I'm trying to convert a beeping noise into an input my arduino can respond to. This will be used as a method of estimating distance because the frequency of the beeps increases as an object gets closer. The frequency of the sound is constant, but the beeping increases as the object approaches. I used pulseIn() for my ultrasonic sensors, but I'm afraid that the arduino will confuse the frequency of the sound as the period rather than spacing of the beeps.
Attached is what the response looks like on the oscilloscope with the solid looking blocks representing the sound and the curve representing the "off" period between beeps.
"RC" stands for Resistor + Capacitor. It's a type of simple analog filter, probably the simplest. It can be either high-pass or low-pass but in this context, I think low-pass is what you need.
I would try an analogRead(). You will get somewhat random values as it may sample the audio at a high part of the wave or a low part. But if you do a hundred analogRead()'s in a hundredth of a second and all of them are near zero then you can be pretty sure that the "beep" wasn't active during that time period.
By taking 100 readings and doing some processing on them, you've built a digital signal processor that could be generalized as a "low pass filter" but it's so much easier to do this in software than selecting resistors and capacitors and testing them in a circuit.
You have a sound signal - your first task is to get something useful out of it, i.e. finding out when a "beep" begins and when it ends. Then the time between the start of two beeps gives the frequency you're looking for (hope I understand that part correctly).
The hard part is filtering the correct frequency from the sound, ignoring everything else. I don't think a simple RC filter or so is going to cut it - instead you'd need a filter that passes a very specific frequency, as in the frequency you're looking for. That gives you no signal when the beep sound isn't there, and a signal when the beep sound is there.
As you now have your sound signal cleaned up, you can transform that AC on/off signal into a DC block wave, after which it gets easy to read with an Arduino and do things like timing on it. This can indeed be done with a simple RC filter.
A simple filter may work since presumably you know the beep frequency so can tune the filter fairly well.
But what you really want is an sound detector/follower circuit. Google will show you plenty of examples if you want to make one. Otherwise there are plenty of breakout boards like Sparkfun Sound Detector you could buy.
