Dodoultra is the first DSP (Digital Signal Processor) based bat detector.
This device samples incoming ultrasounds in range 10 Khz ~ 125 Khz converting analog signals into digital. The integrated DSP processes ultrasound and produce udible waves in two different ways:
- Digital heterodyne.
- Frequency shift.
Improvements:
Using digital heterodyne system, sampled signals are digitally processed and the result is a low noise, a better precision.
Dodoultra is the first device to use frequency shift technology.
Therefore it's possible to understand the more powerful wavelenght and on this basis a digitally synthesized (DDS) wave is generated with a reduced factor of 10 frequency. The result is a best precision than that obtained in analogic zero crossing system.
A graphic display offers a wide data visualization and an encoder allows an easy menu navigation.
Here some simple recording tests made with frequency shift: freq1 freq2 ethero
Introduction to bat detectors
Bat detectors
Bat detectors are used to detect the presence of bats mainly converting their ultrasound signals to audible frequencies.
A major limitation of acoustic bat detectors is their range which is limited by the absorption of ultrasound in air. At mid range frequencies around 50kHz, the maximum range is only about 25 to 30 metres in average atmospheric conditions when bats fly. This decreases with increasing frequency. The usable range of bat detectors decreases with humidity and in misty conditions the maximum range can be very low.
Waves...
Human ears are normally limited to the range from 20 Hz to 16 kHz, bats emit calls from about 8 kHz to 160 kHz even if the greatest part of sounds are limited to around 10 kHz to 125 kHz.
Ultrasounds
There are three types of bat echolocation call, Frequency Modulation (FM) Constant Frequency (CF) (sometimes called Amplitude Modulation) and composite calls with both FM and CF components.
The FM call is heard as rapid dry clicks and the CF call as peeps. It varys in frequency due to the Doppler effect as the bat flies fast. A heterodyne bat detector exaggerates the Doppler effect. As the bat making the CF calls swoops over the detector the pitch falls.
Several species of bat use a composite FM and CF call starting with a rapid falling FM call which slows to become a CF call at the end, giving a "hockey stick" shape to the graph. This makes the call sound different on a bat detector.
Pipistrelles generally use the hockey stick call for general echolocation, but use only the FM part at times.
Bat detector types
Till now there were three types of bat detector in common use, the Heterodyne, Frequency Division and Time Expansion. Some bat detectors combine two or all three types. The new DSP based series open a new field in ultrasound research increasing new digital methods.
Heterodyne
Heterodyning is probably the oldest, most widely used, most successful, and most self-build method of transforming bat calls. A heterodyne function is often built into the other types of detector too.
A "heterodyne" is a beat frequency such as can be heard when two close musical notes are sounded together. The user selects a frequency to which the detector will “listen” through a narrow frequency window, with the selected frequency representing the center of the window. When a signal is detected it is mixed with a signal from an input oscillator. It produces a signal with two frequency peaks which is filtered so that only the low frequency peak remains. For instance, a bat calls at 45 kHz and an internal frequency of 43 kHz produces output frequencies of 2 kHz and 88 kHz. The 88 kHz frequency is inaudible and is filtered out and the 2 kHz frequency is fed to a loudspeaker or headphones. The internal frequency is displayed on a dial or on a display.
Unfortunately, the limited bandwidth to which the detector listens blurs the duration, absolute frequency, and the frequency time course of the original call in the eterodyned signal, thus rendering it unacceptable for spectral analysis. The advantages are in good signal to noise ratio due to the narrow bandwidth and that it works in real time, exaggerates the frequency changes of a bat call, is easy to use, and is the least expensive. It is easy to recognise a doppler shift in CF calls of flying bats due to their speed of flight.
The disadvantages of a heterodyne bat detector are that it can only convert a narrow band of frequencies, typically 5 kHz, and it has to be continually retuned, and can easily miss species out of its current tuned range.
In the field the operator guesses the likely species to be present and tunes the frequency accordingly. Many users will start listening around 45 kHz. If a bat is seen or a bat-like call is heard, the frequency is tuned up and down until the clearest sound is heard.
All FM calls tend to sound like clicks, but the start and end frequencies and the call repetition pattern can give clues on the species.
Frequency division
Frequency division bat detectors produce a sound that is a fraction of the bat call frequencies. This is done by converting the call into a square wave, a counter 'counts' the zero crossings, after a defined number of passages (usually 10) a pulse is produced. The common frequency division offers a lot of problems in sound identification i.e. sounds with great harmonic spectrum are wrongly interpreted and the coming out frequency is usually much greater (double or more) than the expected one.
