SPDIF

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

SPDIF transports stereo digital audio signals on PC audio cards, CD players, DVD players, car audio systems, and other systems that transmit or receive stereo digital audio.

For S/PDIF, two 192 bit blocks (left and right channel data) are 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 that both pieces of audio hardware accept.

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) and 48khz (DAT- Digital Audio Tape).

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, the SPDIF protocol does not directly support 24 bit audio.

In order to transmit S/PDIF signals with 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 IEC standard 60958-3 defines the SPDIF Specification 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 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 effective 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 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, which is comprised of 192 bits per audio block. This translates to 192/8 = 24 bytes available (per audio block).

S/PDIF Interface Applications

The S/PDIF interface has two primary purposes:

• The S/PDIF interface transfers compressed digital audio (as defined by the IEC 61937 standard) and carries the signal from 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 interconnect commercial (professional) audio equipment.

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

SPDIF Digital transference’s main advantage (.vs original analog transmissions) is noise immunity.

For example, if a CD-ROM drive has a poor quality D/A (digital-analog) converter, it can generate all kinds of undesirable hisses, 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 typical internal computer chassis electromagnetic interference.

Therefore, if a sound card has an SPDIF input and the CD-ROM drive has an SPDIF output, opt for a crisp, clean SPDIF connection instead of a shaky analog connection.

This will accommodate the A/D conversion from within the sound card itself, not 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 a shielded coaxial type that arrests and grounds virtually all audio spectra noise (including white, pink, and brown).

The ultimate by far, however, is the 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 yet, it is getting more common for the backside 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, 1 coaxial output, 2 optical SPDIF inputs and 1 optical SPDIF output. An accessible cover (only remove to install the optical fiber) usually protects them.