The audio acquisition equipment consists of the following,

• 24 Sennheiser MKE 2-5-C miniature electret microphone
• Custom-built microphone power box
• 3 PreSonus Digimax preamplifier/digitizer
• 1 Mark of the Unicorn 2408mkII PC interface
• 1 Windows PC with Cakewalk SONAR recording software

Each component is described in detail below.

4.1 Sennheiser MKE 2-5-C

The Sennheiser MKE 2-5-C is a high quality miniature electret microphone. It has an approximately linear   frequency response between 20Hz and 20kHz and omni-directional characteristics. It has a very high sensitivity of 31mV/Pa, making it a good choice for use in microphone array applications where the desired source is not in the immediate vicinity of the array. The diameter of the microphone is 6mm.

4.2 Custom Microphone Power Box

The disadvantage of the MKE 2-5-C is that requires a separate DC bias voltage in order to operate.

It cannot be plugged directly into the microphone preamplifiers. Due to the large number of microphones and the high cost of biasing modules produced by Sennheiser, it was decided to build a custom power box into which all microphones were input. A 12V DC adaptor is used with appropriate circuitry to supply each microphone with the correct biasing voltage. The power box also merges the cabling for all microphones into a 12m multicore cable that runs to the rest of the audio acquisition equipment. The use of the multicore cable considerably simplifies the microphone cabling requirements and increases the freedom of microphone placement.

4.3 PreSonus Digimax

The PreSonus Digimax is a high-quality, 8-channel microphone preamplifier, with integrated 24-bit digitization. Separate gain controls are provided for each input channel and the digital output is via a single ADAT lightpipe (optical fibre) containing all 8 channels. The Digimax is able to sample at rates of 32kHz, 44.1kHz and 48kHz, using either its own internal clock, or a wordclock supplied by an external source. The Digimax uses an external power supply to ensure noise-free amplification and digitization across all input channels. The 3 Digimax units in the Smart Meeting Room are housed in a 19” rack. The 24 input channels in the multicore cable from the microphone power box are fanned out inside the rack and connected to the individual inputs of the 3 Digimax units.

4.4 Mark of the Unicorn (MOTU) 2408mkII

The MOTU 2408mkII is a flexible interface for PC hard disk-based audio recording. It consists of a 19” rack-mounted I/O unit connected via a firewire-like (audiowire) interface to a PCI card installed in a PC. The I/O unit has 24 inputs and outputs, divided into three banks of 8-channels. Each bank has a range of input and output options, including analog, S/PDIF, TDIF, or ADAT lightpipe and all inputs and outputs can operate simultaneously. The PCI card can accommodate up to three I/O units, allowing up to 72 simultaneous input and output audio channels. Driver software installed on the PC containing the PCI card allows configuration and acquisition to be controlled through software.

In the Smart Meeting Room acquisition system, the ADAT lightpipe outputs from the 3 PreSonus Digimax units are connected to the 3 input banks on a single MOTU 2408mkII I/O unit.

4.5 Windows PC

The PC used with the MOTU 2408mkII is a Windows based variant for compatibility with the MOTU 2408mkII driver software. The recording software used to control audio recording is Cakewalk SONAR. A graphical user interface allows the user to easily configure recording sessions on the MOTU 2408mkII and to view and export the acquired data. It is also compatible with the synchronization hardware described in section 10.