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 Forum index » DIY Hardware and Software » Developers' Corner
Amplitude filtering? (and more)
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Kenneth



Joined: Apr 16, 2009
Posts: 43
Location: Stockholm, Sweden

PostPosted: Sat May 16, 2009 3:43 pm    Post subject: Amplitude filtering? (and more) Reply with quote  Mark this post and the followings unread

Normally we do filtering on the frequencys, like lowpass, hipass and bandpass. But the amplitude, VCA? yes, but that works the same on all input. Compressing? Well, that's one thing you can do using this idea + a lowpass. But think of the ability to "bandpass" or "equalize" ceartain amplitudes, do this:

(input)--(FM modulator)--(Filter)- -(FM demodulator)--(output)

I'm sure this is not a new idéa as a technology, no Idéas are new anymore Smile

But in music? Is it used before?

Waveshaping is one intresting usage, compression of course and maybe more. If someone got the equiment to test it? I would do it right away if I could Cool


(pobably a lot of distortion that comes from the modulations)

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Well, got things to do anyway, just got my hands on a LM358 (from a telephone modem 75/1200 BAUD anno 1987) and a NE558 (286/386 ISA HD/RS232/Centronics controller card) I do not buy components so far, I'm in the learning so I use what I get =)

I'm thinking of a lowpassfilter box with a built in LFO to control sweeps that I can use together with my Yamaha Electone... Any suggestions of a simple contsruction?

I do not care about the best performances, I'm looking for simplicity, most important is that I understand how it works, a 2'nd order filter + triangle LFO for sweeps is good enough (I will work something out myself, I'm sure but suggestions is nice anyway)

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One thinking, the NE558, it can be used as four timers in a sequence, 4-step sequencer? (who can build the smallest analog sequencer? yeha, special desings on chip, but using "store stuff" =)

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Another Idéa, (this is as weird as the punch card sequencer). I found a "digital pot" (microwawe owen front panel). It works click click click... when you turn, it's digital. When it is in a fixed position it is no connection but when you are in the middle of two clicks it is leading, no resistance. So it gives pulses, one for each click and it is a 360 degree turner. Have a electric engine rotating the "pot" (it's not a pot, I know, it's a switch but it looks like a pot, have three connectors for no reason at all) input voltage is the amplitude of your choise for the pulse. Speed of engine is of course voltage controled... (this is not a project, just a weird idéa)

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(instead of several posts I been thinking of the latest days)

BTW: Blue Hell, if you connect the rubberband to a piezo and then play the rubberband and hit the piezo with the teaspoones, then you can use that for "VCO", the rest, standard modular synth stuff...
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Kenneth



Joined: Apr 16, 2009
Posts: 43
Location: Stockholm, Sweden

PostPosted: Mon May 18, 2009 3:01 am    Post subject: Re: Amplitude filtering? (and more)
Subject description: Powering?
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Kenneth wrote:

Well, got things to do anyway, just got my hands on a LM358 (from a telephone modem 75/1200 BAUD anno 1987) and a NE558 (286/386 ISA HD/RS232/Centronics controller card) I do not buy components so far, I'm in the learning so I use what I get =)

I'm thinking of a lowpassfilter box with a built in LFO to control sweeps that I can use together with my Yamaha Electone... Any suggestions of a simple contsruction?

I do not care about the best performances, I'm looking for simplicity, most important is that I understand how it works, a 2'nd order filter + triangle LFO for sweeps is good enough (I will work something out myself, I'm sure but suggestions is nice anyway)


Working on the filter, I think I make this tread "my tread", I post all my stuff here, so expect anything Smile

When looking at descriptions it is almost always +-15 volts that is used. But I don't have any 15 volts supply. But I got a few 12 volt cards (cheap shit CD/DVD-players got them as a stand alone part, I found several of those in the garbage, just a few years old ones). I know I can use one of them instead of 15 volts but but somwhere I read something like (about a filter) "you can run under 12 volts but you wont get it to self oscillate". Is it always like that or was it just for that particular contruction I wonder. Is it not just a matter of compensation? (like use other resistors and because of that maybe also diffrent caps)

If that does not work, I got a +-24 volts card, can I use that and make it output 15 volts? How do I do that?

Another option is to use 4*9 volts batterys and connect them so I get +-18 volts, maybe that is the best option? is 18 volts to much? (no risk for shocks if I use batterys Shocked )

Or shall I go for 20*1.5 volts, that will give me exactly +-15 (if I use large batterys, R14 or R20 they will probably last for years, but a bit clumpsy)

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EDIT: I'm looking at the clone of the early MS-20 filter that I consider and found some Sweds have done a bit of work on it earlier. Within that material I found this "+/-12V matning: För de som vill använda 12V matning så kan ni ändra på R17 till ca: 17 kOhm (från 22k) och R6 till ca: 240k (från 300k)" Smile

That answers one of my questions, 12 volts works fine with some resistance adjustments. But I'm still considering the battery solution, it's much safer and you will not have any distortion from the mains AC (50 Hz in Sweden)

Last edited by Kenneth on Mon May 18, 2009 7:36 am; edited 4 times in total
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DrJustice



Joined: Sep 13, 2004
Posts: 2048
Location: Morokulien
Audio files: 3

PostPosted: Mon May 18, 2009 3:41 am    Post subject: Re: Amplitude filtering? (and more)
Subject description: Powering?
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Kenneth wrote:
Working on the filter, I think I make this tread "my tread", I post all my stuff here, so expect anything Smile

That's cool Cool

I've not heard about (FM) -> (modifier) -> (FM demodulation) being used in audio synthesis before. Maybe you've got a first here?

On you last question, those 'digital pots' are usually called rotary encoders, and are commonly used on synthesizer front panels these days. The three terminals are a common pin and two phases of square wave that encoded the direction of rotation.

I'll let you work on the remaining points in your original post. I'm sure other members will chime in as well.

DJ
--
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Kenneth



Joined: Apr 16, 2009
Posts: 43
Location: Stockholm, Sweden

PostPosted: Mon May 18, 2009 5:35 am    Post subject: Reply with quote  Mark this post and the followings unread

Ah, nice (all parts), thank's for replying Smile

I used an analog Ohm metre to test the encoder, that's why I thought it was only one and the same output on both terminals.

Yeha, I later realised it was the thing used in "alpha wheels" ever since the first digital syntesizers in the 80's.
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