S/PDIF (Sony/Philips Digital Interface Format) 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 fibre optic cable with TOSLINK connectors. S/PDIF interconnects components in home theatres and other digital high fidelity systems.
S/PDIF is based on the professional AES3 interconnect standard.[ 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 formats (such as Dolby TrueHD and DTS-HD Master Audio) which require greater bandwidth like that available with HDMI or DisplayPort.
S/PDIF is a data link layer protocol and 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).[2]
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
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.[3] S/PDIF remained almost identical at the protocol level,[a] 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.[4]
| AES3 balanced | AES3 unbalanced | S/PDIF | |
|---|---|---|---|
| Cabling | 110-ohm STP | 75-ohm coaxial | 75-ohm coaxial or 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 | 1,000 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. |
| Max. resolution | 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.
With one exception, S/PDIF protocol is identical to AES3. The channel status bit differs in S/PDIF. There is one channel status bit in each subframe, making 384 bits in each audio block. The meaning of the channel status bits is completely different between AES3 and S/PDIF. For S/PDIF, the 192-bit block for each channel 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.
| Bit | Unset (0) | Set (1) |
|---|---|---|
| 0 | Consumer (S/PDIF) | Professional (AES3) (changes meaning of control word) |
| 1 | Normal | Compressed data |
| 2 | Copy restrict | Copy permit |
| 3 | 2 channels | 4 channels |
| 4 | — | — |
| 5 | No pre-emphasis | Pre-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) | |
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.[6][7][8] If the DAC does not have a stable clock reference then noise will be introduced into the resulting analogue signal. However, receivers can implement various strategies which limit this influence.[8][9]TOSLINK cables, unlike coaxial cables, are immune to ground loops and RF interference.[10] TOSLINK cables may suffer permanent damage if tightly bent.
S/PDIF is unidirectional, so it lacks flow control and retransmission facilities. This limits its utility in data communications.
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SPDIF, or the Sony/Philips Digital Interconnect Format) is used to carry or transport digital audio signals in consumer electronic equipment such as CD and DVD players, PC audio cards, and car audio systems over short distances. The digital signal can be carried over fiber optic (TOSLINK connectors) or coaxial cable (RCA connectors) and is also commonly used in home theater and stereo systems. SPDIF is based on the AES3 interconnect standard is capable of two 192 bit blocks (split into left and right channel data) which are divided into 12 words of 16 bits each. The first six bits of each word are a control code and SPDIF is also referred to as IEC 958 type II.
What Are the Uses of SPDIF?
The most common use of the SPDIF interface is to transport compressed digital audio. The audio must meet the requirements defined by the IEC 61937 standard. The interface is commonly used to connect the output of DVD and other audio players to home theater receivers which support DTS or Dolby Digital surround sound capability. The interface is also used to transport two channels of digital audio from a CD player to a receiver in uncompressed format. When SPDIF is used to couple the digital sound from a personal computer to a receiver, only stereo sound is supported unless the computer has specialized support such as DTS Connect or Dolby Digital Live installed.What is the AES/EBU Standard?
The AES/EBU (Audio Engineering Society and European Broadcasting Union) standard (better known as AES3) was first developed in 1989 and later revised in 1995, 1998, and 2003. AES3 is the complimenting standard to S/PDIF and is primarily used in the professional music industry and for applications requiring the highest degree of sound quality possible. The standard supports PCM encoded audio that is delivered in DAT and CD format. The most accepted formats used are 44.1 kHz (CD) and 48 kHz (DAT) and supports running and processing at any rate. The standard also uses recovering moves synched with the clock rate. This in turn is then handled by encoding the data using the Biphase mark code (BMC).AES3 Balanced Specifications
Cabling 110-ohm STP (70 ohm coaxial unbalanced)
Connector 3-pin XLR (BNC unbalanced)
Output Level 2 to 7 V peak to peak (1 to 1.2 V unbalanced)
Min. Input Level 0.2 V (.32V unbalanced)
Max. Distance 100 m (up to 1,000m using the AES3 unbalanced standard)
Modulation Biphase mark code
Subcode Information ASCII id. text
Max. Resolution 24 bits
What Are TOSLINK Audio Connectors?
TOSLINK (Toshiba Link) is the standardized fiber optic connection system used throughout industry. The connectors are also known as optical audio cables and used as one of the possible connectors for SPDIF. TOSLINK is most commonly used to connect CD/DVD players, computers, video gaming consoles, DAT recorders, and MiniDisc to AV receivers for playback on a home theater system. Although known for SPDIF connections, TOSLINK connectors support a number of different standards and media formats at bit rates ranging up to 125 MB/sec (originally 3.1 MB/sec).What Are the SPDIF Hardware Specifications?
