Heterodyne; Connection To Recording Device; Headphones - Batbox Duet User Manual

2.5kHz), and also into the range of an audio recorder
(MP3, minidisc, tape etc.).
The Batbox Duet also measures the amplitude of the sonar before
dividing by 10 and then reinstates it at the output. This creates an
identical waveform to the original signal but reduces the pitch to an
audible frequency. Having analysed the recorded calls on a computer,
our peak frequency of 2.5kHz, would be exactly 1/10th of the original bat
sonar (25kHz). To ascertain the original peak-frequency, simply multiply
the analysed result by 10.

HETERODYNE

Heterodyning involves the mixing of two signals, one being the bat call
from the ultrasonic microphone and the other produced by a local-
oscillator in the detector. These signals are mixed and filtered in such
a way as to produce a difference between them at the output of the
detector. The difference is low enough in frequency to hear. For example,
if the detector oscillator (frequency control) is set to 42kHz and a
frequency of 40kHz supplied to the microphone, a difference of 2kHz will
be heard at the output.
The digital frequency-counter of the Duet gives the exact frequency of
the local-oscillator to the nearest 1kHz. When the detector is tuned to the
peak frequency of a bat call there is, in theory, zero frequency output.
In practice, because the bat is in motion the calls will always be heard,
but the lower the pitch of the output, the closer the local-oscillator
frequency is to that of the bat's peak-frequency.

CONNECTION TO RECORDING DEVICE

(MP3/minidisc/tape, etc.)
Batbox Duet will record both frequency division and heterodyne, at
the same time, so that when doing the computer analysis, it is possible
to listen simultaneously to the heterodyne calls whilst looking at the
frequency divided calls. This will be useful to those with some experience
of identifying bats with a heterodyne bat detector, like the Batbox III.
A stereo recorder will record F/D in the left channel and heterodyne in the
right. Connection to a mono recorder will result in F/D only,
i.e. without heterodyne.
The recording device should have a line level input. If not, and your
recorder has only a microphone input, a special attenuation lead may be
required, in order to reduce the signal level from the detector.

HEADPHONES

Although it is possible to listen through the built in monitor speaker,
listening with headphones provides greater sensitivity and clearer
reception, for hearing bats. Both channels on the Batbox Duet
headphone socket are the heterodyne function.