Digital Audio

Digital audio uses the same electronic techniques as digital video, so you already understand most of the subject. Digital audio in Television conforms to the AES/EBU standard, for Audio Engineering Society/European Broadcasting Union. AES/EBU digital audio carries two channels (they can be a stereo pair, Sum and Difference, or two entirely different signals). 

Sum and Difference

Some systems, rather than using distinct left and right audio signals, use a system of sum and difference, which will bring back memories of Y, R-Y, B-Y matrixing. One channel carries the sum of the left and right stereo signals, and can be received by mono equipment as the monaural mix. The other channel carries the left channel minus that right channel. Remember, this is easy to do in electronics by inverting one of the components, and adding them together. The L-R signal is often hidden on a subcarrier that is ignored by monaural equipment. Stereo equipment re-matrixes the two to recover left and right information. FM radio and TV audio use this system, for compatibility with monaural receivers.

Sample Rate

AES/EBU audio can use sample rates of 32, 44.1, or 48 Khz (thousand samples per second). 32khz is considered by many to be adequate for television audio. 48khz is the "Cadillac" sampling rate, and 44.1khz is directly compatible with CD audio, and some early digital-audio-for-television rates. 48 thousand samples per second is the most common sample rate.

Bits

Audio samples are usually coded as two-bytes (16 bits), giving the signal 65,536 levels (2 to the 16th power). AES/EBU allows for up to 24 bits, giving, well, and astounding amount of levels!

24-bit audio seems absurd at first, but in the production environment, audio is used in complex mixes. A music track that is 20db below a narration track, or a background effect that is at -40 ("down in the mud") isn't using the full stretch of bits, so more is better.

There are many ways to move audio digits around.

		Consumer equipment uses the SPDIF format (Sony/Phillips Digital InterFace) using RCA connectors, or Optical cable.
		AES/EBU audio uses ordinary xlr-3 audio connectors, or BNC connectors on video cable.
? ? ?	? ? ?	AES/EBU audio can also be carried on NO cables. Details in the next section.