Frequency compression in a Binary and Acoustic Tech detector



The new DSP detector "Dodoultra" has only just come out and the company Binary and Acoustic Technology in the USA is already announcing a similar DSP-based system to be implemented in one of their detectors. From reading their website it is not entirely clear to me when this detector will come out and how exactly the DSP-based conversion process works. We don't know for sure if the process is the same as used in the Dodoultra. On their site, one can download a so-called "frequency-compressed" recording of western Pipistrelles.

compression

Unfortunately, the time-expanded version of the same recording is in fact a recording of free-tailed bats near a cave, so there is no original to compare it to. Still, we can more or less safely assume the original is a steep FM signal with a QCF tail. What we see in the compressed recording is that the FM sweep doesn't come through very well. The QCF part is at 60 kHz, so we expect a second harmonic at 120 kHz, which we find instead at 180 kHz, or is the sound in between the second harmonic? Low amplitude calls have a tendency to drop in frequency, down to very low frequencies, which is obviously undesirable. It could well be that the sound file was just to give an impression of the sound quality, so we are looking forward to seeing more recordings with the future detector.




Last updated 20.5.2009