Analog Vocoder Ideas
| I'm not sure if I'll ever build this vocoder. Lately ebay has had the Electrix WarpFactory vocoder (a DSP design) going for around $160 - $200. I'd spend about 3/4 of that just on parts, and come up with something that wouldn't work as good. | |
| I've been kicking around some ideas for building an analog vocoder, and did some design work but haven't built anything yet. So a big caveat here is I don't yet know whether this will work! | |
| My design philosophy was to make a 18 channel vocoder with each channel spaced at 1/3 octave apart and made up of three staggered two pole bandpass filters to obtain 18 db/octave slopes. There are three cascaded Multiple Feedback Bandpass filters with slightly staggered center frequencies (see response below). MFB filters are sensitive to component tolerance, but with the Q of each stage kept below 10 it should be managable. | |
| After picking up Don Lancaster's Active Filter Cookbook to learn more about filter theory, I found that going to a state variable filter for each filter state would solve the problem of filter tunability and parameter interdependence. The price, of course, is parts count: the state variable filter requires three opamps per stage. Whereas the unworkable MFB design used 4 opamps in the filters for each channel, using the state variable would require 12. | |
| Not shown (yet) is the output stage, which will be an opamp summing up the the output currents of the OTA's |
| Schematics:Lowpass filter (1)Bandpass filters (15) Env.follower and VCA (17) Highpass filter (1) |
Calculations:To obtain a "maximally flat" response for a 3rd order bandpass response, we chose Q=7 and stagger factor a=1.085 (from Active Filter cookbook). The stagger factor means you design for the corner frequencies of your three stages to be fc/a @ Q=7, fc @ Q=3.5, and and fc*a @Q=7. Real cool 3rd Order Staggered MFB Bandpass filter Component Calculator
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