Recording calls

While it is interesting and exciting to be out in the dark listening and watching bats, its great to be able to share your experience with someone who was not there.  Perhaps you are unsure about the species you were hearing, especially if you could not see them well.  Maybe the calls they made weren't typical of the species.   Recording the sounds for later playback or analysis can bring a new dimension to your interest, and give you greater insight into the amazing capabilities of bat sonar.  It can also be used (with a considerable degree of caution) to HELP identify or discriminate between different species.

Some alternative techniques

Perhaps the best way of recording bat calls is to record the original bat sound.  Avisoft BioAcoustics make ultrasound recording equipment that links directly to a computer via the USB port, enabling the actual sound from the bat to be recorded. This professional standard equipment is expensive (£2000 upwards) and you would need to consider the practicality of its use in the field.  (i.e. you need to carry the recorder, microphone and a laptop around). However equipment is currently under development (e.g. BATBOX Griffin) that makes this more practical - and more economical.

Less expensive approaches are based on recording the output from your bat detector.  Any bat detector with a signal output can be connected to a sound recorder to record signals.  (see sound recorders below.)
Heterodyne detectors are not ideally suited to this kind of work as they lose the most important part, the frequency of the original signal; however time expansion detectors and frequency division detectors change the signal in a very precise and predictable way that supports detailed analysis.  The BCT have produced a PDF document about digital recording with bat detectors HERE

   

Using a time expansion detector

Using a time expansion detector allows very good quality recording and faithful analysis of the signal.  A professional quality set up would typically include a Petterson Time expansion detector, such as the D240x (£1085).  To go along with this you would need a recorder; The Edirol R-09 recorder records at high quality onto SD cards up to 8G: Price £300 inc 1G SD card.  You would then need a sound analysis package, and the software recommended for the D240x is the BatSound package, priced at £250.  (All available from Alana Ecology.)  Digital recorders have the advantage of allowing you to transfer the recorded and encoded signal directly to your computer.

"Budget" using a frequency division detector

Detectors such as the BatBox Duet ( £275 ) and even cheaper, the BATON can be used to good effect with a sound recorder.  To keep costs down and because the signal quality is not so good you can use a cheaper recorder such as an MP3 recorder or MiniDisk.  The Sony MiniDisc transfers signals to the computer via a USB cable. Sony has a program called ‘Connect’ which you can download from their website FOC. This enables a fairly smooth extraction of the data from the disc and stores it into a library. The data can then be converted to WAV format for analysis.
For sound analysis you could choose BatScan (about £100) or WaveSurfer which is a free open-source program.

Many MP3 players offer speech recording via a built-in microphone.  The quality of this is nowhere near good enough for bat recordings, and a "line in" input is required. Barry says "recorders need to be of a certain quality to be able to usefully record bat calls.  For WAV recording it will need 16 bit sampling at 44.1 KHz (or higher); For MP3 it will need to sample at 320kbps (160kbps per channel) at 44.1 KHz (or higher). "  However there are some (example below) that have the right characteristics.

TREKSTOR Organix 2. A compact music player that can play mp3, wav, ogg, and wma files, and can record from internal microphone and also from a line-in socket direct to mp3 format.  Has a clear bright two colour (blue / yellow) OLED display
Available in 4G and 8G versions from Bat Planet.
Features
Menu options to adjust the following:
line-in record gain to suit your detector and also microphone gain
record quality for line-in and microphone (choose 96, 128 or 192 kb/s)
power save mode (off after 3, 5, 10, 15, 20 sec., never)
Brightness is controllable to give good battery life and suit working in the dark.
The rechargeable (via USB) Li ion battery gives up to 50H continuous use.
When connected to the USB port the player appears as a drive. Recording files are stored in their own folder and easily moved or copied to your computer.
As with most MP3 players the menu system takes a little getting used to.  There is no sound in the earpieces when recording, and there is no record level indicator.  This makes it rather difficult to use with bat detectors like the BATON that turn off the speaker when the line-out or headphone socket is in use. Its chunky and gives the impression of being robust - tough as old boots!

Check your mp3 recorder can be easily set up to record.  My MuVo 200 takes ages to set up so is no good in the dark!

Signal processing

Advances in digital signal processing are making new approaches possible.  In particular wavelet transforms are a powerful strategy recently discovered for use in digital signal processing.  Another new technique, Waveform Similarity Overlap Addressing (WSOLA) allows data reduction by recognising that most sound signals don't change much between one wave and the next.  WSOLA allows a representation of the original signal to be built using fewer waves.  IF the original signal can be represented by one tenth as many waves, and if these waves are then 'stretched out' by a factor of ten we get a true representation of the original signal, but at one tenth the frequency.  This would allows us to convert bat sounds at 20 - 120kHz down to the human hearing range at 2 - 12 kHz in real time.  A similar result is produced by "frequency compression" where the signal is analysed and a similar signal synthesized at a lower frequency.  This is currently used in DSP hearing aids!

Sound Editing

Sooner or later you will want to do some editing - to remove hiss or wind noise and clean up your bat call recordings.  Audacity is a great free tool for doing these jobs, and can also record directly and change from one format to another.  If you want to save the sound file you need to "Export" it.  You will also need to download the (free) LAME encoder.