The S/PDIF interface is also used to carry [[Data compression#Audio|compressed]] digital audio for [[surround sound]] as defined by the '''IEC 61937''' standard. This mode is used to connect the output of a [[Blu-ray]], [[DVD player]] or computer, via optical or coax, to a [[home theatre]] amplifying receiver that supports [[Dolby Digital]] or [[DTS Digital Surround]] decoding.

==Hardware specifications==
[[File:Digital coaxial audio cable (orange).jpg|thumb|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 stakeholders to have at least sufficient similarities between the two interfaces to allow the use of the same, or very similar, designs for interfacing [[Integrated circuit|IC]]s.<ref>Finger, Robert A. 1992 'AES3-1992: The RevisedTwo-ChannelDigital Audio Interface', J.AudioEng.Soc., Vol.40, No. 3, 1992 March, p108</ref> S/PDIF is nearly identical at the [[Communications protocol|protocol]] level,{{efn|Consumer S/PDIF supports the [[Serial Copy Management System]], whereas professional interfaces do not.}} but uses either [[coaxial cable]] (with [[RCA connector]]s) or [[optical fibre]] ([[TOSLINK]]; i.e., JIS F05 or EIAJ optical), both of which cost less than the [[XLR connector|XLR connection]] used by AES3. The RCA connectors are typically colour-coded orange to differentiate from other RCA connector uses such as [[composite video]]. S/PDIF uses 75&nbsp;Ω coaxial cable while AES3 uses 110&nbsp;[[Ohm (unit)|Ω]] [[balanced]] [[twisted pair]].

Signals transmitted over consumer-grade TOSLINK connections are identical in content to those transmitted over coaxial connectors. Optical provides electrical isolation that can help address [[Ground loop (electricity)|ground loop]] issues in systems. The electrical connection can be more robust and supports longer connections.<ref>{{cite web |url=https://blog.teufelaudio.com/spdif-connections-get-connected-not-confused/ |title=SPDIF connections: Get connected, not confused |access-date=2024-05-15}}</ref>

{| class="wikitable"
|+ Comparison of AES3 and S/PDIF<ref>{{cite web |url=http://www.nu9n.com/images/AES3-SPDIF-notes.pdf |access-date=18 January 2011 |title=Interfacing AES3 & S/PDIF |page=2 |year=2001 |author=Dennis Bohn |publisher=[[Rane Corporation]]}}</ref>
|-
! rowspan=2 |
! colspan=2 | AES3
! colspan=2 | S/PDIF
|-
! Balanced
! Unbalanced
! Copper
! Optical
|-
! Cabling
| 110&nbsp;Ω STP
| 75&nbsp;Ω coaxial
| 75&nbsp;Ω coaxial
| [[Optical fibre]]
|-
! Connector
| 3-pin [[XLR connector|XLR]]
| [[BNC connector|BNC]]
| [[RCA connector|RCA]] or [[BNC connector|BNC]]
| [[TOSLINK]]
|-
! Output level
| 2–7&nbsp;V peak to peak
| 1.0–1.2&nbsp;V peak to peak
| 0.5–0.6&nbsp;V peak to peak
| {{N/A}}
|-
! Min. input level
| 0.2&nbsp;V
| 0.32&nbsp;V
| 0.2&nbsp;V
| {{N/A}}
|-
! Max. distance
| 1000&nbsp;m
| 100&nbsp;m
| colspan=2| 10&nbsp;m
|-
! Modulation
| colspan=4| [[Biphase mark code]]
|-
! Subcode information
| colspan=2| [[ASCII]] id. text
| colspan=2| [[Serial Copy Management System|SCMS]] copy protection info.
|-
! [[Audio bit depth]]
| colspan=2| 24 bits
| colspan=2| 20 bits (24 bits, optionally){{citation needed|reason=Not addressed in above ref|date=September 2022}}
|}

==Protocol specifications==
S/PDIF is used to transmit digital signals in a number of formats, the most common being the 48&nbsp;kHz [[sample rate]] format (used in [[Digital Audio Tape]] and [[DVD]]s) and the 44.1&nbsp;kHz format, used in [[CD audio]]. In order to support both sample rates, as well as others that might be needed, the format has no defined [[bit 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 protocol differs from [[AES3]] only in the channel status bits; see {{section link|AES3|Protocol}} for the high-level view. 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. For S/PDIF, the 192-bit status word is identical between the two channels and is divided into 12 [[Word (data type)|words]] of 16 bits each, with the first 16 bits being a control code.

