AES3

AES / EBU ( Audio Engineering Society / European Broadcasting Union ) is a common name for the specification of the interface for transmitting digital stereo, two-channel or mono audio signals between various devices according to the standard AES3. It is used mainly in the professional recording studio environment. Not to be confused is the name given to the encryption algorithm Advanced Encryption Standard.

AES3 uses symmetric cable with 110 ohms or in the AES3 id specified unbalanced coaxial cable with 75 ohm characteristic impedance and twisted- pair cable. As connector is used in balanced cables XLR and unbalanced BNC coaxial cables. With the use of high-quality coaxial cables ( and connectors with 75 ohm characteristic impedance) can be the lengths of over 300 meters.

Due to the increasing quality in LAN systems (also LAN cable category 5 or 6, colloquially known ) are in the beginning of the 21st century increasingly high-quality twisted- pair cable used for AES / EBU cabling. Your wave impedance is within the specified values ​​for AES3.

Structure of the interface

The transmission format of AES3 interface is divided into blocks, frames and subframes, as shown in the adjacent figure. A block contains 192 frames, it consists of 2 sub-frames, each frame. An audio sample with 16, 20 or up to 24 bit dynamic and with 4 information bits transmitted per subframe. The information bits in a so-called channel state is transmitted, among other things, includes information about the type of audio data. The remaining bits of a 32 -bit time slots are used to synchronize the frames and the protection against errors. A stereo channel can be transferred per frame.

The usual sampling frequencies of the audio signal is 32 kHz, 44.1 kHz, 48 kHz and 96 kHz. In addition, sampling rarely used as 88.2 kHz and 192 kHz are supported. The word length of the samples may be in the range of 16 bits to 24 bits.

The channel coding of audio data is done using biphase mark encoding. The signal to be transmitted is no direct component and can be transmitted to avoid hum loops electrically isolated by pulse transformers. Electric uses the AES3 interface level and driver according to the standard EIA- 422 ( RS-422). By the type of channel coding can be recovered at the receiving end of the sampling clock by a PLL signal from the AES3.

Furthermore, not only PCM encoded audio signals are transmitted over this interface transfer, but also for example Dolby E or Dolby Digital encoded multichannel audio data - but these definitions are no longer part of the AES3 specification and are available in standard SMPTE 337m set.

That defined in the AES3 bit stream may also be tunnelled in various other interface protocols, that is, the AES3 signal is again wrapped within the other protocol, and possibly also multiple bundles. Examples include MADI, IEEE 1394, AES50.

The interface AES / EBU has been standardized by several organizations in approximately the same way:

  • By the AES as AES3
  • Of the EBU Tech - 3250E
  • By the IEC as IEC 60958-4

The AES42 standard for digital microphones is based on the standard of AES3.

Signal quality

The digital signal transmission can especially with long and inferior cables lead to a signal mutilation: If the recipient of the bit stream with the help of the checksum no longer 100 % reconstruct it comes to signal degradation. It manifests itself by loss of amplitude resolution ( dynamics) and temporal desynchronization ( Glitch ). The signal sounds rough and then optionally is interrupted by clicks. Remedy in this case, higher-quality cables that are specified for the digital transmission ( wave impedance of 110 ohms) and the avoidance of adapters and extensions in the digital signal path.

Generally, however, the quality of digital transmission is lossless and therefore in most circumstances much better than with analog transmission.

Synchronization of the audio clock

If several digital sources used in a production environment, so the PCM streams have to be gen-locked. In this case, let professional PCM signal sources (A / D converter, digital tape machines, etc.) by a so-called house sync generator externally betakten to ensure synchronization of the PCM Bitstöme. Desynchronized digital signal sources manifest themselves as audible clicks through to the noise with a complete loss of signal (see also jitter).

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