The Dodoultra: a DSP-based frequency-shift detector
There's a new kind of detector in town. The company Dodotronic in Italy, who have this extinct bird on their logo, are selling a new type of bat-detector. Here is the website of Dodotronic. In the meantime the company have given us some more information on the Dodoultra.
The detector appears to be something like a very small computer with a "sampling board" that samples continuously at 250 kHz (12 bits). The microphone signal is digitised immediately and this DIGITAL sound is then converted into either a heterodyne or a frequency-shifted output. When I say immediately, I really mean immediately: this detector has a silicon microphone. The production of silicon microphones has only started very recently and most have found their way into a new generation of hearing aids that produce less noise, are more directional and less sensitive to interference of magnetic fields. Dodotronic have managed to lay their hands on an ultrasonic version of the silicon microphone that has a linear frequency response up to 70 kHz and still a good sensitivity up to 130 kHz. A silicon microphone is in fact a chip with a "membrane" directly on it. The movements of the rigid silicon membrane, due to the incoming sound are digitised at the spot. In the Dodoultra, this happens with 250 kHz and 12 bits. Ivano Pelicella, one of the makers of the Dodoultra, wrote to me that the microphone will only stop working if it is completely wet, but works fine even under great humidities.
As you read the word "digitised" you are probably thinking the Dodoultra is a time-expansion detector, but it's not. As mentioned before, you can do two things: heterodyne and frequency-shift. Contrary to conventional detectors though, all this is accomplished digitally. In the heterodyning process the detector finds the most intense frequency itself and shows it on the dial. The user can then change the dial to judge the sound at other frequencies.
Of course, most readers will be interested in the frequency-shift function on this detector. This system is not as precise as time-expansion, but seems to be a better alternative to heterodyning and frequency-division. In real-time, a Fast Frequency Transform of 512 points (FFT) is calculated of the microphone signal. The frequencies that show up are then all devided by 10 so they are within the audible range. This is different from frequency-division, because here only the first harmonic is divided by 10 (or another factor). The whole process is also more sensitive than frequency-division conversion, so we are probably more likely to see more realistic starting frequencies of bat calls.
A good call has also been to make the gain adjustable (as in the Pettersson D240) as the loudest horseshoe bat you can get may well be 15 dB below most pipistrelles and you might want to be able to record both species well.
An FFT-size of 512 points with a sampling frequency of 250 kHz imply the frequency resolution is 500 Hz, which is quite good. However it also implies a time resolution of only 512/250=2 ms. I would guess the system interpolates, or in other words, the FFT-window moves only a couple of samples (fewer than 512) between calculations. This will become clearer as soon as we will see the first recordings with the Dodoultra.
The detector is real-time, so you will hear digitally transformed pulses as the bat is passing by, which really is a novelty. In fact, you don't hear the transformed sound itself, but a sound that was synthesized by the detector to follow the original as closely as possible.
The detector doesn't seem to be intended for scientific analysis of bat pulses, but depending on its performance it could offer a better alternative to heterodyning and frequency-division. It is very interesting to see the emergence of the first DSP detector and we hope to see more developments in this area of course.
The current price of the Dodoultra is 250 euros, including VAT and shipping.
Currently, Danilo Russo is testing the Dodoultra to see what the recordings look like and whether its design has the right curves to impress the Napolitan girls at night time. He will report back soon, I think!
Dodotronic are also working on a "detector" for professional use. In fact, the whole detector will be replaced by a digital microphone you can plug in any device with a USB port. We will keep you posted.
Last updated 25.5.2009