S/PDIF
SPDIF stands for “Sony/Philips Digital Interconnect Format”. It is a digital audio format, it is also called S/PDIF, S/P-DIF, and IEC 958 type II.
What is SPDIF used for? It is used on systems which receive or transmit stereo Digital Audio such as PC audio cards, CD players, DVD players, car audio systems, and others.
For S/PDIF, the left and right channel data (two 192 bit blocks) are divided into 12 words of 16 bits each. The first 6 bits of the first word are a control code.
The Audio Data Rate of SPDIF
There is no specific data rate or resolution for the SPDIF protocol. The data rate “hand shake” is determined in the actual equipment containing the SPDIF connectors from the mutual SPDIF signal accepted by both pieces of audio hardware.
It is flexible since it uses the Bi-phase mark code (which has either 1 or 2 transitions for each bit). The initial word clock can then be directly extracted from the base signal itself.
The most common SPDIF data rates are the 44.1khz data rate for stereo CD audio and 48khz data rate for DAT -Digital Audio Tape-.
Typical SPDIF transmissions are limited to 16-bit audio due to the limitations of CD audio, but the SPDIF protocol can support up to 20-bit audio.
It may even be adapted to support 24 bit audio in the aftermarket. but this kind of audio is not directly supported by SPDIF.
The unnecessary bits will automatically be “shaved” to zero (off) in order to transmit S/PDIF signals that have less than 20 bits of bit rate sample accuracy.
The Format´s Data Transmission.
SPDIF data transmission is a stream of 32-bit data words. Each data frame consists of 384 words in total (A stereo channel: 192 data words, B stereo channel: 192 data words).
The SPDIF Specification is documented in the German patent EP000000811295B1 and it is defined by IEC standard 60958-3.
The Format is part of a larger collection of IEC-60958 standards (also known as the AES/EBU standard, and designated IEC-958 type II).
It is merely just another adaptation of the original AES/EBU standard that requires cheaper hardware, but at the protocol level the S/PDIF Audio Data Format remains identical to AES/EBU.
The physical connectors were switched from commercial and professional audio equipment (XLR) to either RCA jacks with electrical coaxial cable or TOSLINK optical fiber (sometimes known as “EIAJ Optical”).
It aims to be a more cost-efficient and simpler to use version for consumer applications.
The cabel itself was changed as well: the S/PDIF cable went from a 110 ? balanced twisted pair to the more economical and common 75 ? coaxial cable which is good to up to 10 meters with RCA connectors.
This way, plus the use of cheaper RCA jacks and plugs opened the door for consumers to enjoy audio quality previously previously unheard of except in the most expensive niches.
The real difference between the AES/EBU and S/PDIF protocol is the Channel Status Bit.
There is one channel status bit in each sub-frame,this translates to 192/8 = 24 bytes available (per audio block).
If the Channel Status Bit is not changed then it looks like this:
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
Applications for the S/PDIF Interface.
Since one of the primary purposes of the S/PDIF interface is to transfer compressed digital audio as defined by the IEC 61937 standard, and the other purpose is to carry the signal between the output of a computer or DVD player to a DTS surround or Dolby Digital home theater system, the S/PDIF interface is widely used to inter-connect commercial and professional audio equipment.
Most CD and DVD-ROM drives contain a S/PDIF interface, and today´s high-end sound cards like the Sound Blaster Live also have an external SPDIF output.
When compared to the older analog transmissions,The SPDIF Digital transference has the advantage of noise immunity.
Older Digital/Analog CD-ROM drives generated all kinds of undesirable ambient noise, like hisses or static, and the connection cable acted as a kind of antenna that could pick up unwanted noise generated by electromagnetic interference inside the machine.
But if your sound card has a SPDIF input and your CD-ROM drive has a SPDIF output you will enjoy the clean and crisp sound of the SPDIF connection.
All that is required is a SPDIF I/O data cable that is easy and relatively cheap to acquire.
This cable transfers data via RCA plugs and jacks and has a shielded coaxial type which arrests and grounds virtually all spectra noise (white, pink, and brown).
If you want the best then the SPDIF optical fiber cable is “noise-proof” and, compared to coaxial cable, is far less lossy in how many feet of cable can carry the same signal.
Even if it has not reached the level of the mainstream consumer yet, it is more often that one finds an Optical SPDIF I/O connection carry on the back side of off-the-shelf mini CD-ROM decks.
This optical SPDIF I/O connection array generally consists of 1 coaxial SPDIF input, 1coaxial output, 1 optical SPDIF output and 2 optical SPDIF inputs protected by a cover that only removed if you desire to install the optical fiber.
