Ensemble Circuit Configuration Questions

[Sean Costello]

jorgen.bergfors at idg.se wrote:
> 
> > Also, has anyone tried creating ensemble sounds using techniques besides
> > time delay? Would several parallel allpass circuits generate such a
> > sound? I've tried this in Csound, with little success, but perhaps
> > others have generated nice sounds in this manner.
> 
> Doepfer made a kit (his first product) that used all pass stages instead of 
> BBDs. Apart from this, it is as you described (2 LFOs with 120 degrees phase 
> shift etc).

Very cool! Has anyone ever heard this circuit? I tried simulating it in Csound,
but this only points out how I really need to improve my allpass algorithms, as
the mapping of the control variable to frequency is completely non-linear.
Still, the sounds I was getting are promising enough to keep working on it. Once
I have a linear frequency control over the allpass stages, I should be able to
better judge how this would sound. 

If this works, this would be a great cheap way of getting an ensemble effect.
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). The LFO's would control three 4-stage phase shift sections in parallel.
Each of the phase shift sections could be based around 2 LM13600/13700 IC's (Tom
G. has an example of this on his website), with exponential modulation to get a
nice vibrato sound. Mix the outputs of the phasers together, with no straight
signal, and you might get a nice ensemble sound! Certainly cheaper than using 3
BBD chips + the associated clock circuits. 

In other ensemble-related issues, my attempts to recreate the sound digitally
are finally getting somewhere - but it took awhile. The slower speed (.73 Hz)
still sounds much faster in Csound than it does in my Opus 3. Determining the
ratio of slow vibrato to fast vibrato in the modulation amount was tricky. From
looking at the Opus 3 schematics, I finally figured that the fast modulation
should be about 1/4 the amplitude of the slow modulation; testing this in Csound
made it sound about right. The whole thing still sounds pretty "flangy" compared
to having 3 vibrato-rate LFO's, one for each delay line. On the other hand, my
Opus 3 sounds pretty flangy, too, so I guess this is on the right path.

Does anyone have any preferences for the 2 120-degree LF0 method, versus the
3-independent LFO method? Here's a listing of what synths use what methods
(thanks to Marjan, Mike I, Steve Ridley, Jorgen Bergfors, Peter Blackett for
their info):

3 BBD IC's, modulated by 2 120 degree LFOs:
- Solina/ARP String Ensemble
- Eminent organ
- Korg Polysix
- Moog Opus 3
- Crumar Performer?

3 BBD IC's, modulated by 3 seperate LFO's (one for each IC):
- ARP Omni
- Crumar Performer? (from old JH posting on AH)

3 seperate oscillator/divide down stages:
- Korg Polyphonic Ensembles
- Korg PS-3300
- Korg Lambda (with additional 120-degree LFO, at the fast vibrato speed, for
modulating pitch of each oscillator)

3 phase shift stages, modulated by 2 120 degree LFO's:
- Doepfer thing

2 BBD IC's, modulated by a single triangle oscillator, with the output of the
oscillator inverted for one BBD channel:
- Roland Dimension D

Any additions/corrections to the above list most welcome. Also, what devices
implemented this as a stand alone effect? I know that the Dr. Boehm Phasing
Rotor produced this effect (found this from a JH posting on AH), and it sounds
like the MTI Analog String Ensemble ADL-100 was a similar effect (perhaps
similar to the ensemble effect in Crumar devices).  Any other devices? Do any
digital multieffectors produce such a sound today? 

Sean Costello