[sdiy] Shruthi 4PM (was re something else...)

[rburnett at richieburnett.co.uk]

If you take a look at Olivier's technical description here...

http://mutable-instruments.net/static/documentation/pole_mixing.pdf

...you can see that the cascaded 4-pole filter intrinsically produces 
the four low-pass responses.  So these responses will be of good quality 
and not subject to resistor matching tolerances.

It is the other responses (band-pass, high-pass and notch) that are 
constructed from carefully chosen weighted sums of the input and the 
four lowpass outputs.  So the purity of these "constructed responses" 
depends on how accurate the weightings of the various signals are in the 
final mix.  This is where tight tolerance on the mixing resistors will 
reduce "non ideal" artifacts in the constructed responses, like LF 
bleed-through in the stop-band of a high-pass filter.

-Richie,

On 2013-11-27 23:08, Michael Bachman wrote:
> Olivier
> 
> The filter I have on my DIY voice card uses all 13700s.. I added the
> resonance compensation I saw in your circuits (THANK YOU).  It added
> about 20% back to the signal at mid-high Q.
> 
> I did some A-B sonic and O'scope trace comparisons  and found very
> little side effects with it.  It is a nice add.
> 
> I am intrigued by the multi-mode filter concept. I guess I'll try the
> filer pole mixing concept and do so A-B comparisons on that too.
> Back in 1980, the filter I built for my DIY modular synth  had a ZIP
> socket on the front panel.  You swapped programming headers to change
> between filter types!!  This seems so much better.. (great write up on
> the filter design too)
> 
> Thanks again.
> 
> Mike
> 
> On Wed, Nov 27, 2013 at 5:46 PM, Olivier Gillet <ol.gillet at gmail.com> 
> wrote:
>>>  but does its 4 pole LP setting really have the same qualities as a 
>>> normal 4pole OTA filter?
>> 
>> The difference is that the 2164 is linear so it doesn't bring the
>> slight coloration of OTA filters when they are used closed to their
>> saturation point. I still kept an OTA in the resonance feedback loop
>> since it works very well as the non-linearity element that prevents
>> the resonance from blowing up. Speaking of OTAs as a gain control
>> element for resonance: the fact that it has differential inputs, to
>> subtract the original signal from the feedback signal and get loudness
>> compensation is another thing I like with this configuration.
>> 
>> The same design, without the fancy pole-mixing, is used in my Eurorack
>> module Ripples, and it looks like people are quite happy with it. I
>> also use it on the Ambika (the "4P" voicecard) - this gives a
>> well-behaved, low-part count 4-pole filter.
>> 
>> Olivier
>> 
>> On Wed, Nov 27, 2013 at 11:33 PM, Michael Bachman 
>> <bachmanm50 at gmail.com> wrote:
>>> I am thinking of building the 4p Misson board. (or some variant)
>>> 
>>> What I wonder is, with all those mixing resistors in the circuit
>>> bleeding between pole outputs, what affect it may have.
>>> 
>>> True , if it provides a wide range of nice sounding filter functions,
>>> then it has value to build, but does its 4 pole LP setting really 
>>> have
>>> the same qualities as a normal 4pole OTA filter?
>>> 
>>> Mike
>>> 
>>> On Wed, Nov 27, 2013 at 2:06 PM, Dave Manley <dlmanley at sonic.net> 
>>> wrote:
>>>> 
>>>> 
>>>> Tom Wiltshire <tom at electricdruid.net> wrote:
>>>> 
>>>>> 2) THAT2162 is non-inverting. There are a number of occasions where 
>>>>> the
>>>>> inverting nature of the 2164 is very helpful. Olivier Gillet's 
>>>>> "Four
>>>>> Pole Mission" Xpander-style filter is a good example:
>>>>> 
>>>>> http://mutable-instruments.net/shruthi1/build/4pm
>>>>> 
>>>> 
>>>> This is a very nice implementation.
>>>> 
>>>> One thing I've droned on in the past about is the sensitivity of the 
>>>> different modes to the summing resistor tolerances.  With exact 
>>>> values the calculated responses are obtained.  With real world 
>>>> tolerances perhaps not so much.  Has anyone measured the actual 
>>>> filter responses to see what they look like?
>>>> 
>>>> It would be interesting to see the results from multiple boards.
>>>> 
>>>> Dave
>>>> 
>>>> P.S. yes, before anyone states the obvious, all that really matters 
>>>> is what it sounds like.
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