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Integrating an audio signal
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tohito



Joined: Jun 06, 2009
Posts: 3
Location: Ruston, LA

PostPosted: Sat Jun 06, 2009 10:26 pm    Post subject: Integrating an audio signal Reply with quote  Mark this post and the followings unread

When I learned in an electrical engineering class that a capacitor connected in between the input and the output of an op-amp creates an integrator (the output will be the integral of the input) my first thought was "I wonder what music would sound like if you integrated it." Has anyone done this before?

A friend and I started building one. We're having ground problems, but we managed to get rid of the buzz for long enough to play his guitar through it. What we got out sounds a bit like really strong fuzz, with a really interesting gate effect. But the problem we're having now is that it doesn't seem to pick up the higher frequencies of the guitar very well if at all. Without an oscilloscope on hand to examine the waveforms, I'm not sure exactly what's going on, and my knowledge of electrical engineering is limited, at best.

Has anyone tried this before, and if so can you comment on things like what size capacitor to use?
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Tony Deff



Joined: May 25, 2008
Posts: 42
Location: Suffolk, UK

PostPosted: Sun Jun 07, 2009 1:34 am    Post subject: Reply with quote  Mark this post and the followings unread

Quote:
What we got out sounds a bit like really strong fuzz, with a really interesting gate effect. But the problem we're having now is that it doesn't seem to pick up the higher frequencies of the guitar very well, if at all.


What we have here (i.e. apart from over-driving the amp!) is a problem with semantics, the meaning of words.

If I asked you "tried integrating music with what?", does it begin to dawn? The biggest barrier to understanding a subject is understanding the terminology. Even if English is your first language, you should still use a dictionary, unless, as the Queen of Hearts says in "Alice in Wonderland" — " A word means what I want it to mean!".

In electronics, "integrate" is used in the mathematical sense, which is generally poorly understood by students (yet nobody dares raise a hand to ask the teacher what it means, exactly). It's some sort of set of scary complex mathematical procedures, isn't it?

When you slap a capacitor across an op-amp, then (provided the overall phase shift is not such that it oscillates!) you are integrating with respect to time.

You have re-invented the low-pass filter. The high frequencies zip straight through the capacitor without developing any appreciable signal drop across it, so you whack-up the guitar input to compensate, overload the op-amp and re-invent the fuzz-box as well!

I hope I am not "talking-down" to you. I speak as one who has suffered by being too timid to ask what things meant. The simpler the word, the less inclined you are to show yourself up — so I didn't even understand the meaning of Shakespeare's famous "To be, or not to be" in time for the English Lit. exam!

Soak-up knowledge!
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CJ Miller



Joined: Jan 07, 2007
Posts: 371
Location: 127.0.0.1

PostPosted: Mon Jun 08, 2009 9:44 pm    Post subject: Reply with quote  Mark this post and the followings unread

Funny, this is exactly what was going on with me this past week!

I was trying to understand some circuits - I know a bit about synthesis, but hardly anything of electronics. It was the Serge "Negative Slew" I was looking at. I recognized that there was an amp being used as an integrator and it suddenly caught up with me that I had no idea what this actually meant! Like a lowpass filter I can understand subjectively. So I googled "integration" Saturday night and my head is still swimming. I have gotten myself to understand a fair amount of algebra, but this type of mathematics (calculus) is entirely new to me. It's going to take me a while to wrap my head around this.
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tohito



Joined: Jun 06, 2009
Posts: 3
Location: Ruston, LA

PostPosted: Tue Jun 09, 2009 8:48 am    Post subject: Reply with quote  Mark this post and the followings unread

Ok, so what we have is a type of low-pass filter. That makes a lot of sense now, and explains the sounds we're hearing.

I understand time integrals (having taken more than my fair share of calculus courses), but didn't make the prediction that it would attenuate the higher frequencies. What I was hoping for was to modify the waveform, specifically producing a sawtooth output from a square input.
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Blue Hell
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Joined: Apr 03, 2004
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PostPosted: Tue Jun 09, 2009 9:55 am    Post subject: Reply with quote  Mark this post and the followings unread

tohito wrote:
specifically producing a sawtooth output from a square input.


It would indeed do that, provided that the sawtooth would not clip against the power rails. Furthermore the amplitude of the saw would go down when the square frequency goes up (and so, reversly, for low enough input frequencies the output will always clip). One more drawback, when the square is not precisely square (which it never will be) there will be a growing positive or negative DC offset on the output until the sawtooth does get clipped against one of the power rails (until some equilibrium is reached).

A low pas filter can be seen as a "leaky intgrator" and it will not have these drawbacks, but it will not shape the waves like an integrator does, instead it will round the ramps.

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