What is SPDIF?

SPDIF (Sony/Philips Digital Interconnect Format) is a digital audio format, and is also referred to as S/PDIF, S/P-DIF, and IEC 958 type II.

SPDIF is used for transporting stereo digital audio signals on PC audio cards, CD players, DVD players, car audio systems, and other systems which transmit or receive stereo digital audio.

For S/PDIF, two 192 bit blocks (left and right channel data) is divided into 12 words of 16 bits each. The first 6 bits of the first word are a control code.

SPDIF Audio Data Rate

The SPDIF protocol does not specify a given data rate or resolution. The actual equipment containing the SPDIF connectors must determine the data rate "hand shake" from the mutual SPDIF signal accepted by both pieces of audio hardware.

For greatest flexibility, the SPDIF protocol utilizes the Bi-phase mark code (which has either 1 or 2 transitions for each bit). This allows the initial word clock to be directly extracted from the base signal, itself.

The most common SPDIF data rates are 44.1khz (stereo CD audio) data rate and 48khz (DAT- Digital Audio Tape) data rate.

While typical SPDIF transmissions are limited to 16-bit audio (due to audio CD limitations), the SPDIF protocol actually supports 20-bit audio.

Furthermore, the SPDIF protocol can be after-market adapted to support 24 bit audio (with an extra 4 bits). However, 24 bit audio is not directly supported by the SPDIF protocol.

In order to transmit S/PDIF signals having less than 20 bits of bit rate sample accuracy, the unnecessary bits will automatically be "shaved" to zero (off).

SPDIF Audio Data Format

SPDIF data is transmitted as a stream of 32-bit data words. A data frame consists of 384 words in total, with 192 data words transmitted for the A stereo channel, and 192 data words transmitted for the B stereo channel.

The SPDIF Specification is defined by IEC standard 60958-3, and is documented in the German patent EP000000811295B1.

The SPDIF Audio Data Format is more recently part of a larger collection of the IEC-60958 standards (also known as the AES/EBU standard, and designated IEC-958 type II).

The SPDIF Audio Data Format is merely just an off-shoot adaptation of the (original) AES/EBU consumer use standard (requires cheaper hardware).

As a matter of fact, the S/PDIF Audio Data Format remains identical to AES/EBU (at the protocol level).

The physical connectors were switched from XLR (commercial and professional audio equipment) to either RCA jacks (with electrical coaxial cable) or TOSLINK (optical fiber; also known as EIAJ Optical).

These solutions proved to be far more cost-efficient and simpler to use, for consumer applications, and the actual S/PDIF cable was changed, as well.

S/PDIF cable went from 110 Ω balanced twisted pair, to the more readily available (and economical) 75 Ω coaxial cable (good up to ten meters with RCA connectors).

This, combined with using cost-effective RCA jacks and plugs (over the more costly and less compatible BNC connector) opened the door for consumers to enjoy audio quality previously found only in costly commercial equipment.

The real difference between the AES/EBU and S/PDIF protocol, is the Channel Status Bit. If the Channel Status Bit is not set, then:

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, (comprising of 192 bits per audio block). This translates to 192/8 = 24 bytes available (per audio block). The meaning of the channel status bits

S/PDIF Interface Applications

The S/PDIF interface has two primary purposes:

The S/PDIF interface is used to transfer compressed digital audio (as defined by the IEC 61937 standard), and carry the signal between the output of a computer or DVD player to a home theater system designed for Dolby Digital (or DTS surround sound);
The S/PDIF interface is extensively used to inter-connect commercial (professional) audio equipment.

In addition to most CD and DVD-Rom drives containing a S/PDIF interface, many high-end sound cards now also have an external SPDIF output (i.e. The Sound Blaster Live! Audio card).

The main advantage of SPDIF Digital transference (.vs original analog transmissions), is noise immunity.

For example, if a CD-ROM drive has poor quality D/A (digital-analog) converter; it can generate all kinds of undesirable hiss, static and other unwanted ambient noise.

Also, if the signal between the CD-ROM drive and the sound card is transferred via analog format; the connection cable used can often times act as an antenna of sorts.

It can actually pick up unwanted "stray" noise, which is generated from the typically-occurring internal computer chassis electro-magnetic interference.

Therefore, if your sound card has a SPDIF input and your CD-ROM drive has a SPDIF output- Instead of using a shaky analog connection; opt for a crisp, clean SPDIF connection- you won't be disappointed.

This will accommodate the A/D conversion from within the sound card, itself, and not in the CD-ROM drive.

All that is required is a readily available SPDIF I/O data cable.

This is why the cable used to transfer SPDIF data via RCA plugs and jacks is commonly of a shielded coaxial type; which arrests and grounds virtually all audio spectra noise (including white, pink, and brown).

The ultimate by far, however, is SPDIF optical fiber cable. It is as "noise-proof" as can be expected, and is far less lossy in how many feet of cable can carry the same signal (as compared to coaxial cable).

Though not quite at the abundant and cheap mainstream consumer level quite yet, it is getting more and more common for the the back side of newer, off-the-shelf mini CD-ROM decks to accommodate an Optical SPDIF I/O connection array.

A typical optical SPDIF I/O connection array might consist of 1 coaxial SPDIF input, 1coaxial output, 2 optical SPDIF inputs and 1 optical SPDIF output; usually protected by a accessible cover (only removed if you wish to install the optical fiber).