Frequency Division
The detector divides the incoming frequency (usually by 10) to give an audible output. For example 50kHz (inaudible to the human ear) is output at 5kHz (audible). This method allows you to record bats on all frequencies constantly, reducing the chance of missing a bat or part of a bat's call.

Time Expansion
A time expansion detector digitally records the ultrasonic call and then plays it back at a slower rate (usually 10, 20 or 32 times) so you can hear details of the sound not normally audible. Unlike an FD detector, during the playback currently being recorded, you may be missing other bats calls. However, TE will produce a much better quality sonogram in the sound analysis software.

Any device should have a line in socket and preferably a line out. Using line in and out means you will make the best quality recordings and keep the quality when it comes to transfer to your PC for analysis. It is possible to use microphone and headphone sockets but the recording can become corrupted with variable recording levels of these types of socket.

A device with a backlit display or buttons is very useful for when you are trying to make recordings in the dark.

A device which 'stamps' the date and time on each recording is also very useful for when it comes to sorting for later analysis.

Digital Audio Recorders

• These latest devices like the Edirol R-09 save digital recordings onto a built-in memory card such as an SD card or CompactFlash.
• Sounds can be recorded as MP3 or as WAV.
• They also store huge amounts of recordings - up to 12 hours.
• USB 2.0 connection to PC means that hours of recordings can be downloaded very rapidly.
• They also have built-in microphones for voice notes.

MP3 Recorders

• These devices save digital recordings onto built in memory or a hard disc.
• Recordings are normally stored as MP3s (which can't be used by BatScan, Batsound etc.) - but they usually come with software to convert the files to WAV.
• They can store huge amounts of recordings and can take many hours worth even on the models with the lowest memory.
• Some digital recorders can have a couple of seconds delay between pressing record and the unit actually starting to record. This can mean you miss part of a bat's call. Some have also been found to have a higher battery consumption than other devices.
• The greatest benefit of using one of these systems is when it comes to transfer of the calls to a PC. As they store the recordings as files, you can link the device with a USB or firewire cable and transfer hours worth of recording in minutes.

Minidisc Recorders

• Minidiscs are a digital recording device so there are no problems with recording speed.
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• Minidiscs can store about an hour's worth of recordings on one disc. This can be increased by decreasing the quality of the recording.
• They are usually small and portable but can have small buttons which can be difficult when using in the dark.
• Minidiscs can start recording instantly if using pause between records so there is less chance of missing part of a call.
• Transfer of recordings to a PC can be a long process. If you have made 5 hours worth of recordings then it will take 5 hours to play into you PC before analysis.
• Battery life tends to be quite good. A minidisc will usually run for several hours on only one or two AA batteries.

Tape Recorders

• The most popular method before the arrival of digital recorders due to the low cost and ease of use.
• They tend to be more bulky than a minidisc or MP3 recorder.
• Tape recorders may not always run at a constant speed which can affect the frequency of your recordings. This should be checked by recording a known reference frequency.

Your PC should ideally should have a line in socket for the same reasons as with the recording devices. Although a microphone socket can be used.

Before transferring, check your Windows sound properties and volume controls are setup correctly:

• In the volume control window, choose properties from the options menu. Choose adjust volume for recording and make sure the line in box (or mic if using that instead) is ticked and then click OK.
• The recording volume control window will now come up. Make sure the line in (or mic ) is selected.
• Also note in this window the recording volume level. This may need to be adjusted if your recordings are coming through too loud or quiet.

The two main options at the moment are Batsound from Pettersson Electronik, or BatScan from Batbox Ltd.

BatScan has been primarily developed for use with a Frequency Division (FD) detector. Recordings from a Batbox Duet or Baton detector can used without the need to change any settings. It can also be used with any other FD or Time Expansion (TE) detector with small alterations to the settings. A more economical choice than Batsound, it has fewer functions but is quick and easy to use for simple call analysis.

Batsound, although more expensive, has many more functions to allow you to manipulate your recording to aid identification and analysis. Batsound is not designed for any specific detector - can be used with FD and TE with just some alteration to settings.