> Could you tell us more about the VibraChorus ? From what you've
> told already it seems to use 'mechanical' variable capacitors...

And I did add that my knowledge of the vibrachorus was limited, the gearbox itself was more fun.

The vibra had a single rotating armature with multiple stacked horizontal fans that formed plate capacitors with 16 fixed plate segments, each fan covers about 20 degrees of arc - would you call these 'plate capacitors', no? As the rotating armature moved between the fixed plates/fans, whatever, they fead more signal into it depending on its position. The 16 fixed segments were from an 8 stage delay line (including 0 delay) going up then coming back down through a single rotation - each of the 8 stages was a cascade of RC delay lines so the larger delays had a darker/warmer sound. The output was roughly a vibrato due to scanning through consequetively larger phase shifted delays from the RC filters - the movement from one RC delay to the next was quite smooth since the rotating fans would pick up signal from adjacent delay sections, gradually mixing them as it moved. Chorus was generated by mixing this signal back in with the direct signal. I could not find any information on the phase shift per section of the cascaded delay line by which I mean the actual RC values did not seem to be documented anywhere online.

The rotation speed was fixed as the arm was connected directly to the main gearbox spindle, the different speeds/depths that made up V3/V2/V1/0/C1/C2/C3 purely depended on which of the rotating fans was tapped to the output.

You can find a block diagram on wikipedia and if you search further you can find a diagram of the build, they are quite enlightening as to how it worked but lacked detail of the RC circuits and naturally emulating its complexities is left up to you. I have not looked at how Beatrix emulates this. A lot of synths implement a more simple multitap chorus which is a reasonable extraction. Bristol uses an 8 stage delay line with a different filter cutoff on each delay to emphasise the phase shift and taps off up to 4 different positions depending on the C/V selection - it previously also used a chorus circuit which had very different characteristics. The current algorithm is also not optimal, only the C3 position sounds half reasonable, V1/V2/V3 are weak. I think this algorithm, even with just C3, sounds better than a multitap chorus but then I would naturally have a preference.

Regards, Nick.

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