1474 Chapter 39
39.5 AES42 (AES3-MIC)
AES42 (AES3-MIC) is a variant on AES3 designed to
meet the needs of interfacing microphones that have
direct digital outputs. The first most significant differ-
ence is that the transmitter and receiver use center
tapped (on the cable side) transformer, which allow a
digital phantom power (DPP) of +10 (+0.5, –0.1) Vdc at
250 mA to be supplied to the microphone. No more
than 50 mVp-p ripple is allowed on the DPP. The
microphone may draw no less than 50 mA, or more than
250 mA from the DPP, and may not present a load in
excess of 120 nF to the DPP. The microphone must not
be damaged by the application of any of the analog
microphone phantom powers specified by IEC 61938
including common 48 V phantom. The techniques
described by this standard may be applied to portable
AES3 output devices other than microphones, however,
AES42 only covers microphones.
Optionally a modulation from +10 to +12 V
(resulting in a peak current of 300 mA) may be applied
to the DPP for remote control purposes. This modulated
signal thus travels in common mode from the
AES3-MIC input back to the AES3-MIC microphone
over the same cable that is carrying the AES3 audio data
from the microphone to the AES3-MIC input. Because it
is sent in common mode, the data rate must be far slower
than that of AES3 to avoid interference. If the AES3
frame rate (FR) is 44.1 kHz or 48 kHz, the bit rate of the
remote control signal is FR/64 bits per second (bit/s).
For a FR of 88.2 kHz or 96 kHz the remote control bit
rate is FR/128 bit/s. For a FR of 176.4 kHz or 192 kHz
the remote control bit rate is FR/256 bit/s. As a result,
the remote control bit rate is 750 bit/s if the AES3 FR is
48 kHz, 96 kHz, or 192 kHz, and 689.06 bit/s if the FR
is 44.1 kHz, 88.2 kHz, or 176.4 kHz.
The remote control signals are sent as required,
except if used for synchronization, in which case they
will be sent on a regular basis of not less than six times
per second.
The following information is based on AES42-2006.
It is always advisable to obtain the latest revision of the
standard.
39.5.1 Synchronization
There are two primary possible modes of operation for a
microphone meeting the AES42 Standard, Fig. 39-12.
Mode 1 allows the microphone to free run at a rate
determined by its own internal clock. No attempt is
made to lock the microphone’s clock rate to an external
clock, and if such a lock is desired, sample rate conver-
sion must be performed external to the microphone.
This technique is the simplest way for an AES3-MIC
microphone to operate, and does not require the use of
the optional remote control signal.
Mode 2 uses the remote control signal to send data
back to the microphone that allows its sampling rate to
be varied, and phase locked to an external reference. The
mode 2 microphone (or other AES3-MIC device)
contains a voltage controlled crystal oscillator (VCXO),
which has its frequency controlled by a digital to analog
converter (DAC). The DAC receives control information
via the remote control signal from the AES3-MIC
receiving device. The receiving device compares the
current sample rate of the microphone to the external
reference and uses a phase locked loop (PLL) to generate
a correction signal, which is sent back to the micro-
phone. This results in the sampling rate of the micro-
phone becoming frequency and phase matched to the
reference signal. If multiple microphones or other
AES3-MIC mode 2 sources are locked to the same refer-
ence, this has the additional advantage of providing a
consistent and near zero phase relationship between the
sampling times of the various sources. When multiple
microphones sample correlated signals, for example, in
stereo or multichannel recording techniques, this results
in stable imaging.
If the receiver does not support mode 2 operation,
the mode 2 microphone automatically reverts to mode 1
operation.39.5.2 Microphone ID and Status FlagsAES42 defines the use of the user data channel in AES3
to optionally allow the microphone to identify itself and
send back status information. Imagine the benefits in a
complex setup of not having to worry which input a
given mic is plugged into. The receiving device could
use the microphone ID information to automatically
route the microphone to the correct system input, no
matter to which physical input it was connected.39.5.3 Remote ControlAES42 defines three possible sets of remote control
instructions, simple, extended, and manufacturer
specific. If a device supports the extended instruction
set, it must also support the simple instruction set. If a
device supports at least the simple instruction set, it
must have predetermined default settings it enters if no
instructions are received on power up. If a device has
switches on it, those will have priority over received
instructions.