With the existence of various audio and video standards comes the need to have specific interfaces. These interfaces need to have certain definite physical characteristics of the connection between electrical equipments. S/PDIF stands for Sony/Philips Digital Interconnect Format, also known as Sony/Philips Digital Interface, it is used for the digital transfer of audio data over either optical or electric cable. It is a standard for digitally transferring audio signals between stereo components and other such devices with the help of an optical or electrical cable.

At the initial stage, the signals were within the cabinet and could leave only after conversion into an analog signal. Now the goal is to keep these signals in a digital domain for as long as possible, which is the only way to guarantee a high sound quality. Communication of various devices with each other in the digital domain is required to achieve this quality. It allows the transfer of the digital signal from one device to another, without the ordeal of converting it to an analog signal.

Here is how it happens, when an audio CD is inserted into the CD-ROM drive, this drive needs to convert data that it reads in digital format, into an analog audio signal. This analog signal is then sent to the sound card via a suitable cable. There are two points that can lower the quality of sound, the first point being where the CD-ROM drive can use a poor quality D/A (digital-analog) converter and thereby generate noise, and the second point is when seeing the transmission between the CD-ROM drive and the sound card carried out in an analog format, the cable used for connecting may pick up noise from electromagnetic interference inside the computer.

Therefore, if your sound card has a S/PDIF input and your CD-ROM drive also has a S/PDIF output, instead of using the analog connection you can opt for a S/PDIF connection. This connection will allow the A/D conversion to take place in the sound card and not in the CD-ROM drive. The connection is set up via two wires and a small two-terminal plug.

Characteristics

S/PDIF or the standard digital audio interface IEC958 of EBU (European Broadcasting Union) has specified characteristics. The audio format is linear 16 bit by default, but can be expanded up to 24 bit. Control information bit is VUCP (Validity, User, Channel Status, and Parity). The bit V signals the validity of the audio sample. Bit U is user free coding for song run-time as well as track numbers. Bit C is for sampling rate, copy permit and emphasis. P bit is for detecting errors in order to ensure quality reception. Communication is one way from the transmitter to the receiver. Coding format is a bi-phase mark. Bandwidth occupation is from 100kHz to 6MHz but without DC. Signal bit rate as per the sample frequencies are 2.8MHz, 2MHz and 3.1MHz for 44.1kHz, 32kHz and 48kHz respectively. As it is the IEC (International Electro-technical Commission) standard that defines the characteristics of S/PDIF, the IEC specifications should be used instead of the S/PDIF standard.

Quality of sound in detail

As read above, there are two things that can cause a difference in the sound of the digital interfaces, they are:

Clock Phase Noise (Jitter)

Clock jitter in a digital audio system is defined as modulation on the master sample clock. It is the time variation of a periodic signal in electronics, usually in relation to a clock source, jitter can have large effects on the audio quality due to incorrect sample timing. They are required in almost every electric system today, as digital data is processed and transmitted at a high speed and the conversions are done between analog and digital signals at a higher rate. It measures the spectrum of the clock signal. The often-used jitter measurements are period jitter, cycle-to-cycle jitter, and accumulated jitter. Period jitter is most often encountered. Hence engineers are required to pay special attention to the quality of clock signals.

Errors

This usually causes very significant changes in the sound, often loud popping noises. Any data loss or errors in either are a sign of a very broken link, which is probably intermittently dropping out altogether.

Electrical Interface

The electrical interface for S/PDIF signals can be either 75 ohm coaxial cable or optical fiber (usually called TOSLINK). Usually the consumer models use coaxial cable interface and semiprofessional/professional equipments use optical interface. The electrical signal in the coaxial cable is about 500mVtt.

Coaxial cables

They are cables with an inner conductor surrounded by a tubular insulating layer made of a flexible material, with a high dielectric constant. They are a type of wire used for carrying a wide range of transmissions from the source to the device. They are basically cables and connectors of 75 ohm impedance and used to connect DVD players, Compact Disk (CD) and some DAT recorders to multichannel surround sound (Dolby Digital, DTS, etc) equipment. S/PDIF uses 75 ohm coaxial cable and RCA connectors. The 75 ohm coaxial cable is quite a low cost option, since the same cable is used for video transmission. Coaxial S/PDIF connections function well within a distance of 10 to 15 meters.

Optical cables – TOSLINK

The original creator of TOSLINK is Toshiba; TOSLINK was created in order to connect their CD players to their receivers. This idea was soon adopted by other CD player manufacturers. The intended use for the optical cable determines which type needs to be selected. Uses can range from use as submarine installation, power lines, attaching to aerial telephone poles, use in conduit, burial in trenches, and placement within paved streets. Early TOSLINK systems used the raw PCM data from the CD player; the S/PDIF standard has now become nearly universal for all the audio streams.

Characteristics

• High quality optical cable TOSLINK plugs, contain an attractive silver metal finish and gold plated inner pin.
• The TOSLINK optical cable are generally used to connect devices such as a DVD player, home cinema amplifier, etc.
• A single TOSLINK optical cable can transport a full multichannel digital audio stream such as Dolby Digital 5.1 or DTS.

Comparing with AES/EBU

Another digital audio standard frequently called AES/EBU, officially known as AES3, is also used for carrying digital audio signals between various devices. AES/EBU is a standardized, balanced digital interface with 110 ohm impedance cable. S/PDIF is a successor of the AES/EBU developed keeping in mind the consumer products. Today we find both these interfaces side by side – AES/EBU for professional use, and S/PDIF for consumer products. We find that sound cards and CD drives normally have S/PDIF connectors. Some TVs and other devices may also incorporate S/PDIF connectors. The two formats are quite compatible with each other, differing only in the sub-code information and connector. The professional format sub-code contains ASCII strings for source and destination identification, whereas the commercial format carries the SCMS.