Understanding A Guitar Pedal Circuit (MXR Envelope Filter)

Hey every one, how's it going ?

My friend asked me f I can build him a guitar pedal, and I thought it would be a nice learning experience for me.

I had some trouble understanding the use of the logic gates (NOT gate) and the electrical switch.
The usage for audio data with these digital switches passed in my head, that being a ASK modulation. But how in the hell you use a guitar signal on logic gates, and how would the result look?

Oh, and another thing:
Would J1 be the input from the guitar to the pedal, what is the J3 input is made for?

Any way, here is the circuit:

Quite easy to understand (and a common use), you simply apply negative feedback to the ones you want to act as a 'linear' amplifier (this biases them in an analogue region) - such as R13 etc.

Bear in mind we're not talking HiFi quality here, but it's as good as you need for a guitar effect.

An unbuffered CMOS inverter like the 4069 can behave like an inverting opamp over a limited range of voltages and frequencies. For example, A2c has a 200K input resistor and a 240K feedback resistor, for a gain of 1.2. Similarly, A2a and A2f have capacitors in the feedback loop, so these are integrators, a form of lowpass filter/signal shaper. A1d is taking an analog signal and turning it into a digital control signal for the analog switches, so it is acting as a comparator. The analog switches insert the integrators into the signal chain under signal conditions determined by the center and lower signal circuits.

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