Toslink Adapter

S/PDIF specifies a Data Link Layer protocol and choice 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 ), the two companies being the primary designers of the S/PDIF format. It is part of a larger collection of international standards known as IEC and defined by IEC 60958 (often referred to as AES/EBU), 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.

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.

Hardware specifications

S/PDIF was developed from a standard used in the professional audio field, known as AES/EBU which is commonly used to interconnect professional audio equipment. 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 jacks) or optical fibre (TOSLINK, i.e., F05 or EIAJ Optical), both of which cost less. 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 common in commercial applications. S/PDIF is, for all intents and purposes, a consumer version of the AES/EBU format.

Note that there are no differences in 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. Connections longer than 6 meters or so, or those requiring tight bends, should use coaxial cable, since the high light signal attenuation of TOSLINK cables limits its effective range. On the other hand, TOSLINK cables are not susceptible to ground loops and RF interference like coaxial cables. One 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.

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, but not all equipment supports this, and might ignore these extra bits.

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".

The channel status bit in the S/PDIF

There is one channel status bit in each subframe, making 192 bits in each audio block. This means that there are 192/8 = 24 bytes available in each audio block. The meaning of the channel status bits are completely different between AES/EBU and the S/PDIF.

For S/PDIF, the 192 bit block 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 (do 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 non-recordable CDs), the L-bit is not defined and recording is prevented by alternating bit 2 at a rate of 4-10Hz.

See also

• Word clock
• ADAT Lightpipe
• AES/EBU
• I²S

References

1. ^ Joseph D. Cornwall (December 31, 2004). "Understanding Digital Interconnects". Audioholics.com . http://www.audioholics.com/education/cables/understanding-digital-interconnects . Retrieved on 2007-07-12 .  

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