{| class="wikitable"
|+ S/PDIF control word components<ref>{{webarchive |url=https://web.archive.org/web/20190228070051/https://www.av-iq.com/avcat/images/documents/pdfs/digaudiochannelstatusbits.pdf/ |title=Understanding/Analyzing Digital Audio Channel Status Bits}}</ref>
|-
! Byte
! Bit 
!width=45%| Unset (0)      
!width=45%| Set (1)
|-
! rowspan=7 | 0
! 0 
| Consumer (S/PDIF) 
| Professional (AES3)<br />(changes meaning to [[AES3#Channel status word|AES3 channel status word]])
|-
! 1 
| Normal PCM   
| Compressed data
|-
! 2 
| Copy restrict     
| Copy permit
|-
! 3
| 2 channels        
| 4 channels
|-
! 4 
| —              
| —
|-
! 5 
| No pre-emphasis   
| [[Pre-emphasis]] 50/15
|-
! {{nowrap|6–7}}
| colspan="2" | Mode, defines subsequent bytes; values other than zero are undefined.
|-
! rowspan=2 | 1
! 0–6
| colspan="2" | Audio source category indicating the type of source equipment (general, CD-DA, DVD, etc.)
|-
! 7
| colspan="2" | L-bit, original or copy{{efn-ua|(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 DVD-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&nbsp;Hz.}}
|-
! rowspan=2 | 2
! 0–3
| colspan="2" | Source number
|-
! 4–7
| colspan="2" | Channel number
|-
! rowspan=3 | 3
! 0–3
| colspan="2" | Sampling frequency: {{n-ary|0000|2}}: 44.1&nbsp;kHz, {{n-ary|0100|2}}: 48&nbsp;kHz, {{n-ary|1100|2}}: 32&nbsp;kHz
|-
! 4–5
| colspan="2" | Clock accuracy: {{n-ary|10|2}}: 50ppm,{{n-ary|00|2}}: 1100ppm, {{n-ary|01|2}}: variable pitch (requires compatible receiver)
|-
! 6–7
| colspan="2"   {{n/a|Undefined}}
|-
! rowspan=3 | 4 
! 0
| Word length 20 bits
| Word length 24 bits
|-
! 1–3
| colspan="2" | Sample length (0: undefined, 1–4: word length minus 1-4 bits, 5: full word length)
|-
! 4–7
| colspan="2"   {{n/a|Undefined}}
|-
! 5–10
! 0-7
| rowspan=2 colspan="2" | EAN-13 code (possibly in binary-coded decimal)
|-
! rowspan=2 | 11
! 0-3
|-
! 4–7
| colspan="2"   {{n/a|Undefined; padding on 13-digit EAN code}}
|-
! 12–13
! 0-7
| rowspan=2 colspan="2"   {{n/a|Undefined}}
|-
! rowspan=2 | 14
! 0–3
|-
! 4-7
| rowspan=2 colspan="2" | ISRC (encoding unclear; ISRC is 2 alphabetic, 3 alphanumeric and 7 numeric, which is 26<sup>2</sup> × 36<sup>3</sup> × 10<sup>7</sup> ≈ 2<sup>48.164</sup> and so obviously fits into 7.5 bytes, but a naive 5 ASCII + 7 BCD would be 8.5 bytes)
|-
! 15–21 
! 0–7
|-
! {{nowrap|22–23}}
! 0–7
| colspan="2"   {{n/a|Undefined}}
|}
{{Notelist-ua}}

=== Data framing ===

S/PDIF is meant to be used for transmitting 20-bit audio data streams plus other related information. 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.

To transmit sources with less than 20 bits of sample accuracy, the superfluous bits will be set to zero, and the 4:1&ndash;3 bits (sample length) are set accordingly.

