Bat detector briefing
Bat detectors are indispensable research tools for converting the otherwise inaudible, ultrasonic calls of bats. This is most useful to determine bat activity, and increasingly to identify bats to species. Bat detector conversions employ three main technologies:

Heterodyning (HET): The most common method, HET is a very sensitive, narrow band technique whereby the user selects a short range of frequencies to transform using a manual tuning control. Setting a HET detector to 40kHz makes echolocation calls between 35-45kHz audible, in real time. Most bat calls are broad-band, sweeping thru many frequencies over time, so only a portion of the call transformed by the HET system is re-played. Depending upon the dialed frequency of the detector and the characteristics of the bat call (i.e., the duration of the call within the dialed frequency range), the tonal output of the detector will vary, sometimes producing a "chirping" sound (indicating a flatter, longer call) or a "ticking" sound (for a shorter, steeper call). With practice, it is possible to identify species based upon the dialed frequency and the detector output.

Frequency division (FD): FD detectors use a broadband conversion, transforming the entire bat call in real-time. The original high frequency call is converted to a "square" wave and divided by a user-selected ratio (usually 4, 8, 10, 16, or 32). The detector counts how many times the original high-frequency sound wave cycles from negative to positive, producing an output signal for every 4, 8, 10, 16, or 32 zero-crossings. This results in an output that is 1/4th, 1/8th, 1/10th, 1/16th, or 1/32nd of the input frequency. Low frequency calls are better represented at low division ratios, and high frequency calls should be recorded at higher division ratios. Though FD detectors are less sensitive than HET (or TE) units, they are capable of producing accurate representations of a bat call, including important information about the shape, slope, and characteristic frequency; parameters often used in species identification.

Time expansion (TE): TE conversions record the original high-frequency, broadband call and can play it back at a slower speed, allowing digital outputs to capture the entire signal, retaining important information about original signal strength (amplitude) and spectral components (e.g., harmonics). TE detectors collect the very best ultrasonic recordings, retaining all call parameters commonly used in species identification. Users of a D240x select how much of the original signal to record (0.1, 1.5, or 3.4 seconds) and the detector or software expands the call by a factor of 10, producing a detailed output 1, 15, or 34 seconds long which can be digitally analyzed by popular software programs, including SonoBat. Direct Recording Detectors like the D500x is a relatively new subcatagory of TE-based gear designed for long term passive monitoring that can record continously to memory cards for weeks on end.

Time Expansion or Frequency Division?
A frequency division (FD) detector transforms the ultrasonic calls in real time; the calls are heard through the detector at the same time they were emitted by the bat. A time expansion (TE) detector first stores a portion of the ultrasonic signal in its digital memory and then either replays it mmediately for you at a slower speed or stores it for later analysis; it does not transform the calls in real time.

Since the TE detector actually stores the entire original signal, the output of such a detector provides the same information as that of the original signal. 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. A 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, TE detectors are more sensitive and offer the user better possibilities to aurally analyze the transformed calls. If you require a broadband bat detector, a TE detector is usually the best choice.