[sdiy] Real piano keyboards: any use?

cheater cheater cheater00 at gmail.com
Sun Feb 1 11:16:31 CET 2009 

Scott,
My take on the slew rate thing is thus: I didn't call the filter's
time constant the slew rate - this would be incorrect as an LPF limits
the slew rate non-linearly (albeit in a way that is still very nice
and very predictable). I think slew-limiting is happening, since the
slew is lowered indeed. Well, if there's a sine at 1/2 fc, then it can
be theoretically of any amplitude, therefore attaining any slew.
Practically, it would become distorted and then clipped, foiling this
attempt at punching a hole in my theory ;) Consider that I was talking
about slew-limiting of a gate by a near-DC lpf. Anything above fc gets
attenuated , and in a typical A-weighted signal the frequencies are of
similar amplitude, but faster and faster frequency, therefore their
slew gets higher and higher. Their attenuation also becomes higher and
higher... their slew gets lowered. Perhaps it is not limited by some
constant as is the case with a physically imperfect op-amp, but it is
still limited by some complex phenomenon. For example, in mathematics,
a list of functions can be limited by a certain maximum, but that
maximum can be expressed as a function over the same domain, instead
of a single number. For example the family {f(x) = x^n: x>0, x<1, n=2,3,4...}
is bound by the function y=x.
If you look at the constant function f(x)=C, then
it's also a function over that domain, and it corresponds directly to
limiting by a constant. Therefore limiting by a function is a good
generalization of this concept I think.


On Fri, Jan 30, 2009 at 3:10 PM, Scott Nordlund <gsn10 at hotmail.com> wrote:
>
> 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
>>> 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 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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