A digital microphone (today) is still an analog microphone capsule with very early conversion into the digital domain.
|general setup for digital microphones|
A proposal for a standard for digital microphones was first introduced in 1999 by Steven Harris (AES reprint 4518 (B-8):
- A/D conversion remotely controlled
- optional synchronization
- phantom power supply over the AES3 output
- use of regular XLR connector
In 1999 the AES proposal AES42-2001 was introduced. Now external synchronization was further detailed. A new connector, called XLD, was designed. This was necessary because connecting an analog device to a digital microphone input could destroy the analog device because of the supplied voltages. The next standard revision followed in 2006 (AES42-2006).
As the signal stream the regular AES3 stream is used, with the 'user bits' used for information about microphone and settings.
See: AES3 Channel Status
The digital phantom power is 10V +/- 0,5V with maximum current of 250mA. It is called DPP (digital phantom power). The voltage carries also a modulated signal for microphone control (common mode modulation of +2V with a data transmission of 689 to 750 bps). For input and output connection of an AES42 line always a transformer is needed for the coupling of the phantom power (the usual procedure with resistors as in analog equipment would not work because of the high current necessary for the digital components).
Sampling rates are 44,1kHz, 48kHz and rates up to 192kHz with 24bit resolution. A digital microphone is compatible to AES42 if the microphone complies to the DPP (digital phantom power) and the AES3 specifications.
There are two synchronization modes: Mode1 and Mode2:
- Mode1: no synchronization supplied to the microphone, eventually a sample rate conversion is needed at the input stage of the following device
- Mode2: the master clock in the receiving device sets the synchronization for the connected microphone
Command set for control information via modulated transmission over phantom power:
- signal attenuation
- directional pattern
- lo-cut filter
- mode2 on/off
- additional extended command set