I just dont get that Goertzel algorithm or the application from Microchip.

What i intend is this, i have 4 frequencies 350,420, 480 and 620 Hz. I need to find if a certain frecuency is happening.

These are by the way typical Multifrequency Signaling frequencies so detection has to be fast, like 40mS to determine wheter the frequency is present or not in a given signal.

Microhip has an app about DTMF detection that seems to have a lead, i just need to find different Frequencies than those in DTMF. The Microchip app is made to 18F devices, i intend to use a cheaper 16F628A. Would this be impossible?.

I tried using external components like a LM567 but i would need 4, since each lm567 can only get one of the signals, due to bandwidth restrains.

Ok has anyone done something similar?, can anyone explain the algorithm?

Thanks for any help!!