==== IEC 61937 encapsulation ====
IEC 61937 defines a way to transmit compressed, multi-channel data over S/PDIF.<ref>Digitalton - Schnittstelle für nichtlinear-PCM-codierte Audio-Bitströme unter Verwendung von IEC 60958 - Teil 1: Allgemeines (IEC 61937-1:2007 + A1:2011); Deutsche Fassung EN 61937-1:2007 + A1:2011</ref>

* The control word bit 0:1 is set to indicate the presence of non-linear-PCM data.
* The sample rate is set to maintain the needed symbol (data) rate. The symbol rate is usually 64 times the sample rate.
* Data is packed into blocks. Each data block is given a IEC 61937 preamble, containing two 16-bit sync words and indicating the state and identity (type, validity, bitstream number, length) of encapsulated data present. Padding is added to match full block size as required by timing.

A number of encodings are available over IEC 61937, including Dolby [[Dolby Digital|AC-3]]/[[E-AC-3]], [[Dolby TrueHD]], MP3, AAC, [[ATRAC]], [[DTS Coherent Acoustics|DTS]], and [[WMA Pro]].<ref>{{cite web |title=FFmpeg: libavformat/spdif.h File Reference |url=https://ffmpeg.org/doxygen/trunk/spdif_8h.html |website=ffmpeg.org}}</ref><ref>{{cite web |title=Representing Formats for IEC 61937 Transmissions - Win32 apps |url=https://learn.microsoft.com/en-us/windows/win32/coreaudio/representing-formats-for-iec-61937-transmissions |website=learn.microsoft.com |language=en-us |date=15 May 2023}}</ref>

==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]].<ref name="pozzoli">Giorgio Pozzoli. "[http://www.tnt-audio.com/clinica/diginterf1_e.html DIGITabilis: crash course on digital audio interfaces]" tnt-audio.com.</ref><ref name="dunn_hawksford">Chris Dunn, Malcolm J. Hawksford. "[http://www.aes.org/e-lib/browse.cfm?elib=6773 Is the AES/EBU/SPDIF Digital Audio Interface Flawed?]" AES Convention 93, paper 3360.</ref><ref name="tracy">{{cite web|archive-url=https://web.archive.org/web/20170701142216/http://www.audiocraftersguild.com/AandE/npt.on.jitter2.htm|archive-date=1 July 2017|first=Norman|last=Tracy|url=http://www.audiocraftersguild.com/AandE/npt.on.jitter2.htm|title=On Jitter, the S/PDIF Standard, and Audio DACs.}}</ref> If the [[digital-to-analog converter|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.<ref name="tracy" /><ref name="Lesso">{{cite journal|first=Paul|last=Lesso|url=http://www.wolfsonmicro.com/documents/uploads/misc/en/A_high_performance_SPDIF_receiver_Oct_2006.pdf|archive-url=https://web.archive.org/web/20140604060307/http://www.wolfsonmicro.com/documents/uploads/misc/en/A_high_performance_SPDIF_receiver_Oct_2006.pdf|archive-date=4 June 2014|title=A High Performance S/PDIF Receiver|publisher=Audio Engineering Society|year=2006}} AES Convention 121, paper 6948</ref>

==See also==
* [[ADAT Lightpipe]]
* [[I2S|I<sup>2</sup>S]]
* [[McASP]]

==Notes==
{{notelist}}

==References==
{{reflist}}

== External links ==
{{Commons category}}
*[http://www.epanorama.net/documents/audio/spdif.html S/PDIF at Epanorama.net]
*{{webarchive |url=https://web.archive.org/web/20220911082917/http://www.hardwarebook.info/S/PDIF |title=S/PDIF at hardwarebook.net}}
*[http://www.minidisc.org/spdif_c_channel.html More about channel data bits]
*[https://www.ranecommercial.com/legacy/note149.html Interfacing AES3 and S/PDIF]

{{AVconn}}
{{Audio and video interfaces and connectors}}
{{Digital audio and video protocols}}
{{Sony Corp}}
{{List of IEC standards}}
{{Computer bus}}

{{DEFAULTSORT:S PDIF}}
[[Category:Audio communications protocols]]
[[Category:Computer hardware standards]]
[[Category:IEC 60958]]
[[Category:Digital audio transport]]
[[Category:Digital audio connectors]]