About SP-DIF or S/PDIF

1996 DJ Greaves, Mixerton ST.

S/PDIF is the Sony and Philips Digital Interconnect Format. It can carry a stereo pair of channels with a sampling rate of up to 96 Ksps (kilo-samples per second) and with a sample precision of up to 24 bits. The S/PDIF output from CD player fits inside this envelope, being only 16 bits per sample at 44.1 Ksps. S/PDIF receivers can often automatically adapt to the rate and precision being delivered to them. When different sampling precisions are used, the most significant bit is always at the same position in the frame. This means that if the transmitted precision is greater or lower than the receiver can render, then the least significant bits become automatically dropped or padded with zeros respectively.

The physical link for S/PDIF carries a Biphase Manchester Coded stream. Manchester Coding is a class of line coding methods which combine a data stream with a clock on a single channel where there are up to two transitions on the line for each bit conveyed. With Biphase Manchester, there is a line transition at each end of a bit period and a central transition if the data is a one. For CD audio at 44.1 Ksps the line rate is 5.6448 megabaud and the effective data rate is 2.8224 Mbps or 352.8 kilobytes per second.

RCA/phono sockets are commonly used for copper S/PDIF links, using a line level of about 0.5 volts and transformer isolation at both ends. As mentioned below, pro-audio devices may often use XLR connectors to carry the signal and they also use an AES/EBU extended subcode (see below).

S/PDIF was first used in the commercial hi-fi world to interconnect CD mechanisms to external DACs, however at least one hi-fi company today makes hi-fi separates which are interconnected using S/PDIF, including an S/PDIF pre-amplifier with digital input and output and active loudspeakers with S/PDIF input and digital cross-overs. S/PDIF is now widely found on DAT players, PC soundcards, CD-ROM drives and home theatre separates.

Apart from the pair of audio channels, S/PDIF also carries a subcode similar to the subcode on CDs which indicates the current track number and current time within the track. For S/PDIF the only widely used subcode component is SCMS, the serial copying management system. This enables a stream to be marked as an original or a copy. A DAT or CD-R recorder sold for the consumer market or digital recording studio component should mark as a copy anything it records from the digital input and is not supposed to allow the user to make copies of material which is already marked as a copy. Many contemporary DAT recorders my be switched between the consumer and professional mode. Other components of the subcode that delineate tracks from each other are also widely supported, including cue sheet information that gives track titles and artist names, ISRC recording codes, and so on.

For professional digital audio, where cable runs between studios are often needed, the precursor to S/PDIF known as AES-EBU is widely used. The two formats are compatible with each other in terms of conveying the audio content, differing only in the subcode information and connector. The professional format subcode contains ASCII strings for source and destination identification, whereas the commercial format carries the SCMS. Professional equipment uses balanced XLR connectors to carry S/PDIF over differential pair cable, as commonly used for low impedance microphones. A normal balanced to unbalanced cable will allow interconnection and may devices nowadays have both XLR and RCA connectors. However, it is important not to use `low microphony' cable, such as guitar leads, for digital interconnections (including MIDI actually) since these cables have very poor high frequency transmission capabilities.

A third physical media often used is optic fibre, including low-grade plastic optical fibre (POF). Optical fibre has the advantage that, since it is non-conducting, earth-loops cannot be generated and the fibre link is immune to hum and noise pickup. In practice, the poor quality optical fibre components often used can tend to introduce data-dependent jitter in the process of separating clock from data in the Manchester decoder at the receiver. This can cause a measureable degredation of the conversion back to analogue format, but advanced design of the circuits using low bandwidth phase-locked loops ameliorates the problem. The electrical isolation advantage of fibre is sometimes quoted as its saving grace, but actually transformer-coupled coaxial links provide isolation that is just as effective.


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