You can do this with your exisiting heterodyne converter and audio
recorder a lot cheaper than a 722. You already have all that you
need. I have a similar bat detector to yours, price-wise. On these
the sensor is a highly tuned to 40kHz piezo device, and has a peak
sensitivity bandwidth of about 37kHz to 42k.
Record the audio output of the heterodyne detector. I used a HiMD for
testing. Set the oscillator frequency to something like 35kHz. This
all depends on the type of bat you're after, you want the oscillator
to be about 1-2kHz below the lowest incoming frequency
I then put a scope on the audio output, and could see the oscillator
frequency breakthrough. You could also read this from the knob, less
accurate but still workable unless you need this calibrated for
analysis.
Capture the wav from the HiMD. The use the freeware Spectrum Lab
http://people.freenet.de/dl4yhf/spectra1.html to mix the signal with
the oscillator frequency. You must make sure the spectrumlab output
file is sampling at least at 96kHz, which is not the default on that
program. You can even go to 192kHz if you have a wider band sensor,
or you are chasing signals > 48kHz.
Take output file and just change the sampling frequency (not
resample, just change) from 96kHz to 22kHz. This will time-expand the
signal by 4.36, which is enough to make the signal audible. You are
now the proud owner of a poor man's time expansion bat detector at no
extra cost, though you are limited by the narrow bandwidth of the
sensor compared with the rich man's version
http://www.alanaecology.com/acatalog/Pettersson_D_240x_Bat_Detector.ht
ml
The tone colour of the result is a lot better. I have done this with
a jangling keys test - where the original
http://www.suffolkbirds.co.uk/audio/mono-batty38khzkeys.mp3
is a strange sort of sound, that signal upconverted and time-expanded
http://www.suffolkbirds.co.uk/audio/battyupcovnert-4xexpand.mp3
is a much more recognisable metallic sound.
I did try modifying my detector to divide down the oscillator with a
CD4040 chip to the audio range, so I could feed the main output to L
channel and oscillator divided by 8 to right output, but you need
decent screening to keep the harmonics out of the microphone
amplifier. So I leave that for another day when I have found some
reliable bat locations. I seem to find noctule bats when I am audio
recording at dusk but never when I have a bat detector to hand :)
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