Abstract: A new and more efficient filtering system (e.g., digital microphone decimation filter architecture system) is described. A key to this architecture is the use of two parallel filter paths. Each path operates at the output sample rate, and comprises a shorter FIR filter followed by a series of allpass stages (e.g., implementing IIR filters). The FIR filter is designed to remove all but the last octave of out-of-band noise. The allpass stages are designed such that when the two paths are summed together, the out-of-band noise for the final octave cancels out, leaving only the desired sign.

Abstract: An electronic music synthesizer including a keyboard control system for enabling multiple independent voice channels (voices) to be controlled by the keyboard in a musically pleasing manner. The control system responds to a keyboard of M keys to control N voices where N<M; e.g. M=64 and N=10. The voice channels are preferably identical to one another, each being comprised of voltage controlled elements such as an oscillator (VCO), amplifier (VCA) and filter (VCF). Typically, the control system selects an available voice channel and in response to a key depression, supplies a DC control voltage thereto whose level is nominally linearly related to the note corresponding to the depressed key. In addition to the control voltage, the control system supplies a gate signal to the selected voice channel indicating the time duration of the key depression.

Abstract: A current controlled variable transconductance amplifier having a differential input stage and a current source output stage is employed with negative feedback, either through a buffer amplifier or directly, to provide various all-pass, low-pass or high-pass filter configurations, each with controlled bias current to the differential input stage for current control of the variable transconductance amplifier to provide dynamic control of phase shift and filter cutoff frequency.