[<<][synth][>>][..]Mon Dec 20 14:16:15 EST 2010

How to control an analog synth digitally? What is the best design, assuming that we want to keep the signal path analog? * PROBLEM: Exponential-scale control is necessary for both amplitude and frequency somewhere in the control chain. The question is where to implement it? * FREE: BJTs cost about $0.02. Physically, the V_be vs I_c characteristic of a BJT is very near to exponential over a large current range, but it requires calibration due to the unknown curren scaling factor and junction temperature. At room temperature we have a scale of e / 26mV or 20mV per octave, which means standard 1V / octave needs about 50x voltage attenuatio.n * FREE: On-chip processing power is free. Even on tiny < $1 microcontrollers, since the control rate is so low. * NOT FREE: Digital to analog resolution. The cases that are readily available (PWM, delta-sigma or even ladder DA) all have linear error sensitivity, not exponential. * The cost of an analog exp converter is mostly the calibration: temperature compensation, trimpot component cost, manual (mechanical) setup cost. The collector current is mostly propertional to exp(V_BE/V_T) where V_T is the thermal voltage[1] equal to k T / q. The scaling factor depends on the circuit biasing. So, essentially, all things being equal, the voltage input that goes into a BJT based exponential converter has two parameters: scale (octaves per volt proportional to temperature) and offset, the biasing current of the network, determined by say 5% resistors. This affine transformation can be done in the digital domain. LM394N matched pair data sheet[2] contains some interesting "log/exp based application notes. A true linear-to-exponential converter schematic can be found in the National AN-30 application note[3]. Using a transistor array for temperature coupling, perform the following: - Factory mismatch calibration: determine whole function transfer: from D-A to controlled frequency and measure frequency (if separate). This takes care of controlling and measuring BJT differences. - On-line temperature calibration: measure output vs. feedback frequency. So, can it be built with only one transistor? The schematics I find all use a matched pair to get rid of the initial biasing problem, i.e. what current corresponds to 0V input. [1] http://en.wikipedia.org/wiki/Boltzmann_constant#Role_in_semiconductor_physics:_the_thermal_voltage [2] http://www.futurlec.com/Datasheet/Linear/LM394N.pdf [3] http://www.national.com/an/AN/AN-30.pdf

[Reply][About]

[<<][synth][>>][..]