professional and research work, as they allow a complete analysis of the bats' calls at a later time. Sampling frequency for digital/TE detectors Research in 2010 observed that frequencies used by bats can be has high as 250 kHz.). The Nyquist–Shannon sampling theorem observes that the minimum sampling frequency required to record a signal successfully must be greater than twice the bandwidth of the signal. To record a bandwidth of 250 kHz therefore requires a sampling frequency in excess of 500 kHz. Modern Time-Expansion capable units typically sample at between 300 kHz and 700 kHz. In general, faster is better, though a higher sampling frequency does use more storage space. Other bat detector types Zero crossing analysis ZCA is most commonly associated with the Anabat bat detector from Titley Scientific. The original bat calls are digitised and the zero crossing points used to produce a data stream which is recorded on a memory card. There are sophisticated timing and trigger controls and the device can be set to respond to bat calls, so that many hours of recording are available in unmanned situations. The purpose of ZCA is to reduce the amount of data which must be recorded to the memory and may be considered as a simple form of lossy data compression. Historically, to achieve long recording times, such information reduction has been necessary due to memory capacity limitations and memory cost. The solid state ZCA recording is analysed by custom software to produce a time/frequency plot of each call which can be examined for species recognition in a similar way to FD or TE recordings. How it is used The ZCA detector is usually placed in a bat roost or bat flight path and left for a number of days to collect data. Thus it is less labour-intensive than using a manned bat detector in real time. Pros and cons While the ZCA detector can also be used in real time, its value is for remote recording over long periods. The analysis is similar to that for FD recordings, but there is no amplitude data included. However it does accurately record each zero crossing point, rather than only one in ten. As with all recording devices triggered by an input, a ZCA detector recording automatically is prone to ultrasonic interference from insects such as crickets. Filters can be written to select a characteristic frequency of certain species and