Bat Detectors: Frequency Division
Frequency division detectors work by amplifying the signal from the bat, then frequency dividing it with a digital IC. The amplitude (loudness) of the signal is lost in this process, and the result is a crackly sound, which you can hear in this example (bottom of page).
Each time the incoming signal (blue line) passes through zero a one is added to a counter. When the counter reaches a predetermined value (normally 16 or 32) the output signal changes, as shown here. This means that a bat call at 48kHz (maybe from a pipistrelle) will cause an output signal at 48/16 = 3kHz. This output signal is easily audible. Frequency division detectors will not pick up very quiet sounds - the call must exceed a certain threshold before they start to work.
A frequency division detector will allow you to listen for all types of bat at the same time. It doesn't need tuning in to a particular bat species. The ability to cover the whole bat spectrum is most useful if your detector has a good quality sensor. Cheaper detectors that use piezodetectors do not greatly benefit from this approach. Here you can see that frequency division results in a reduction in the observed frequency range. The blue line, representing a pipistrelles call, has a range of almost 10kHz but when divided (as here) by 10 the sound you hear has a range of only 1kHz.
Because of this reduction in frequency range the sound from an FD detector can sound thin, quiet and not very interesting when compared to a heterodyne or time expansion detector.
The output from frequency division detectors can be recorded for later spectral analysis, which is not ideally suited to heterodyne detectors as with these the frequency content is changed by an unknown amount. By contrast in f-div detectors the incoming frequency is scaled by a known factor - often 10, 16 or 32.
Detectors such as the BatBox Baton and Duet use the frequency division with amplitude restore principle to put back the lost loudness information. This provides even more information for analysis, and a nicer sound. More expensive detectors use capacitance microphones that are expensive and fragile – and sometimes not very sensitive!
Here you can play the sound of a pipistrelle recorded from a simple Frequency Division detector.