Hi,
* J1 is the input ;-)
* C7, C8 are input filters, they suppress high frequencies. There is no series resistor, thus one can not calculate a upper cutoff frequency.
* R11 ensures zero level on idle
* IC2 (R12, C18...)is an integrator for the frequencies of interest.
* IC1 is a very low frequency feedback for the integrator to prevent the integrator from run away. It ensures precise zero level on idle.
* IC3 is a 2nd order lowpass filter.
now I have to guess:
I assume IC4 combined with IC5 is some kind of peak_detect_and_hold circuit. But I´m not sure.
Maybe it is used to make the peak value readable at the 7107 display. .. then the value decreases slowly.
***
If you want to use a microcontroller, most of this analog circuitry can be replaced by software... with higher precision...
Klaus