AES-3,  AES-EBU,  Digital Audio

The AES-3 standard was specified in 1985, revised in 1992 and in 2003.

- 110-ohm twisted pair cabling with XLR connector
- 75-ohm coaxial cable with BNC or RCA connector
- optical fiber

Basic Specification for the 100-Ohm balanced version:
- balanced transmission
- 2 channels are transmitted together (channel A and channel B)
- XLR connector, Pin1 = screen, PIN2 = signal (+), PIN3 = signal (-)
- impedance: 110 Ohms (± 20%)
- signal level 2 - 7 Vpp
- cable length: 100m and more
- not limited to one sample rate and one bit rate
- extended channel status information
- transmitted signal is free of any DC component
- transformers can be used / are used
- clock information and the sample clock is carried within the signal
- signal can be reversed in polarity without data loss

The second AES-3 specification is the transmission over 75 Ohm coax cable. It is called AES3-id.
The difference between the AES3 standard and the European version AES-EBU is only the use of coupling transformers at both ends of the transmission. These coupling transformers are optional for AES3 but mandatory for AES-EBU.

The AES signal is transmitted via the bi-phase-mark coding scheme. With the bi-phase-mark coding each bit boundary is marked by switching the signal polarity. A digital ´1´ is determined by an additional transition in the middle of a ´1´ bit. A digital ´0´ doesn't change polarity.
The two channels are called channel A and channel B and are embedded into their subframes within one AES3 frame. Channel A and channel B are transmitted alternately.
Originally 20 audio bits were defined and 4 aux bits. These aux bits are used today to transmit 24 audio bits.
Each subframe starts with a SYNC header consisting of 4 bits, the so-called X-preamble, Y-preamble and z-preamble. A subframe starting with the sync header 'X-preamble' is marked as channel A, a subframe starting with the sync header 'Y-preamble' is marked as channel B. A subframe consists of 32 bits.

Two subsequent subframes of channel A and channel B form one AES3 frame. Both channels A and B can carry a stereo signal or two completely different audio signals.
The AES-3 transmission system (in 110-Ohm and in 75-Ohm) are matched impedance systems. With these systems the transmission distance is not a function of the transmitter, it is only a function of the cable losses and the characteristics of the receiver. 110-Ohm AES-3 supports transmission distances up to 100 meters. The 75-Ohm coax AES-3 version supports up to 1000 meters (with cable EQ).

Additional Data

A parity bit (P) is included to detect transmission errors.

The validity bit (V) is defined to mark invalid or defective sample values. Samples flagged invalid should not be used any further in the signal processing. But the validity bit is used in a slightly different way today. The validity bit is also set to '1' to mark compressed audio like MP3 etc.

Each subframe contains also one user bit (U).

The channel status information (C) is carried in an entity of 192 subframes. Each subframe carries just one channel status bit. After 192 subframes the channel status information is complete. A new cycle starts with the sync header 'Z-preamble' on channel A.

Channel Status Information
There are two types of channel status information for two different user groups. These basic channel status information is set in the first bit of the channel status information: '0' indicates the consumer standard, used for S/PDIF and TOSLINK, '1' indicates a professional signal. In the consumer data stream the channel status information is used for copy protection. For professional use, information about the signal source, quality, type etc. can be transmitted.
Reading the '0' as consumer data, several professional devices don't accept the data stream at all.

See: AES-EBU Channel Status Byte Settings

Equipment Classification

All AES3 and AES-EBU equipment is classified in the groups A, B1, B2, B3, C1, and C2. The differences are the way, how the equipment handles the data and what data are transmitted at all.

AES3 equipment group A
Equipment of group A simply passes the input signal without any processing to the output.

AES3 equipment group B1
Group B1 equipment only deals with the audio signal, no channel status data is transmitted. (This kind of equipment can cause problems in a signal chain, because following devices may not recognize the data stream.)

AES3 equipment group B2
Group B2 equipment decodes channel status, but does not transmit the channel-status data.

AES3 equipment group B3
Equipment of group B3 reads and transmits channel status information and reacts to the channel status data. - The preferred equipment characteristics.

AES3 equipment group C1
Equipment of group C1 is terminal end equipment as digital–analog converter. It decodes audio signal only, no channel status data.

AES3 equipment group C2
Equipment of group C2 decodes digital audio and reacts on the received channel data information. - The preferred equipment characteristics.