PREVIOUS NEXT UP TOP
received from chris brown on aug 16 1995 copyright 804w

[ written before sept 87 ]

A Midi Controlled 4x8 Matrix Mixer

During the last two years I have been developing music on a homemade signal processing instrument that allows parameters of an improvising musician's sound to continuously control the electronic alteration of that sound. This arises from the philosophy that the most expressive controllers for manipulating a new electronic instrument may be those that musicians have already learned to control with great detail, and for which strong physical/aural correspondences have already been developed.

In my system "patches" relating the sound parameters to the control of specific effects are sequenced, so that these relationships change rhythmically during the course of a piece. For example in one section a voltage tracking the performer's pitch may be used to control delay time (so that high sounds are repeated with longer delay than low ones) , and in the next section it may be used to control the delay regeneration (so that high sounds resonate longer than low ones). The same parameters are also used to control which of eight signal processing effects I use are gated into the final output mix. Within one patch this may mean that each specific frequency will have a specific combination of effects associated with it, and so a melody will imply a specific sequence of effects combinations that track with it.

I have found this particular aspect of the system to be musically very rich, and would like to develop an instrument to do this that is compatible with the MIDI environment, making it more generally useable by other composers.

My idea is to build a signal routing submixer, rather than an automated mixdown station for recording and PA uses, which is being developed by commercial manufacturers (such as the currently available device from AKAI) . As such it would not include any faders, pots, or other physical override of its electronic controls, nor any equalization or special monitoring functions. I plan to use 32 on board VCA's, made with DBX 2150 series chips (7 pin inline and available in quantity from DBX for a little over $3 each), controlled by a multiplexed 8-bit DAC with hardware refresh.

The DAC uses MIDI note values from 0-31 (or other 32 note blocks, readdressable with minidip switches) to address each VCA. The voltage level is converted from the velocity byte value, giving a resolution of 127 volume levels, from fully off to a gain of 2. The VCA board should also have 44 SPDT analog switches that can configure the 32 VCA's in two basic ways: 1) there are eight inputs, which could be either 8 different instruments or 8 different effects, all with the same input (my current system is usually used in this way), each input being connected through a VCA to four output mixers for either a quadraphonic sound system or for later mixdown to two; 2) there are four inputs for four separate instruments (or mixes of instruments) each connected through eight different VCA's to eight outputs, which will be connected to the inputs of 8 (or less) signal processing effects. The second arrangement could be used for ensemble pieces and would allow for any instrument to pass through any effect or combination of effects - this is something that I haven't been able to do yet with my current system and want badly to be able to do!

I would imagine construction involving two separate PC boards. The first contains the MIDI-> control voltage and address circuitry, as well as the DAC, the multiplexers, and sample and hold buffers. This is the board that I would like to realize at STEIM (and that I need help with!). The CVs could come out on a card edge, or have a set of plug in jumpers to connect them to the VCA board, which will hold the 32 VCAs. the 44 analog switches, eight output mixers, and eight input buffers. This board I am in the process of designing and am hoping to have operational by September 1987.

Two board construction would allow for the first board to be used to control other "second" boards besides the VCA matrix, should I or someone else get around to making another. While at STEIM I would hope to be working on software for controlling the board, and coordinating it with a analog inputs to a computer (probably MacIntosh, though the software/hardware package should be relatively portable to other machines). For non-interactive uses of course ordinary MIDI sequencing software could be used.

Finally the two boards will require a hefty power supply and and enclosure with 14 input/output jacks ( probably phone). The whole would hopefully be perhaps as small as a Commodore 64 disk drive, and not larger than the proverbial breadbox.


TOP OF DOCUMENT