SPDIF and AES3 were developed at the same time with the goal to better connect professional audio equipment. Due to desires of standards committees that worked on each protocol, both protocols were designed to have a number of similarities and were almost identical at the protocol level. SPDIF can almost be thought of as the consumer variant of the AES3 standard. Similarly, audio signals that are transmitted over TOSLINK connections are identical to those sent over the coaxial SPDIF connectors. SPDIF specifications include:Cabling: 75-ohm fiber or coaxial cable
Connector: TOSLINK or RCA
Output Level: 0.5 to 0.6 V (peak to peak)
Minimum Input Level: 0.2 V
Maximum Distance: 10m
Modulation: Biphase Mark Code
Subcode Information: SCMS Copy Protection Information
Maximum Resolution: 20 Bits (24 Bits is optional)
SPDIF Audio Data Rate
Although the SPDIF protocol doesn’t specific a max resolution or data rate, the equipment which uses the SPDIF connectors has to determine the data rate “hand shake” from the SPDIF signal that both pieces of the audio hardware accept. To ensure greater flexibility, SPDIF uses a bi-phase mark code that includes either one or two transitions for each bit. This feature allows the initial word clock to be extracted from the base signal. The most common SPDIF data rates are 44.1khz for stereo CD audio and 48khz for digital audio tape (DAT). The most typical SPDIF transmissions are limited to 16 bit audio due to the limitations of CD even though it is designed to support 20 bits and 24 using after-market adapters. To support transmitting signals that are less than 20 bits, the left-over bits are automatically shaved off to zero.SPDIF Audio Data Format
SPDIF audio data format is sent as a stream of 32 bit data words with a data frame consisting of 384 words total. There are 192 data words transmitted for both stereo channels (A and B). The most current IEC standard defining the SPDIF audio format is 60958-3 and is also specified in the German Patent EP000000811295B1.The format has also been included in a collection of IEC-60958 standards and is known as the AES/EBU standard and later designated IEC-958 type II. At the protocol level, the S/PDIF (SPDIF) audio data format continues to remain identical to AES/EBU.The primary difference between the SPDIF and AES/EBU protocols remains the Channel Status Bit. In each sub-frame in the format, there is a single channel status bit located in each sub-frame. These are made up of 192 bits per audio block that results in 192/8 = 24 bytes being available in each block. If the Channel Status bit is not set:
0- Consumer/professional
1- Normal/compressed data
2- Copy prohibit/copy permit
3- 2 channels/4 channels
4- n/a
5- No pre-emphasis/pre-emphasis
There is one channel status bit in each sub-frame, which is comprised of 192 bits per audio block. This translates to 192/8 = 24 bytes available (per audio block).
SPDIF Limitations
In the SPDIF specification, the receiver doesn’t control the data rate of the communication. Slip is avoided by synchronizing the conversion with the source clock. As a result, SPDIF is not able to decouple the final signal from influence through the characteristics of the source. Due to the short range; however, the audio signal can normally be transmitted without data loss. The source clock in SPDIF is also known to wander or jitter. If any distortion or noise is introduced into the data cable, the process of clock recover can be further influenced. Also, if there is not a stable clock reference, then noise will get introduced into the resulting analogue signal if there are not any strategies introduced to limit the influence.SPDIF also lacks retransmission facilities and flow control that impacts its use in data communications applications. If TOSLINK cables are being used for SPDIF and are tightly bent there will be a negative impact on overall performance. Also, TOSLINK have a light-signal attenuation that limits the effective range for SPDIF when compared to RCA.
When Should You Use SPDIF?
Digital audio connections should be used whenever possible to obtain the highest sound quality. A common question that arises for consumers is when to use SPDIF and when to use HDMI. If you are connecting two pieces of audio equipment, then you will likely have to use SPDIF since there will not be an HDMI connection on the audio data source. If the two pieces of equipment don’t have SPDIF connections, then RCA cables will be the back-up; however, on most modern consumer audio equipment either optical TOSLINK or coaxial connections will be available. When connecting video and audio between devices, then the type of connection will primarily depend on the connection options available on both pieces of equipment. It is also becoming more common to have a mix of both HDMI and SPDIF connections on home theater setups based on the maximum number of HDMI connections available on the theater receiver and television.SPDIF in Consumer Electronics
In order to save money on SPDIF, the physical connectors were shifted to RCA jacks for coaxial cable connections or TOSLINK for fiber connections which are also known as EIAJ Optical. These proved to be much more cost effective and easier to use for common consumer applications. Similarly, the S/PDIF cable was changed from 110 Ω balanced twisted pair to the cheaper and more common 75 Ω coaxial cable that is good for ranges of up to 10 meters when using RCA connectors. When combined with the less expensive RCA connectors, was able to help consumers enjoy the higher quality audio previously only see in expensive commercial audio equipment.SPDIF on Home Computers
Not all home computers are sold with “ready-t-use” SPDIF connectors. For desktops, the rear panel of the PC will be the location for the coaxial or optical connector if installed (some have both). On laptops, the connector will be combined with the headphone plug or jack and have the word “SPDIF” written near it if capable of supporting the standard. Not all laptops will have the label written beside the jack; however, forcing the end-user to check the computer’s specifications to see if supported. Another potential indication of SPDIF support is if the inside color of the jack is black it likely supports the standard and green if it does not.http://www.tech-faq.com/spdif.html
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