Standardized digital audio interface
S/PDIF
(
Sony/Philips Digital Interface
)
[1]
[2]
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
or
BNC
connectors, or a
fiber-optic cable
using
TOSLINK
connectors. S/PDIF interconnects components in
home theaters
and other digital
high-fidelity
systems.
S/PDIF is based on the
AES3
interconnect
standard
.
[3]
S/PDIF can carry two channels of uncompressed
PCM audio
or
compressed
5.1
surround sound
; 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 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
[
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]
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
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
[
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]
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
ICs
.
[6]
S/PDIF is nearly identical at the
protocol
level,
[a]
but uses either
coaxial cable
(with
RCA connectors
) or
optical fibre
(
TOSLINK
; i.e., JIS F05 or EIAJ optical), both of which cost less than the 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 Ω coaxial cable while AES3 uses 110
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
issues in systems. The electrical connection can be more robust and supports longer connections.
[7]
Comparison of AES3 and S/PDIF
[8]
|
AES3
|
S/PDIF
|
Balanced
|
Unbalanced
|
Copper
|
Optical
|
Cabling
|
110 Ω STP
|
75 Ω coaxial
|
75 Ω coaxial
|
Flexible lightpipe
|
Connector
|
3-pin
XLR
|
BNC
|
RCA
or
BNC
|
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
|
1000 m
|
100 m
|
10 m
|
Modulation
|
Biphase mark code
|
Subcode information
|
ASCII
id. text
|
SCMS
copy protection info.
|
Audio bit depth
|
24 bits
|
20 bits (24 bits, optionally)
[
citation needed
]
|
Protocol specifications
[
edit
]
S/PDIF is used to transmit digital signals in a number of formats, the most common being the 48 kHz
sample rate
format (used in
Digital Audio Tape
) and the 44.1 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
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
words
of 16 bits each, with the first 16 bits being a control code.
S/PDIF control word components
[9]
Byte
|
Bit
|
Unset (0)
|
Set (1)
|
0
|
0
|
Consumer (S/PDIF)
|
Professional (AES3)
(changes meaning to
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
|
6?7
|
Mode, defines subsequent bytes; values other than zero are undefined.
|
1
|
0?6
|
Audio source category indicating the type of source equipment (general, CD-DA, DVD, etc.)
|
7
|
L-bit, original or copy
[A]
|
2
|
0?3
|
Source number
|
4?7
|
Channel number
|
3
|
0?3
|
Sampling frequency:
0000
2
: 44.1 kHz,
0100
2
: 48 kHz,
1100
2
: 32 kHz
|
4?5
|
Clock accuracy:
10
2
: 50ppm,
00
2
: 1100ppm,
01
2
: variable pitch (requires compatible receiver)
|
6?7
|
Undefined
|
4
|
0
|
Word length 20 bits
|
Word length 24 bits
|
1?3
|
Sample length (0: undefined, 1?4: word length minus 1-4 bits, 5: full word length)
|
4?7
|
Undefined
|
5?10
|
0-7
|
EAN-13 code (possibly in binary-coded decimal)
|
11
|
0-3
|
4?7
|
Undefined; padding on 13-digit EAN code
|
12?13
|
0-7
|
Undefined
|
14
|
0?3
|
4-7
|
ISRC (encoding unclear; ISRC is 2 alphabetic, 3 alphanumeric and 7 numeric, which is 26
2
× 36
3
× 10
7
? 2
48.164
and so obviously fits into 7.5 bytes, but a naive 5 ASCII + 7 BCD would be 8.5 bytes)
|
15?21
|
0?7
|
22?23
|
0?7
|
Undefined
|
- ^
(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 Hz.
Data framing
[
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]
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 4:1–3 bits (sample length) are set accordingly.
IEC 61937 encapsulation
[
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]
IEC 61937 defines a way to transmit compressed, multi-channel data over S/PDIF.
[10]
- 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
AC-3
/
E-AC-3
,
Dolby TrueHD
, MP3, AAC,
ATRAC
,
DTS
, and
WMA Pro
.
[11]
[12]
Limitations
[
edit
]
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
.
[13]
[14]
[15]
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.
[15]
[16]
See also
[
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]
Notes
[
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]
References
[
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]
- ^
"S/PDIF Information"
.
Intel
. 21 July 2017
. Retrieved
3 April
2018
.
- ^
"S/PDIF"
. Retrieved
3 April
2018
.
- ^
"SoundSystem SixPack 5.1+ True 6 Channel + Digital In & out ? Stuff Worth Knowing"
(PDF)
.
TerraTec
. 5 July 2001. p. 43
. Retrieved
18 January
2011
.
- ^
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...
- ^
"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.
- ^
Finger, Robert A. 1992 'AES3-1992: The RevisedTwo-ChannelDigital Audio Interface', J.AudioEng.Soc., Vol.40, No. 3, 1992 March, p108
- ^
"SPDIF connections: Get connected, not confused"
. Retrieved
15 May
2024
.
- ^
Dennis Bohn (2001).
"Interfacing AES3 & S/PDIF"
(PDF)
.
Rane Corporation
. p. 2
. Retrieved
18 January
2011
.
- ^
Understanding/Analyzing Digital Audio Channel Status Bits
at the
Wayback Machine
(archived 2019-02-28)
- ^
Digitalton - Schnittstelle fur nichtlinear-PCM-codierte Audio-Bitstrome unter Verwendung von IEC 60958 - Teil 1: Allgemeines (IEC 61937-1:2007 + A1:2011); Deutsche Fassung EN 61937-1:2007 + A1:2011
- ^
"FFmpeg: libavformat/spdif.h File Reference"
.
ffmpeg.org
.
- ^
"Representing Formats for IEC 61937 Transmissions - Win32 apps"
.
learn.microsoft.com
. 15 May 2023.
- ^
Giorgio Pozzoli. "
DIGITabilis: crash course on digital audio interfaces
" tnt-audio.com.
- ^
Chris Dunn, Malcolm J. Hawksford. "
Is the AES/EBU/SPDIF Digital Audio Interface Flawed?
" AES Convention 93, paper 3360.
- ^
a
b
Tracy, Norman.
"On Jitter, the S/PDIF Standard, and Audio DACs"
. Archived from
the original
on 1 July 2017.
- ^
Lesso, Paul (2006).
"A High Performance S/PDIF Receiver"
(PDF)
. Audio Engineering Society. Archived from
the original
(PDF)
on 4 June 2014.
AES Convention 121, paper 6948
External links
[
edit
]
Wikimedia Commons has media related to
S/PDIF
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Interfaces are listed by their speed in the (roughly) ascending order, so the interface at the end of each section should be the fastest.
Category
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