Published: TV Technology magazine

Issue: 20 April 98



by  Joe Fedele



Defining the Issues - What is AES Audio?             



  AES audio (a.k.a AES/EBU) is a digital audio transmission standard developed jointly by the Audio Engineering So-

ciety (AES) and The European Broadcasters Union (EBU). Each AES bitstream is capable of carrying two audio chan-

nels, which may be used as either a stereo pair or two distinct audio paths as a PCM datastream carries the digital 

information. Distribution of the digital bitstream can be facilitated via coaxial cable or standard twisted-pair audio ca-

ble.

  The coax method is defined in the AES-3id and SMPTE 276 standards and specifies 75-ohm unbalanced cable with a 

1 V peak-to-peak level. AES-75 uses standard video DAs, BNC connectors, routers and patching for its distribution 

through a facility. Distribution of AES-75 range in distances of between 1,000 feet for unequalized lines and 5,000 

feet or better when reclocking and equalization methods are employed.

  Balanced 110-ohm AES is defined in the AES3-1992 standard. Line levels of between 3 and 7 volts are typical for 

short runs of twisted-pair cable. Both XLR- and DB25-type connectors are used for termination between units. AES 

balanced audio is intended for shorter runs when compared to that of coaxial cable with typical distances of between 

1,000 feet with no EQ and 3,000 feet or better when reclocking and gain control are used.

  Conversion between the two impedances is easily accomplished via passive adapters and other devices. A variety of 

attenuating and non-attenuating matching transformers, ranging in price from about $50 to $80 each, are available 

from several vendors. 

  Digitized sampling rates include 32 kHz for Digital Audio Broadcasting (DAB), 44.1 kHz for CD players and con-

sumer products, and 48 kHz for high-end digital VTRs and other applications. Quantizing levels of 20 or 24 bits are 

also specified in the AES standards.

  Synchronization of all AES sources is important for its proper operation as both sampling and phase of the audio is 

locked to a common source. In A/V applications, the audio must be locked to the video reference as this is vital to en-

sure that audio and video frames line up correctly.

  Both asynchronous and synchronous routers may be used for AES switching and distribution. Asynchronous analog 

routers that work with both balanced and unbalanced lines are the most cost-effective systems. Synchronous routers 

tend to be up to 50 percent more expensive but their retimed and reclocked inputs reduce the popping and clicking 

noise common in asynchronous ones.