audio schematics for filters to isolate fundamental or harmonic tones?

I know what a freaking string does. For playing around you do not need to make it that complicated. I do not think commercial application is the goal here.

Bob

A: Which note are you playing? B: How much of its timbre do you want to filter out? How much do you want to keep?

BTW, did you know that, for most instruments, and the human voice, you can entirely delete the fundamental, and the sound will still be recognizable?

So, what are you trying to accomplish? Or is this homework? ;-)

Good Luck! Rich

Thanks for the explanation - that makes sense.

I have a couple of applications/projects I'd like to do.

Application #1 would be pitch tracking. This could be for controlling a tone generator circuit - ie play a low 'E' on the guitar, and you hear a low 'E' from the tone generator. I know there are MIDI pickups and interfaces for this, but I'm interested in working on something from scratch and keeping it cheap/simple if at all possible. If I want to simply control a synth with a guitar I can buy a MIDI interface & keyboard. Also, another thing I might use pitch tracking for is to control a color organ type device - different notes would light up different LEDs.

Application #2 would be an effects pedal that removes the fundamental tone of whatever note is currently being played, or in a different setting, ONLY plays the fundamental tone of whatever note is currently being played.

This would be for whatever note is currently being played. One application would be pitch tracking. Another would be an FX pedal.

keep?

It would be nice if this could be variable (ie controlled by a knob)

It would be nice to control the amount that is being filtered - so you could filter out a range around the note.

Build cool gadgets to control tone circuits, light organs and DIY effects pedals.

Thanks for your reply and have a good weekend

get one of those korg "guitars" with the nylon strings and MIDI output.

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Bye.
   Jasen

I'm afraid that pitch tracking of a guitar will never be cheap or simple with analog methods. Over the years I've seen a lot of schemes attempted (and tried a few of my own), and none of them exactly set the world on fire. Normally, you would need to have a good pitch tracker to control the filter cutoff to remove harmonics, not the other way around. I recommend you try a simple zero-crossing or threshold-type detector to get a feel for this "non-2nd harmonic" problem. Feed the signal into a comparator and listen to the "square" wave output (and watch it on a scope). If everything worked as hoped, the output would have only one zero-crossing per cycle and would be relatively easy to convert to a DC level representing pitch. But alas, this only seems to be true for certain portions of the note, breaking at others into a buzz as the second harmonic confuses the detector. Since it isn't truly twice the fundamental, you can't find a threshold that will produce only one zero-crossing per cycle.

You don't need pitch tracking for this... plain old static filters will do fine, a sharp (and stable) bandpass for each note.

Again, you'd need the output of the pitch tracker to control the filter cutoff. Catch 22!

Believe me, many, many folks have spent many, many hours on this problem.

Best regards,

Bob Masta D A Q A R T A Data AcQuisition And Real-Time Analysis

Scope, Spectrum, Spectrogram, Signal Generator Science with your sound card!

Thanks for the explanation - I think I understand.

Hmm. Well in that case, what would stop you from setting up such a bandpass filter for each note and using it to trigger a tone generator instead of an LED? This would serve the purpose of simple pitch tracking, no? It would only be capable of tracking notes in half step intervals (no string bends or anything) but that would probably serve my purpose. Would that work?

Thanks again...

What does one use this for? A pitch extractor to control a 6 voice synthesizer, one voice per string? There was a guitar synth (forgot the brand! sorry!) with a 'hexaphonic' pickup that tried to isolate the signal from each string, so if one played the E string on the 6th fret, the computer didnt think you were playing the A string on the

1st fret.

42 fixed filters

Bob

Nothing but the cost. You'll probably have to tune each filter to allow for variations in component tolerance, which can be a bind. In the old days you used to have to wind inductors, but these days I guess you can use gyrators (with op-amps).

This is one of those things that sounds good at first, but when you get down to details there are some problems (besides the cost, as another post mentions). The main problem is that you will need very sharp tuning to isolate each note. If you build a simple bandpass for each note, the "passband" must be very narrow in order to reject adjacent notes... but then it will reject the desired note if it is not dead-on. So you need a multi-pole bandpass that has a flatter top and still has steep slopes. These get complex and fussy to tune.

But suppose that is not a problem. (Actually, there are digitally tuned single-chip filters that could handle this, where the fitler frequency is controlled by a clock. This could be made very accurate using a crystal reference.)

Then you will discover the hidden "gotcha": The response time of a filter is proportional to the inverse of its bandwidth, which means there will be a lag between the time you pluck the string and the time the peak comes out of the bandpass.

Then, if that wasn't bad enough, remember than the bandpass output is still AC, and for a guitar most of the notes of interest are in the 100 Hz or less range. If you are going to convert this to a DC value you will need further smoothing after you (full-wave) rectify it, further adding to the delay.

OK, maybe with one filter per note you don't really need to convert to DC, just some sort of a threshold comparator. But you will have to have some way to keep the subsequent synth oscillator going between comparator peaks, which means a time delay on the shut-off... probably much better than delaying the onset.

Anyway, as a rule of thumb, figure that an overall 10 msec onset delay will probably be the upper limit of tolerability. You might be able to tolerate more if you use the synth as a backup instrument that echoes your leads with a slight reverb-like lag. A 10 msec lag is roughly what you'd get from a 100 Hz passband, but the actual passbands you need are much narrower because each semitone is only 6% from its neighbors. So at 110 Hz you need passbands that are only 17 Hz wide, which means a lag of roughly 59 msec... pretty bad. And it gets worse for lower notes.

Still, despite all this gloom and doom, I strongly encourage you to play around with this idea. Start with a simple single-note bandpass and see what you can do. You might come up with some clever workaround for the above problems that other folks haven't thought of yet, or you might not... but you'll have a fun and educational experience that will almost certainly lead to other insights and maybe to other inventions.

Best regards,

Bob Masta D A Q A R T A Data AcQuisition And Real-Time Analysis

Scope, Spectrum, Spectrogram, Signal Generator Science with your sound card!

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