4.2 Background to Internationally Standardized Interfaces

Within the audio field the Audio Engineering Society (AES) has been a lead body in determining digital interconnect standards. Although the AES is a professional society, and not a standards body as such, its recommendations first published in the AES3-1985 document ¹ have formed the basis for many international standards documents concerning a two-channel digital audio interface. The Society has been instrumental in coordinating professional equipment manufacturers' views on interface standards although it has tended to ignore consumer applications to some extent, preferring to leave those to the IEC (see below). A consumer interface was initially developed by Sony and Philips, subsequently to be standardized by the IEC and EIAJ, and as a result there are many things in common between the professional and consumer implementations . Before setting out to describe the international standard two-channel interface it is important to give a summary of the history of the standard, since it will then be realized how difficult it is to call this interface by one definitive title.

Other organizations that based standards on AES3 recommendations were the American National Standards Institute (ANSI), the European Broadcasting Union (EBU), the International Radio Consultative Committee (CCIR) (now the ITU-R), the International Electrotechnical Commission (IEC), the Electronic Industries Association of Japan (EIAJ) and the British Standards Institute (BSI). Each of these organizations formulated a document describing a standard for a two-channel digital audio interface, and although these documents were all very similar there were often also either subtle or (in some cases) not-so-subtle differences between them. As time has gone by some of the most glaring anomalies have been addressed. A useful overview of the evolutionary process that resulted in each of these standards may be found in Finger ² . The documents concerned were as follows : AES3-1985 ¹ , ANSI S4.40-1985, EBU Tech. 3250-E ³ (1985), CCIR Rec. 647 (1986) ⁴ , CCIR Rec. 647 (1990) ⁵ , IEC 958 (1989) ⁶ (with subsequent annexes), EIAJ CP-340 (1987) ⁷ , EIAJ CP-1201 (1992) ⁸ , and BS 7239 (1989) ⁹ .

As mentioned above, the roots of the consumer format interface were in a digital interface implemented by Sony and Philips for the CD system in 1984. This interface was modelled on the data format of AES3, but used different electrical characteristics (see section 4.3.4) and is often called the SPDIF (Sony-Philips Digital Interface). Although audio data was in the same format as AES3, there were significant differences in the format of non-audio data. In 1987 the EIAJ CP-340 standard subsequently combined professional (Type I) and consumer (Type II) versions of the interface within one document and included specifications for non-audio data which aimed to ensure compatibility between different consumer devices such as DAT and CD players. This interface was by no means identical to the original SPDIF and led to some differences between early CD players and later digital devices conforming to CP-340. CP-340 has now been renumbered and is called CP-1201.

Slightly later than CP-340 the IEC produced a document that eventually appeared in 1989 as IEC 958. The consumer version (with its subsequent annexes) was an extension of the SPDIF to allow for wider applications than just the CD, and the professional version was essentially the same as AES3. It also allowed for, but did not describe in detail, an optical connection. As will be seen later, interpreting this standard to the letter seemingly allowed the manufacturer to combine either consumer or professional data formats with either 'consumer' or 'professional' electrical interfaces. It did not originally state that a particular electrical interface had to be used in conjunction with a particular data format, although the situation was made clearer in the revised IEC 60958 standard ¹⁰ . Some confusion therefore existed in the industry over whether consumer devices could be interconnected with professional and vice versa, and the answer to this problem is by no means straightforward, as will be discussed.

Concerning key similarities and differences between the other documents listed above, one should note that EBU Tech. 3250-E is a professional standard, the only effective difference from AES3 being the insistence on the use of transformer coupling. Tech. 3250-E was revised in 1992 to define more aspects of the channel status bits, to allow a speech quality coordination channel in the auxiliary bits, to give an improved electrical specification and to specify which aspects of the interface should be implemented in standard broadcast equipment. CCIR Rec. 647 was a professional standard that did not insist on transformers , but was otherwise similar to the EBU standard. The 1990 revision contained some further definitions of certain non-audio bits, including use of the auxiliary bits for a low quality coordination channel (see section 4.5). CCIR Rec. 467 became ITU-R BS.647 (1992) when the CCIR was reborn as the ITU-R. BS 7239 was identical to IEC 958. ANSI S4.40 was identical to AES3. (The formal relationship between ANSI and AES standards has recently been broken.)

It may reasonably be concluded from the foregoing discussion that a device which claimed conformity to AES3, ANSI S4.40, EBU 3250 or CCIR 647 would have been be a professional device, but that one which claimed conformity to IEC 958, EIAJ CP-340, 1201 or BS 7239 could have been either consumer or professional. It was conventional in the latter case to specify Type I or II, for professional or consumer. (In modern IEC nomenclature , the consumer application is defined in 60958-3 and the professional application is in 60958-4.) Because it will be necessary to refer to differences between these standards in the following text, the cumbersome but necessary term 'standard two-channel interface' will be used wherever generalization is appropriate.

Some revisions of the original documents have taken place, the most important of which will be found in AES3-1992 (with revisions in 1997 and amendments up to 1999) ¹¹ and IEC 60958 (superseding IEC 958 and covering general specifications, software delivery mode, consumer and professional applications in four parts ). Because these two standards are the most comprehensive and form the primary focal points for international standards activity relating to two-channel interfaces, most of the following discussion will deal primarily with AES3 for professional interfaces and IEC 60958 for all other purposes. (The British Standard now follows the IEC standard directly in content and nomenclature, being denoted BS EN 60958 (2000).)

Owing mainly to the efforts of four UK companies, a further standard was devised to accommodate up to 56 audio channels. Originally called MADI (Multichannel Audio Digital Interface), it is based on the AES3 data format and has now been standardized as AES10-1991 ¹² . It also appears as an American Standard: ANSI S4.43-1991. This is only a professional interface and is covered further in section 4.11.