Now, back to the topic in the subject. What about instead of the normal approach of timing a pulse, with sensitive amps, ... Using the more Pic-friendly method of connecting recievers, and transmitters more or less directly to pins, and getting the gain instead from Many pulses. Basically, take a maximal length shift register, connect that to the output, now connect the reciever to the PIC, which counts on it's fingers every time it hears a little bit of noise, and works out when it may have been sent, hence the range.