[sdiy] ot: rotating speaker simulation or stupid approach

[jhaible]

> At 16:34 02/07/2003 +0200, Czech Martin wrote:
> >I can record the transfer charactericstic of a loudspeaker with some
> >microphone in some room (the impulse response, if you like).
> >I can repeat that, turning the speaker a few degrees of axis.
> >I can do this , say for 23 steps of 15 deg to make a full circle.
> >I must be carefull in order to get the right time shift for
> >each impulse response vs. all the others.
> >I think the impulse response will be not too long, perhaps
> >some 10ms.
>
> I doubt this will work, because an essential part of the rotating speaker
> effect is the way the sound dopplers as the horns move. If you record the
> impulse response you'll just get the effect of the room/horn-as-filter
> sound, with a different frequency response available as the horn points in
> different directions. But the dopplering won't be present, and
> interpolating between different impulse responses won't create it.

I think the Doppler effect _will_ be reproduced, as the location in the
simulated room changes, and so does the delay time between the location
of the horn and the listener. Interpolating along a tapped delay line
reproduces the Doppler effect nicely. Only problem is that with a simple
crossfade for the interpolation you get a little phase cancellation. Which
does *not* destroy the reproduction of the Doppler effect, but might add
some
unwanted extra timbre modulation.


> The rotating speaker effect is *not* the same effect as simple panning.

Certainly not, and you can be sure Martin knows this. (;->)

But then again, the Leslie effect as known from a 112, or similar closed
cabinets, is *not* the same as a simple rotating sound source.
If you "open" a Leslie, and have the horn rotate in free air, the sound is
very different. If you do this, you have the Doppler effect, volume
modulation and panning of a simple rotating sound source, more or less. As
said in my
previous mail, emulating this with electronics is quite simple. But this is
not
the classic Leslie effect!

In a real Leslie, the reflections inside the enclosure will give you a mix
of upshifted and downshifted frequency at the same time: As the horn
moves _away_ from you, it will move _closer_ to the adjacent wall
of the enclosure at the same time!

The Dynacord CLS 222, which is, while certainly not perfect, arguably
the best electronic Leslie emulation so far, uses two BBDs for the
horn, producing an upshift and downshift at the same time, with different
mean delay times for both BBDs. (Just speaking of the treble rotor here;
the bass part uses another pair of BBDs.)

I guess this covers the first reflection inside the enclosure, together with
the
direct sound.
Martin's approach will cover a lot of reflections, and I'm confident that
this has the potential to give a very realistic emulation. Doppler effect
included, of course.

JH.