Handbook for Sound Engineers

Digital Audio Interfacing and Networking 1467

designed for use at distances up to “a few hundred
meters.”

If improved performance beyond that of CCITT V.11
is desired, it is suggested but not required that the circuit
in Fig. 39-8 be used.

Series capacitors C 2 and C 3 block external dc from
flowing through the transformers. This protects the
transformers from damage or performance degradation

if dc is applied to them. The AES42 (AES3-MIC)

Digital Interface for Microphones Standard calls for

digital microphones to be powered by 10Vdc digital

phantom power that is a variation on the phantom

power scheme used for analog microphones. This

provides another excellent reason to provide dc

blocking on all AES3 inputs and outputs, transformer

based or not, to prevent damage if such a phantom

power scheme were to be applied. Structured wiring

using Cat5 or high rated cable and RJ45 connectors is

now a permitted alternative interconnect scheme for

AES3 signal. These interconnects are also used for

Ethernet, which may have power over Ethernet (PoE)

applied. They are also used for plain old telephone

service (POTS), which will have 48 V battery and 90 V

ring signals. If structured cabling is used for AES3,

consideration must be given to the survivability of the

line driver and receiver circuits if accidentally intercon-

nected to PoE or POTS lines.

Transformers will make possible higher rejection of

common mode interfering signals, electromagnetic

interference (EMI), and grounding problems than

common active circuits. The European Broadcasting

Union (EBU) in its version of this standard (EBU Tech.

3250-E) requires the use of transformers. This is the

major difference between the standards. It is common to

see the AES3 Standard referred to as the AES/EBU

Standard even though that is not strictly correct since

AES3 makes the transformers optional, while the EBU

requires them.

Figure 39-7. AES3 Channel Status Data Format. Note both
the bits and bytes are numbered starting with 0.

Byte Bit 0 1 2 3 4 5 6 7

a b c d e

f g

h i r

j

k r

r

Alphanumeric channel origin data

Alphanumeric channel destination data

Local sample address code

(32-bit binary)

Time-of-day sample address code

(32-bit binary)

Reliability flags

Cyclic redundancy check character

0 1 2 3 4 5 6 7 8 9

10

11

12

13

14

15

16

17

18

a. Use of channel status channel

b. Audio/nonaudio use

c. Audio signal emphasis

d. Locking of source sample frequency

e. Sampling frequency

f. Channel mode

g. User bit management

h. Use of auxiliary sample bits

i. Source word length and source encoding history

j. Future multichannel function description

k. Digital audio reference signal

r. Reserved

22

19

20

21

23

Table 39-1. Channel Status Data Format Details

Byte 0

Bit 0 0 Contents of the channel status block con-

form to IEC 60958-3 “consumer use” Stan-

dard. Ignore the rest of this table. (See Note

1.)

1 Contents of the channel status block as to the

AES3 “professional use” Standard.

Bit 1 0 Audio words consist of linear PCM samples.

1 Audio words consist of something other than

linear PCM samples.

Bits 2–4 Encoded Audio Signal Emphasis

Bit 2 3 4

State 0 0 0 No emphasis indicated. Receiver defaults to

no emphasis but may be manually overrid-

den.

1 0 0 No emphasis used. Receiver may not be

manually overridden.

1 1 0 50/15 Ps emphasis used. Receiver may not

be manually overridden.