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playing the desired sound file. The signal is then replayed at its original speed and frequency
regardless of the time expansion factor (TE) setting.
Note: In order not to interfere with the continued streaming of samples to the CF card at high
sampling frequencies, the update and display of the tuned frequency and clock as well as the
update of the battery indicator is disabled while recording with the two highest sampling
frequencies, 750 and 768 kHz.
The frequency division system
Frequency division means that the frequency of the ultrasound is changed with a constant
factor of 10. A frequency of 40 kHz becomes 4 kHz, a frequency of 50 kHz becomes 5 kHz
etc. Using this system, any activity over the entire frequency range 10 – 200 kHz can be
constantly monitored. Furthermore, the shape of the transformed signal closely follows that of
the original signal, as shown in the figure below.
The frequency division is obtained by counting the number of “zero-crossings” made by the
original signal. An output signal with one zero-crossing for each ten zero-crossings of the
original signal is then generated. Normally, this gives an output signal with a frequency of one
tenth of the original frequency. However, if the original signal has a strong harmonic
component, extra zero-crossings will occur and the resulting output frequency will be higher
than that corresponding to the fundamental frequency (the system transforms the harmonic
frequency component, not the fundamental). This situation is relatively uncommon, except for
e.g. the Rhinolophus species. Nevertheless, it is important to remember that it occasionally
may occur.
The time expansion system
In a time expansion system, a sequence of the ultrasonic signal is captured and stored in a
digital memory after which it is replayed at a slower speed. That way the frequency becomes
