The  D21m high-density audio interface system has been designed to provide the highest quality analogue, digital and signalling interconnections between external equipment and all current and future  DSP cores, including those serving the Vista range of consoles.

The system is based on a 19" 3U rack which can house up to 12 audio interface cards and one or two HD (high-density) cards. Each HD card carries up to 192 channels in Vista systems- and up to 384 channels in Router systems at sampling rates of up to 96kHz, and is connected to the DSP core by standard CAT-5 twisted twin cables. The system automatically detects newly inserted audio cards and assigns the required number of audio channels to them when the system is reconfigured, either by pressing the front panel "reconfig" button or remotely. All functions and configurations of the D21m are controllable remotely from the associated console or a workstation.

An LED display on the front panel indicates failed or missing cards, and error messages are sent to the console over the control interface. The D21m rack provides space for two power supply units in redundant operation for critical applications.