Auxbar: Audio Crossbar Network
All three units are shown in the "Grand Central Station" photo at right. In the foreground, the matrix of DB-9 connectors is the Sexbar, a 32-channel serial crossbar network. To the right is a dedicated New Micros 68HC11 FORTH board, with the Vixbar, or video crossbar, implemented in its kluge area (the fat shielded cable bundle emanating from the back of the board terminates at a panel of 24 RCA connectors, providing 16 video inputs and 8 outputs).
The large board stack at the rear of the photo is the Auxbar, which allows on-the-fly establishment of up to 8 simultaneous connections among any of 32 sources and 32 sinks. These are invoked by serial commands to the microcontroller that runs the unit, with a typical format like this:
speech cellular aconnect
This looks up the numeric channel addresses for the speech synthesizer and the cellular phone's microphone input, finds the first free bus (of 8 available), and performs a connection between them that persists until issuance of an explicit disconnect command or a reset of the whole unit.
Overall Auxbar performance is excellent, with no observable insertion loss, introduced noise, or distortion. This is largely attributable to the wizardry of Steve Sergeant, an audio engineer who did the entire analog circuit design for us... low-noise amplifiers on both input and output sides of the system that normalize everything to line levels and standard impedance.
The crossbar chips used in the Auxbar are MT8816, each an 8x16 matrix of FET switches. Each of the two boards carries a pair of these, with the 8 "column" lines bussed together and the 16 "row" lines tied to a bank each of inputs and outputs. The column lines are further brought to RCA jacks on the edge of the board, allowing them to be jumpered to one or more additional boards to expand the crossbar network. Clearly, this increases the channel count but does not affect the number of connections that can be active at any given moment (8).
To better understand the architecture of this system, please refer to the "crossbar backgrounder" on the Sexbar page.
Here's a schematic of the core circuitry of the audio crossbar network.
