Bat Echolocation Researc h - Bat Conservation International

It is important that the microphone used has a reasonably
flat frequency response and a high signal-tonoise
ratio.
A time-expansion detector requires some mechanism
to start and stop the recording process. In the manual
triggering mode, a button is used to start and stop
recording. This gives the user full control over the
recording, but also requires manual operation of the
detector. In the automatic triggering mode, the detector
is normally recording continuously. When a sufficiently
strong signal is detected by the triggering circuitry of
the detector, the recording is interrupted after a certain
time. The recording parameters can be adjusted to
ensure that the time-expansion memory contains a
recording of the signal from a certain time prior to the
triggering instant to a time after the triggering instant.
This avoids cutting off the beginning of the first pulse.
Furthermore, the triggering source can be either the
entire signal frequency range or a narrowband signal.
The former makes the time-expansion system sensitive
to all ultrasonic signals, while the latter will make the
detector react only to signals within a certain frequency
band. This can be used to avoid triggering by undesired
signals such as low-frequency noise.
The fact that time-expansion detectors store the
original signal in the memory allows for other important
applications. It is possible in some detectors to replay
the signals at the original speed, rather than a lower
speed that is used under normal circumstances. If a
suitable ultrasonic loudspeaker is available, the timeexpansion
detector can be used for playback
experiments of ultrasonic signals. When the signal is
replayed at its original speed, it can also be fed to the
heterodyne input of the detector, if available. The resulting
signal from the heterodyne system will be exactly
the same as that recorded in real time originally,
enabling the user to carefully listen to the heterodyned
signal over and over again. This is of value for immediate
identification of bats in the field. Both of these
features are available in some bat detectors, e.g., the
Pettersson Elektronik D240X.
HIGH-SPEED RECORDING
With a time-expansion bat detector, it is also possible
to make recordings of the original signal, albeit in a
slowed-down version. As mentioned, the limited memory
size constrains the maximum recording time.
Although the recording time feasible with time-expansion
bat detectors is usually sufficient for recording bat
calls, there are situations when longer recordings are
required. In such cases, high-speed recording of the
original signal can be an alternative to the time-expansion
technique.
High-speed recording can be done in a number of
different ways, for example, using a computer with a
high-speed data acquisition card. Other methods
include using a high-speed digital audio tape recorder or
Section 3: Ultrasound Species Identification
instrumentation tape recorder, a device supporting the
DVD audio format or a specialized high-speed recording
device, using a hard drive or a flash memory card as
storage medium.
Recording ultrasonic signals directly requires a fairly
large memory size. Sampling at 300 kHz with a resolution
of 8 bits requires about 18 MB of memory per minute
or 1 GB per hour. However, with the size of today’s hard
drives, storing large files should not be an issue, and it is
possible to obtain many hours of recording time.
In addition to the increased recording time, the highspeed
recording technique also means higher sound
quality than most time-expansion bat detectors, since
the latter usually incorporates both an A/D conversion
and a D/A conversion. In order to analyze the signal
with a computer, a second A/D conversion is made. In
most high-speed recording devices, only one A/D conversion
is needed, which is obviously an advantage.
Since high-speed recording is simply a recording
technique and not an ultrasound transformation technique,
ultrasonic calls are not made audible, at least not
in real time. Consequently, it is often practical to also
use a simple, real-time transformation bat detector such
as a heterodyne or frequency-division model. This will
allow the user to obtain information about ultrasonic
activity in real time and thus determine when to start
and stop the recordings.
Although a high-speed recording device can be made
relatively small, a time-expansion bat detector with an
audio cassette or digital audio tape recorder is usually a
less expensive solution, so the former should not be considered
a direct substitute for a time-expansion detector.
EXAMPLES OF SYSTEMS FOR TIME-EXPANSION
AND HIGH-SPEED RECORDING AND PLAYBACK
There are a variety of systems for recording and playing
back signals with time-expansion or high-speed
techniques. Factors to consider when purchasing such a
system include the degree of ruggedness for field work,
the sound quality, and the maximum recording time.
Time-Expansion Bat Detector and a Tape Recorder
A time-expansion bat detector and a tape recorder
(regular or a digital audio tape recorder) usually constitute
a highly portable, rugged, and inexpensive solution.
For optimum sound quality, the tape recorder should
have a manual recording level control. In the case of a
regular cassette tape recorder, high-quality cassette
tapes, such as CrO 2 or metal types, should be used.
For automated, unattended recording of bat calls, a
self-triggering bat detector is required. In order to have
the tape recorder make recordings automatically, a
voice-activated tape recorder can be used. Some timeexpansion
bat detectors also have a tape recorder control
output that can be used to make the tape recorder start
and stop automatically. The total recording time in the
automatic recording mode is obviously limited by the
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