ignore others; some (CF species) are more easily filtered, others are nigh on impossible. High frequency recording This can be done by using a high speed digitiser peripheral on a computer such as a laptop. This is not a bat detector as such, but recordings of bat calls can be analysed similarly to TE recordings. This method produces large data files and produces no means of detecting bat calls without the simultaneous use of a bat detector. There are however also more sophisticated systems such as the Avisoft-UltraSoundGate that can replace a conventional bat detector. These advanced systems additionally provide a real-time spectrographic display, automated call parameter measurement and classification tools, integrated GPS functionality and a versatile metadata input tool for documenting the recordings. DSP detectors DSP bat detectors aim to provide an acoustically accurate portrayal of bat calls by using a digital signal processor to map bats' ultrasounds signals to audible sounds; different algorithms are being used to accomplish this, and there is active development and tuning of algorithms going on. One strategy called "frequency shifting" uses a FFT signal analysis in order to find the main frequency and signal power, then using digital simulation a new audible wave is synthesized from the original one divided by a defined value. The processes of Frequency Division and Heterodyne conversion can also be performed digitally. Time domain signal coding This type of bat detector is believed to be in pre-production or experimental and is not available commercially. Research is in progress for analysing many types of ultrasound calls and sounds besides those of bats. A TDSC detector digitises the original calls and derives a two dimensional data string by analysing the parameters of each call with respect to time. This is analysed by a neural network to provide pattern recognition for each species. Non-acoustic detection Visual observation is the obvious means of detecting bats, but of course this can only be done in daylight or crepuscular conditions (i.e., dusk and dawn). Emergence counts are done visually at dusk, using a bat detector for confirmation of species. In lower light conditions a night vision device can be used but the more affordable generation 1 type has a lag time which fails to provide a suitable image of a flying bat. Infrared (IR) cameras and camcorders are used with an IR illuminator to observe bat emergences and bat behaviour inside and outside roosts.