AES

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Overview

The term "AES" is used as an abbreviation for Audio Engineering Society; an international organization that, among other things, sets up standards for connection and interfacing of audio equipment. The AES3 standard is the current worldwide standard for professional digital audio interfacing, and includes a number of formats. The most common is the stereo format that is carried by 110 Ohm shielded twisted-pair cable with XLR connectors in each end (similar to a microphone cable).

History

The first AES stereo digital audio standard was adopted in 1985 and left many parts open for future development and allowed other parts to be left as "not implemented." The first generation of "AES" digital interfaces suffered compatibility problems including channel reversals and incorrect interpretation of informational "status bits" that carried information about the audio content; such as Sample frequency, number of bits (wordlength), and Pre-emphasis. As a result; many pro audio interfaces will pass the audio content of the digital audio signal at the proper sample frequency; even if the status bits embedded in the digital audio signal do not "match" the audio content. In some cases, there is an indication warning of the miss-match, and in other cases a manual setting must be selected to allow the signal to pass. The format called for 5 Volt "TTL" level signals transmitted using differential circuits over shielded twisted-pair cables with XLR connectors.

To address the need for a consumer format, the developers of the Compact Disc Sony-Phillips developed the Sony-Phillips Digital Interconnect Format (S-PDIF) which was derived directly from the AES professional standard with some important differences. The consumer format was essentially the same in the basic format of the digital code; including the "sync" portion at the beginning of each left-right sample section (or "Frame") and the position of the audio data within the frame. But almost all of the non-audio status-bits were arranged differently. Shortly after 1987 with the development of the Serial Copy Management Scheme in response to the RIAA concerns regarding copyright infringement by digital copies, SCMS data was also included in the consumer format. The format was superseded by the IEC 60958 type II standard; which is essentially the same. The format called for 0.5-0.6 volt p-p unbalanced circuitry over coaxial cable with RCA connectors or optical connection via F05 connectors also referred to as "Toslink" connectors.

The AES3 format also includes an unbalanced format at a higher signal level than IEC60958 (1.0-1.2 Volts p-p) transmitted over 75 ohm coaxial cable with BNC connectors.

Basics

for more information on the AES3 format

Please also see serial data, bit clock, and I2S for more details.

By design; the AES3 and S-PDIF formats are similar enough to be "compatible" in many cases. The difference in level has been addressed by the designing the sending and receiving IC's with enough range to discriminate the lower level consumer format without being overloaded by the higher level professional format. For this reason, Lavry AES/XLR digital audio inputs and outputs are compatible with the vast majority of S-PDIF inputs and outputs using simple wired adapters or adapter cables.

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