Signal processing techniques allow the recovery of signals with negative signal to
noise ratio as shown below. The waveforms actually show the same signal both before
and after signal processing, to the same scale. The peak signal level is 25dB below
the peak-peak noise level. The ability to recover signals from a negative signal
to noise ratio allows low cost PIN photodiodes to be used for a low cost rangefinder
rather than having to use avalanche photodiodes (APDs) with associated high voltage
power supplies etc.