Digital Audio

S/PDIF

IEC958 (named IEC60958 at 1998) - 'Sony/Philips Digital Interface'

IEC958 (named IEC60958 at 1998) is the standard for AES/EBU and S-PDIF. S/PDIF is the consumer version of the AES/EBU-interface. The two formats differ only in the subcode information and the connector. S/PDIF was never intended to run through audio cables. It's a 75-ohm format and requires 75-ohm coaxial cable. It is now usual to use standard audio cable for S/PDIF, but it is not recommended and the transmission quality can vary. By specification, the max. length is 25 feet. If good 75-ohm cable with good 75-ohm connectors is used, it is possible to get good results with much longer lines. It depends on the quality of the transmitter, the receiver and the impedance match for the entire line, including the connectors.

Digital audio (SPDI/F and AES/EBU) contain frame rate (timing speed) information in the audio bit stream but not frame number (timecode) information. Therefore timecode can not be 'withdrawn' out of the audio bit stream.

CD & DVD players are the most common domestic audio equipment which is likely to have an S/PDIF output, this single phono (RCA) cable (has been known to use BNC as well) carries a stereo digital audio signal from the player to a digital ready amp or Digital Coaxial Input.

Digital Audio Connection Coax on left, Toslink Optical on the right. Digital Coax uses orange color coded "RCA" connections. These support AC3, Dolby Digital (5.1, etc.), DTS, S/PDIF.

Normal CD Audio

The traditional PCM (Pulse Code Modulation) is the encoding standard used for CD, DVD-Audio and DVD-Video. The Compact Disc CD is the well known optical disk storage media that is designed to store audio, video, and computer data in a digital format. CD's have a capacity to store 650 Mb (Megabytes) of data. The digital information in a standard audio CD is encoded in the PCM format. The main problem with standard PCM technology is that it requires brick wall filters to block frequencies above 20kHz, which is difficult to build and it requires the addition of re-quantization noise for its decimation (down-sampling) and interpolation (up-sampling) digital filters. These problems limit the actual fidelity of the reproduced audio.

PCM systems have a dynamic range and signal resolution fixed by the length of the digital.

When the CD format was created in the early 80's, digital technology was at the beginning and the format was forever frozen at 16bit/44kHz. Uncritical listeners don't notice any of the problems but certain people and most music producers do. Especially the low-level performance of PCM technology, the last 20 or 30 dB, of 16 bit, 44.1 kHz digital audio is more or less unusable for music reproduction.

DSD and SACD

Direct Stream Digital DSD and Super Audio Compact Disc SACD

Philips and Sony have cooperated in developing the Super Audio Compact Disc SACD, incorporating the newest technology and modern optical disc storage. By using a completely different system of encoding, the Direct Stream Digital DSD, it avoids many of the limitations of physics, electronics, and manufacturing technology inherent to the current method of PCM recording used for CDs and other current digital audio applications. DSD uses a different audio coding method during the recording process as the traditional PCM.

The SACD is essentially a DVD, it uses the same sector size, error correction and modulation as a DVD disc and the same file system (UDF plus ISO 9660), but the information contained on the format is using the Direct Stream Digital DSD technology.

AES/EBU

AES/EBU (Audio Engineering Society/European Broadcasting Union) is the name of a digital audio transfer standard. The AES and EBU developed the specifications for the standard.

The AES/EBU digital interface is usually implemented using 3 pin XLR connectors, the same type connector used in a professional microphone. One cable carries both left- and right-channel audio data to the receiving device. AES/EBU is an alternative to the S/PDIF standard.

AES/EBU is not commonly used in consumer products and generally only used in professional audio environments like a digital recording studio.

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However, with technology moving at such a pace together with the new HDTV, DVI & HDMI that are capable of carrying many more channels of sound – who know’s where the next five years are going to take us.

Appropriate Lektropacks Accessories: General Audio.

Also see Analogue Audio & Surround Sound/Dolby Digital.