an analog-to-digital converter and storing the digitised signal in an on-board memory. TE detectors are "real time" devices in that they can be monitored at the time of recording, but there is an inevitable delay while the high speed sampled extract is slowed down and replayed. How it is used In real time mode, with or without an associated heterodyne or FD detector, the slowed down calls can be heard as a drawn-out bat call at audible frequencies. Therefore, fast FM calls can be heard as a descending note instead of a click. Thus it is possible to hear the difference between FM calls which just sound like clicks on the other types of detector. After downloading an audio recording to a computer, the original calls are analysed as if they were still at the original non-expanded rate. Pros and cons The output can be recorded with an audio recorder as with FD detectors, or with more recent units, the signal can be recorded directly to an internal digital memory such as a compact flash card. The whole waveform is recorded with the full call range being preserved, rather than 1/10 of the waveform as in a FD detector. Since both frequency and amplitude information are preserved in the recorded call, more data is available for species analysis. Early units were equipped with small memories which limited the length of time that could be digitised. Once the memory was filled (usually only a few seconds maximum), the unit would then replay the recording at slower rate, typically between 1/10 to 1/32 of the rate of the original recording. While the recorded sample is being played back slowly, nothing is being recorded, so the bat calls are being sampled intermittently. For instance, when a 1-second call is being played back at 1/32 rate, 32 seconds of bat calls are not being recorded. More recent time-expansion recorders use large flash-based memories (such as removable compact-flash cards) and high-bandwidth direct-to-card recording to provide continuous, full-bandwidth real-time recording. Such units can record continuously for many hours while maintaining the maximum information within the signal. Some units are also equipped with an auto-record function and these can be left in-the-field for many days. Some units also include a pre-buffer feature to capture events that happened shortly before the 'record' button was pressed which can be useful for manual surveys. TE detectors are typically used for