[sdiy] Real piano keyboards: any use?

Fri Jan 30 16:10:25 CET 2009

You'd typically expect "LPF" to refer to a linear filter.  Slew
limiting isn't linear and isn't the same as exponential decay, so a
slew limiter (or something with asymmetric rising/falling response)
really shouldn't be called a filter.

> Date: Fri, 30 Jan 2009 08:28:05 +0000
> Subject: Re: [sdiy] Real piano keyboards: any use?
> From: cheater00 at gmail.com
> To: synth-diy at dropmix.xs4all.nl
> 
> No idea. I have never built or considered a circuit like you're describing.
> 
> What I am talking about is...
> Take a normal LPF for a start. A gate goes into that LPF. It's got a
> very large time constant (so it's set to a very low 'cutoff' if you
> can talk about that with DC signals). When the gate triggers, the
> input signal abruptly changes state from say 0 to +5V. The LPF
> slew-limits that, and therefore you get a raising slope. Once the
> output = input (so, the slope is raised), a special trigger feeds back
> to where the gate came from, and turns it off. Then the gate turns off
> to 0V. So again the output of the LPF goes from +5V to 0V in a
> slew-limited fashion, which gives you the release stage.
> 
> Right now you only have one knob that controls the attack and release
> time at the same time. This means the LPF is symmetric, because it
> 'works the same way in both directions', for signals with a positive
> and negative slope. A more sophisticated filter can have a different
> time constant for signals with a positive, and for signals with a
> negative slope.
> 
> Cheers
> Damian
> 
> On Thu, Jan 29, 2009 at 11:31 PM, David G. Dixon
>  wrote:
>> Hey, D.:
>>
>> I tried searching for asymmetric LPFs on the web, but came up empty-handed.
>> I can also find nothing about it in Horowitz and Hill.  So, let me see if I
>> understand you correctly:
>>
>> My vision of a simple AR is two diodes in opposite directions feeding two
>> (variable, or just differently valued) resistors which then come together
>> across a single (typically fairly large) cap to ground.  Obviously, if the
>> voltage input has different polarities it will be shunted across the
>> different diodes, thus rendering the filtering asymmetric.
>>
>> Using active rectifiers rather than just bare diodes would eliminate the
>> diode voltage drop and its associated crossover distortion, which would make
>> the device useful for filtering actual signals rather than just gate inputs.
>>
>> Does this more or less correspond with what you are talking about?
>>
>> Cheers,
>> Dave
>>
>> David G. Dixon
>> Professor
>> Department of Materials Engineering
>> University of British Columbia
>> 309-6350 Stores Road
>> Vancouver, B.C. V6T 1Z4
>> Canada
>>
>> Tel 1-604-822-3679
>> Fax 1-604-822-3619
>>
>> "PERFECTA FINGAMUS SERVIAT NATURA"
>>
>> The information in this email and in any attachments is confidential and
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>> must not be disclosed to any person without the writer's authority.  If you
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>>> -----Original Message-----
>>> From: synth-diy-bounces at dropmix.xs4all.nl [mailto:synth-diy-
>>> bounces at dropmix.xs4all.nl] On Behalf Of cheater cheater
>>> Sent: Thursday, January 29, 2009 1:34 PM
>>> To: synth-diy at dropmix.xs4all.nl
>>> Subject: Re: [sdiy] Real piano keyboards: any use?
>>>
>>> No, if you feed an asymmetric LPF something other than a gate signal
>>> you won't get an AR signal out of it.
>>>
>>> Think of sidechains in compressors. The loudness analyzers are
>>> (usually) asymmetric LPFs fed the rectified signal you put into the
>>> sidechain.
>>>
>>> Cheers
>>> D.
>>>
>>> On Thu, Jan 29, 2009 at 9:09 PM, David G. Dixon
>>>  wrote:
>>>> Ah.  I get it.  It's an AR.
>>>>
>>>> David G. Dixon
>>>> Professor
>>>> Department of Materials Engineering
>>>> University of British Columbia
>>>> 309-6350 Stores Road
>>>> Vancouver, B.C. V6T 1Z4
>>>> Canada
>>>>
>>>> Tel 1-604-822-3679
>>>> Fax 1-604-822-3619
>>>>
>>>> "PERFECTA FINGAMUS SERVIAT NATURA"
>>>>
>>>> The information in this email and in any attachments is confidential and
>>>> intended solely for the attention and use of the named addressee(s).  It
>>>> must not be disclosed to any person without the writer's authority.  If
>>> you
>>>> are not the intended recipient, or a person responsible for delivering
>>> it to
>>>> the intended recipient, you are not authorized to and must not disclose,
>>>> copy, distribute, or retain this message or any part of it.
>>>>> -----Original Message-----
>>>>> From: synth-diy-bounces at dropmix.xs4all.nl [mailto:synth-diy-
>>>>> bounces at dropmix.xs4all.nl] On Behalf Of cheater cheater
>>>>> Sent: Thursday, January 29, 2009 12:27 PM
>>>>> To: synth-diy at dropmix.xs4all.nl
>>>>> Subject: Re: [sdiy] Real piano keyboards: any use?
>>>>>
>>>>> An AR is a non-symmetric LPF that you put a gate into and get an AR
>>>>> envelope out of.[1]
>>>>>
>>>>> An assymmetric LPF is an LPF where the 'slew rate'*is set separately
>>>>> for rising and falling signals.
>>>>>
>>>>> *(actually: time coefficients)
>>>>>
>>>>> [1] That's why different gate designs make your synth sound
>>>>> differently. It's all in the transient!
>>>>>
>>>>> Cheers
>>>>> D.
>>>>>
>>>>> On Thu, Jan 29, 2009 at 8:15 PM, David G. Dixon
>>>>>  wrote:
>>>>>>> It does indeed!
>>>>>>> I think the best way to achieve this kind of control is to have a
>>>>>>> continuous optic distance sensor. This can give you acceleration
>>>>>>> output and position output.
>>>>>>> Imagine having a synth style keyboard with the position going to the
>>>>>>> volume of the voice, and the speed going (through a non-symmetric
>>> LPF
>>>>>>> with a long release) to the volume of the suboctave. This would be a
>>>>>>> great lead/pad patch.
>>>>>>
>>>>>> Did you mean "non-symmetric AR" as in fast attack, long release?
>>> What
>>>>> is a
>>>>>> non-symmetric LPF?
>>>>>>
>>>>>>
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>>>>
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