Ensemble Circuit Configuration Questions

[mbartkow at ET.PUT.Poznan.PL]

Dear all,

I am following the discussion carefully since I plan to build such (possibly
good sounding, no compromise) circuit using analogue circuits. First, I love
the sonic effect, second I have an order for an analogue F/X from my friend 
techno musician.

Three points:

1) 120 degrees-

> The 2 LFOs with 120 degrees phase shift could be based around a single CD4069
> IC, as in the Polysix ensemble effect (thanks to Mike I. for telling me about
> this). 

While direct generating of three sine waves with 120 phase shift maight be tricky
I personally (as an analogue purist) would avoid digital circuits as much as possi-
ble. My idea is to build a normal quadrature pair LFO which outputs two sine
waves shifted by 90 degrees. Now it is very easy to obtain an arbitrarily shifted
sine by mixing these two in appropriate proportions. Note, that other phase shifs
may be experimented with in this way.

2) randomness-

> On the other hand, white noise filtered through a bandpass filter, with center
> frequency of a desired vibrato rate and very high Q, might do the trick.
> Apparently this results in a signal that sounds more vibrato-like than the randi
> generators, or lowpass filtered noise. Perhaps using 1 of these per delay line
> would result in a nice sound.

Passing a white noise through a very narrow band BPF results in a "sine" wave of
fluctuating amplitude. This perfectly mimics only one aspect of the "natural"
vibrato, namely the depth instability. There is also a speed instability which
would be achieved by tuning the center frequency of the filter using another
1/f noise. I suppose that similar effect may be achieved by modulating a speed
and the output amplitude of a VCLFO using the same noise. The latter solution is 
probably cheaper. The depth of both modulations might be manually adjustable.

3) complexity-

What about more delay paths ? Is three a magic number or just the cost compromise ?
I understand that three varied delays is enough mess for the ear to prevent it from
recognising any modulation regularity when periodic modulation is used. What about
4 paths with 90 degrees phase shift or 5 paths with 72 degrees ? OTOH, maybe two is
okay for random modulation ? 

regards,

MB

PS. Does anybody know what exactly are the lenghts of the delay lines used in
vintage staff (especially Solina) ? MN3007 (1024 BBD stages) is acceptably cheap,
but MN3005 (4096 stages) seem to be unreasonably expensive (8 times the price
of 3007). 


--

Maciej Bartkowiak, PhD
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