Time Expansion (TE) and Frequency Division (FD) detectors are both “broadband” detectors. This means they will make the entire ultrasonic range audible all the time (in contrast to a Heterodyne detector, which is sensitive only to a limited range of frequencies at each time). There are, however, several important differences between the two types.
A FD detector transforms the ultrasonic calls in real time, i.e. the calls are heard through the detector at the same time they were emitted by the bat. A TE detector first stores a portion of the ultrasonic signal in its digital memory and then replays it at a slower speed, i.e. it is does not transform the calls in real time. The delay depends on the actual storage time, and if this is very short (up to around 100 msec), the replay is also made fairly quickly and the detector gives an “almost real time” response.
Since the TE detector actually stores the original signal (which is then stretched out in time), the output of such a detector provides the same information as the original signal. Hence, it is possible to make any type of analysis and obtain useful and accurate results. This includes spectral analysis of all types (e.g. spectrogram/sonagram). A FD detector counts the number of cycles of the ultrasonic signal and generates an output cycle (pulse) for each N input cycles, where N is often 10. This effectively divides the frequency by N. Obviously, an FD detector is only capable of tracking one frequency (harmonic) at each time. Usually (but not always!), this is the fundamental frequency. Consequently, it is not possible to perform any harmonic analysis from an FD signal. To do this, TE signals should be used.
TE detectors have a number of other advantages over FD detectors. For instance, the former are more sensitive and offer the user better possibilities to aurally analyze the transformed calls.
To conclude, if you require a broadband bat detector, a TE detector is usually the best choice. However, if the limited storage time of the TE detector is a problem and/or you need a real time response, a FD detector should be chosen. As an alternative to a TE detector, direct high-speed recording of the ultrasonic calls can also be made with the D500X and D1000X detectors. In that case there is virtually no limit on the recording length (storage time).