S/PDIF
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S/PDIF is a Data Link Layer protocol and a set of Physical Layer specifications for carrying digital audio signals between devices and stereo components over either optical or electrical cable. The name stands for Sony/Philips Digital Interconnect Format (more commonly known as Sony Philips Digital InterFace), Sony and Philips being the primary designers of S/PDIF. S/PDIF is standardized in IEC 60958 where it is known as IEC 60958 type II. S/PDIF is essentially a minor modification of the original AES/EBU standard, for consumer use, providing small differences in the protocol and requiring less-expensive hardware.
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[edit] Applications
A 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. Another common use is to carry uncompressed digital audio from a CD player to a receiver. This specification also allows for the coupling of personal computer digital sound (if equipped) via optical or coax to Dolby or DTS capable receivers. This only supports stereo sound, unless the personal computer supports Dolby Digital Live or DTS Connect
[edit] Hardware specifications
S/PDIF was developed at the same time as the main standard, AES/EBU, used to interconnect professional audio equipment in the professional audio field. This was a result of 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 IC's [1]. S/PDIF remained almost identical at the protocol level (consumer S/PDIF provides for copy protection, whereas professional interfaces do not), 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. The RCA connectors are typically color-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 the already far more common (and therefore compatible and inexpensive) 75 Ω coaxial cable, using RCA jacks instead of the BNC connector, which is more common in commercial applications. S/PDIF is, for all intents and purposes, a consumer version of the AES/EBU format.
There are no differences between the signals transmitted over optical or coaxial S/PDIF connectors—both carry exactly the same information. Selection of one over the other rests mainly on the availability of appropriate connectors on the chosen equipment and the preference and convenience of the user. TOSLINK cables do not work well (and may even suffer permanent damage) if tightly bent, and their high light-signal attenuation limits their effective range to 6.1 metres (20 ft) or so. On the other hand, TOSLINK cables are not susceptible to ground loops and RF interference, like coaxial cables.[2] Another deciding factor for many is cost: any standard 75 Ω A/V cable can be used for coaxial connectivity, while TOSLINK requires a specific cable which, until recently, was not very affordable.
| AES/EBU balanced | AES/EBU unbalanced | S/PDIF | |
|---|---|---|---|
| Cabling | 110-ohm shielded TP | 75-ohm coaxial | 75-ohm coaxial or fibre |
| Connector | 3-pin XLR, 25-Pin D-subminiature | BNC | RCA, BNC, or TOSLINK |
| Output level | 2 to 7 V peak to peak | 1 to 1.2 V peak to peak | 0.5 to 0.6 V peak to peak |
| Min Input level | 0.2 V | 0.32 V | 0.2 V |
| Max distance | 100m | 1000m | 10m |
| 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) |
[edit] 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 might be ignored.
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".
[edit] Channel status bit
| Bit num. | If not set means: | If set means: |
|---|---|---|
| 0 | Consumer (S/PDIF) | Professional (AES/EBU) (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) | |
There is one channel status bit in each subframe, making 384 bits in each audio block. This means that there are 384/8 = 48 bytes available in each audio block. The meaning of the channel status bits is completely different between AES/EBU and the 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.
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 4–10Hz.
[edit] Limitations
S/PDIF lacks flow control and retry facilities, which limits its usefulness in applications outside of synchronous playback.
Because the receiver cannot control the data rate, it instead has to avoid bit slip by synchronising its conversion with the source clock. This means that S/PDIF cannot fully decouple the final signal from influence by the analogue characteristics of the source or the interconnect, even though the digital audio data can normally be transmitted without loss. The source clock may carry inherent jitter or wander, and the process of clock recovery may be further influenced by noise or distortion introduced in the data cable[3][4][5]. 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[5].
[edit] Implementations
Many multimedia players (e.g. VLC, MPlayer and FFDshow) support S/PDIF output. FFmpeg has IEC-61937 compatible muxer.
[edit] See also
[edit] References
- ^ (Finger, Robert A. 1992 'AES3-1992: The RevisedTwo-ChannelDigital Audio Interface', J.AudioEng.Soc.,Vol.40,No.3, 1992 March, p108 )
- ^ Joseph D. Cornwall (December 31, 2004). "Understanding Digital Interconnects". Audioholics.com. http://www.audioholics.com/education/cables/understanding-digital-interconnects. Retrieved 2007-07-12.
- ^ 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 Norman Tracy. "On Jitter, the S/PDIF Standard, and Audio DACs."
[edit] External links
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