Doepfer

Is it possible to make an analog filter where you can continuously
vary the cutoff slope?  I am imagining a bandpass effect where the
band gets continuously narrower and wider.  Perhaps a voice is
filtered through, at first it sounds human but you continuously dampen
off all harmonics until it sounds like a sine wave.

> Is it possible to make an analog filter where you can continuously
> vary the cutoff slope?  I am imagining a bandpass effect where the
> band gets continuously narrower and wider.  Perhaps a voice is
> filtered through, at first it sounds human but you continuously dampen
> off all harmonics until it sounds like a sine wave.

I do not remember a filter circuit with adjustable slope. The only solution
I know is panning between several filter stages (e.g. the A-108 VCF with 4
outputs followed by a VC mixer A-135 which is controlled by the A-144
morphing controller). But that's not exactly the same as a control for the
filter slope because one pans only between e.g. the 6dB, 12dB, 18dB and 24dB
outputs of the filter.

But you may get a similar result by increasing the resonance of a bandpass
that has constant amplification at the filter frequency. Depending upon the
filter design the level in the passband or at the filter point remains
constant (or anything in between). In the first case the loudness increases
while the resonance increases (because the power at the filter point is
added to the passband power). In the second case the loudness decreases
while the resonance increases (because the power in the passband decreases
and the level at the filter point is constant). The two behaviours can be
combined to obtain a subjective loudness that remains nearly unchanged while
the resonance increases.

Best wishes
Dieter Doepfer

P.S. We are working on a VCF with adjustable resonance behaviour
(controllable between constant level in the passband or at the filter
point). But it's not yet announced on our website as we have a lot of other
products and modules in the holding pattern have to be released earlier.

You can put a compressor/limiter or AGC circuit after the filter and it can boost or reduce the gain to compensate for the level loss the filter introduces.
-J

On Apr 22, 2010, at 1:24 AM, yahoo@doepfer.de wrote:

>> Is it possible to make an analog filter where you can continuously
>> vary the cutoff slope?  I am imagining a bandpass effect where the
>> band gets continuously narrower and wider.  Perhaps a voice is
>> filtered through, at first it sounds human but you continuously dampen
>> off all harmonics until it sounds like a sine wave.
> 
> I do not remember a filter circuit with adjustable slope. The only solution
> I know is panning between several filter stages (e.g. the A-108 VCF with 4
> outputs followed by a VC mixer A-135 which is controlled by the A-144
> morphing controller). But that's not exactly the same as a control for the
> filter slope because one pans only between e.g. the 6dB, 12dB, 18dB and 24dB
> outputs of the filter.
> 
> But you may get a similar result by increasing the resonance of a bandpass
> that has constant amplification at the filter frequency. Depending upon the
> filter design the level in the passband or at the filter point remains
> constant (or anything in between). In the first case the loudness increases
> while the resonance increases (because the power at the filter point is
> added to the passband power). In the second case the loudness decreases
> while the resonance increases (because the power in the passband decreases
> and the level at the filter point is constant). The two behaviours can be
> combined to obtain a subjective loudness that remains nearly unchanged while
> the resonance increases.
> 
> Best wishes
> Dieter Doepfer
> 
> P.S. We are working on a VCF with adjustable resonance behaviour
> (controllable between constant level in the passband or at the filter
> point). But it's not yet announced on our website as we have a lot of other
> products and modules in the holding pattern have to be released earlier.
> 
> 
> 
> ------------------------------------
> 
> Yahoo! Groups Links
> 
> 
>

> Is it possible to make an analog filter where you can continuously
> vary the cutoff slope?

There are several designs for what are called 'variable slope filters' out there - Google '"variable slope" vcf' and you'll get a number of interesting hits. I've not ever looked into this in much detail, so I'm not sure just how accurately the term 'variable slope' is being used, but I believe the general idea is to 'mix' both shallow- and steep-slope outputs from a VCF - this sounds like a simple enough idea, but the catch is of course that the phase difference between them has to be carefully controlled, and presumably _that_ is where the difficulty lies in designing such a beast!

Tim