Cirrus Logic AN22 User Manual

AN22

Application Note

OVERVIEW OF DIGITAL AUDIO INTERFACE

DATA STRUCTURES

Clif Sanchez & Roger Taylor

The following information is pro vided for conve­nience, but by no mean s con stitu tes the enti re sp ec­ification. Also included is information from the
IEC958 and the new AES3-199x a nd 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 ac­tual sp ecification should be obtained from the Au­dio Engineering Society or ANSI (ANSI
S4.40-1985) for the AES3 document, and the Inter­national Electrotechnical Commission for the
IEC958 docume nt.

The AES/EBU interface is a means for serially
communic ating dig ital audio da ta through a si ngle
transmission lin e. I t pro vides tw o cha nn els f or au­dio data, a me thod for commun icating control in ­formation, and some error detection capabilities.

Cirrus Logic, Inc.
Crystal Semiconductor Products Division

P.O. Box 17847, Austin, Texas 78760
[phone] FAX: [phone]
http://www.crystal.com

The control information is transmitted as one bit
per sample and accumulates in a block structure.
The data is bipha s e encoded, which enables the re­ceiver to extract a clock from the data. Coding vio­lations, de fined as pream bles, are used to identify
sample and block boundaries.

Frames Sub-frames and Blocks

An audio sample i s place d in a stru ct ure kn own as
a sub-frame. The sub-frame, shown in Figure 1,
consists o f 4 bits of preamble, 4 bits of auxiliary
data, 20 bits of audio data, 3 bits called validity, us­er, and channel status, and a parity bit. The pream­ble contains biphase coding violations and
identifies th e star t of a sub-fram e. The audio sa m­ple word length can va ry up to 24 bits and is trans­mitted LSB fi rst. If th e wor d lengt h is grea ter th an
20 bits, the sample occupies both the audio and

Copyright  Cirrus Logic, I nc. 1998

(All Rights Reserv ed)

AN22REV2

FEB ‘98

1

AN22

bit

0347

Aux Data

Channel A

X

Y

8

LSB

Channel B

Figure 1. Sub-frame Format

Channel AZChannel B

Sub-frame Sub-frame

Figure 2. Frame/Block Format

auxiliary data fields. If it is 20 bits or less, the aux­iliary field can b e used for other a pplica tions such
as voice. T he parity bit g enerates even pa rity and
can detect an odd number of transmi ssion errors in
the sub-frame. The validity bit, when low, indicates
the audio sample is fit for conversion to analog.
The user an d channel sta tus bits are se nt once per
sample and, when accumulated over a number of
samples, define a blo ck of da ta. The user bit ch an­nel is undefined and available to the user fo r any
purpose. The channel status bit conveys, over an
entire block, important information about the audio
data and t ransmissi on link. Eac h of the tw o audio
channels has its own channel status data with a
block structure that repeats every 192 samples.

As shown in Figure 2, two consecutive sub-fram e s
are defined as a f rame , cont aining channel s A and
B, and 192 frame s define a block. The preambles
that identify the start of a sub-frame a re different
for each of the two channels with another unique
one identify ing the beginning of a channel sta tus
block.

Sub-frame

Audio DataPreamble

Channel Status Data

Preambles

Y

Frame 0Frame 191

Start of Channel Status Block

Channel A

X Channel BY X

Validity

User Data

Parity Bit

Frame 1

27

MSB

28 29 30 31

VUCP

Modulation and Preambles

The data is t ransmitted w ith biphase-m ark encod­ing to minimize the DC component and to allow
clock recovery from the data. As illustrated in
Figure 3, the 1’s in the data have transitions in the
center, and the 0’s do not, after biphase-mark en­coding. Also, the biphase-mark data switche s po­larity at every data bit boundary. Since the value of
the data bit is determined by whether there is a tran­sition in th e cen te r of th e bit , th e ac tu al po la rity of
the signal is irrelevant.

Each sub-frame starts with a preamble. This allows
a receiver to lock on to the data within one
sub-frame. The re are three defin ed pream bles: one

(2 times bit rate)

Biphase-Mark

Clock

100011

Data

Data

101100110101

Figure 3. Biphase-Mark Encoding

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AN22

Biphase Patterns Channel

X 11100010 or 00011101 Ch. A
Y 11100100 or 00011011 Ch. B
Z 11101000 or 00010111 Ch. A & C.S. Block Start

Table 1. Preambles

for each chan nel an d one to ind icat e the b eginn ing
of a channel status block (which is also channelA).
To distinguish the preambles from arbitrary data
patterns, the preambles contai n two biphase-mark
violations. Bi pha se-mark data is requir ed to transi­tion at every bit period, but each preamble violates
that requirement twice. In Figure 3 each bit bound­ary, indic ated by the d ashed l ines, c ontains a tran ­sition in the bipha se da ta. Each preamble shown in

byte/bit

0

1
2

3
4
5

0123

PRO=1

Reference

Audio

Channel Mode

AUX Use Word Length

Emphasis

11100010

Preamble X

11100100

Preamble Y

11101000

Preamble Z

Figure 4. Preamble Forms

Figure 4 has two bit boundaries with no transition,
which enables the receiver to recognize the data as
a preamble.

block

bit

7

7
15

23
31
39
47

Reserved

Reserved

4

Reserved

56

Lock

User Bit Management

Fs

Reserved

6
7

8

9

10
11

12
13

14

15
16

17
18

19

20
21

22
23

Alphanumeric channel origin data

Alphanu m er i c chan nel destina tion data

Local sample address code

(32-bit bina ry)

Time of day code

(32-bit binary)

Reserved

Cyclic redundancy check character

Reliability flags

Figure 5. Professional Channel Status Block Structure

55

87

119

151

183
191

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