How to split an audio signal for processing?

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mr_monster

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I got an input signal 20Hz - 20KHz. I would like to split it into 2 ranges 20Hz - 3KHz & 3KHz - 20KHz (for example). I am going to process each signal differently and add them together afterwards (via a summing amplifier?). My question is how can I calc. the filters needed for the splitting so that the signals will overlap just a bit and when glued together won't create a huge peak in the overlapping range? -60dB is about as much as I wish to cut.
 

Is your source low resistance (under 200 ohms or so)? There are the typical filters (low pass and high pass) which are built from op amps. These contain higher resistances which will not load your source. Meaning they will not throw off the response curve.

Capacitor values can be found using a website calculator.

Several different filter calculators using op amps:

https://www.wa4dsy.net/robot/active-filter-calc
 


I know that the solution is to form 2-3 pole filters (LPF & HPF) but the answer is what fcut to choose for them? I tried to simulate it and it does not look like after adding the signals the gain is the same at all freq.
 

Yes, there is the point of 3dB drop, and there is the center frequency. These are not necessarily at the same point for the higher order filters. Therefore when combined you may get a hill or two. This makes it more complicated.

It appears unavoidable that each filter topology gives you an advantage somewhere, but then gives you a disadvantage elsewhere.

To get a seamless joining of the two bands, you can expect to do a lot of adjusting of gain, rolloff frequency, etc.... no matter which filter type you choose.
 

If you don't have it, the free FilterPro software from Texas Instrument allows you to readily design active filters of various configurations.
 

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