Cirrus Logic AN22 - Manual

1
Copyright  Cirrus Logic, Inc. 1998
(All Rights Reserved)
Cirrus Logic, Inc.
Crystal Semiconductor Products Division
P.O. Box 17847, Austin, Texas 78760
(512) 445 7222 FAX: (512) 445 7581
http://www.crystal.com
AN22
Application Note
OVERVIEW OF DIGITAL AUDIO INTERFACE
DATA STRUCTURES
Clif Sanchez & Roger Taylor
The following information is provided for conve-
nience, but by no means constitutes the entire spec-
ification. Also included is information from the
IEC958 and the new AES3-199x and TC84 docu-
ments. The AES3-199x and TC84 documents have
not received approval as of the printing of this data
sheet. To guarantee conformance, a copy of the ac-
tual specification should be obtained from the Au-
dio Engineering Society or ANSI (ANSI
S4.40-1985) for the AES3 document, and the Inter-
national Electrotechnical Commission for the
IEC958 document.
The AES/EBU interface is a means for serially
communicating digital audio data through a single
transmission line. It provides two channels for au-
dio data, a method for communicating control in-
formation, and some error detection capabilities.
The control information is transmitted as one bit
per sample and accumulates in a block structure.
The data is biphase encoded, which enables the re-
ceiver to extract a clock from the data. Coding vio-
lations, defined as preambles, are used to identify
sample and block boundaries.
Frames Sub-frames and Blocks
An audio sample is placed in a structure known as
a sub-frame. The sub-frame, shown in Figure 1,
consists of 4 bits of preamble, 4 bits of auxiliary
data, 20 bits of audio data, 3 bits called validity, us-
er, and channel status, and a parity bit. The pream-
ble contains biphase coding violations and
identifies the start of a sub-frame. The audio sam-
ple word length can vary up to 24 bits and is trans-
mitted LSB first. If the word length is greater than
20 bits, the sample occupies both the audio and
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AN22REV2