Square waves sound harsh and contain harmonics which can cause problems in analysis: the amplitude is completely lost and a low noise could be amplified as well as useful signals.
Some frequency division detectors output this constant level signal which renders background noise and bat calls at the same high level. This causes problems with both listening and analysis. The best systems finally multiply this output with the amplitude envelope of the original signal for a good compensation.
As with a heterodyne detector, a frequency division detector works in real time. The main advantage is that bat calls can be heard in their entire call frequency range rather than over a limited frequency range and the call pattern can be measured.
A serious disadvantage with real time listening is that the speed of a bat call remains fast, often too fast for the species to be recognised. The frequency changes of CF calls are not exaggerated as with a heterodyne detector and so are less noticeable.
The zero crossing system tracks the harmonic with greatest amplitude, so no other harmonic information is contained in the output signal. In some species of bat frequency overlap among the harmonics means that the harmonic with most energy may change over the course of the call. This can cause the zero-crossing system to jump between harmonics and a misleading output signal, especially when spectrally analyzed. Besides the use of an input amplitude threshold can decrease the overall sensitivity of the detector and lead to quieter calls being missed. The frequency division method reaches its limitations in very short, high frequency signals.
Frequency shift
The frequency shift derives from and offers a lot of advantages than the classic frequency division: the sound is completely digitally processed. Dodoultra is the first bat detector to use this method.
Incoming signal is not converted in square waves and the main frequency does not come from zero crossingand does not delete all amplitude and misleading high harmonic waves.
Signals coming from the pre-amplifier are converted in frequency domain using FFT (Fast Fourier Transform), therefore the most powerful frequence is extracted and a ten times slower wave is synthesized and digitally reproduced with the same amplitude seen in FFT. This method allow a suppression of environmental noise and even in high harmonic sounds only the most powerful frequency is taken in account.
Time expansion
Time expansion detectors work by digitizing ultrasound at a high sampling rate (al least doube of bandwidth) and replaying them at a lower rate afterwards. This mean that a 120 Khz ultrasound can be recorded only sampling this signal with a sample speed greater than 240 Khz.
Most time expansion systems include a fast 8 bits A/D converter, the output is audible on a loudspeaker or headphones and audio recordings can be made simultaneously for later analysis. The slowed down calls can be heard as a drawn-out bat call at audible frequencies. Therefore fast FM calls can be heard as a descending note instead of a click. Thus it is possible to hear the difference between FM calls which just sound like clicks on the other types of detector.
After downloading an audio recording to a computer, the original calls are analysed as if they were still at the original non-expanded rate.
The output can be recorded with an audio recorder as with frequency division detectors. However the whole waveform is recorded with the full call range being preserved, rather than 1/10th of the waveform as in a frequency division detector.
While the recorded sample is being played back slowly, nothing is being recorded, so the bat calls are being sampled intermittently. For intance, when a 1 second call is being played back at 1/32 rate, 32 seconds of bat calls are not being recorded. Time expansion detectors are typically used for professional and research work, as they allow a complete analysis of the bats' calls afterward.
Sampled time is limited by the RAM memory, sampling rate and number of bits of the AD converder, so it's not possible to fulfill a continuous recording.
Records of bat sound can be done in some device on Flash memory (SD card or Compact Flash).
Recording pre elaborated signals coming from time division or etherodyned signals can be done but there's an information loss and this method is not valid for scientific research.
DSP detectors
DSP bat detectors utilise a digital signal processor to map bat ultrasound signals. The frequency shift allows a FFT signal analysis in order to find the main frequency and signal power. In using DDS a new audible wave is synthesized from the original one divided by a defined value.
Detector type
Noise level
Freq. range
Real time
Sygnal analysis
Precision
Harmonic consider.
Etherodyne
low
Narrow band
yes
bad
middle
yes
Digital etherodyne
low
Narrow band
yes
bad
high
yes
Frequency division
high
full
yes
Low quality
low
no
Frequency shift
very low
full
yes
Middle quality
high
yes
Time expantion
Related to sampling rate and n° of bits
full
no
yes
high
yes
Digital recording
Related to sampling rate and n° of bits
full
no
yes
high
yes
Operating instructions
Dodoultra is controlled with:
2 volume button
1 ON – OFF button
1 encoder with push button
Data is visualized in a backlighted graphic display.
Dodoultra is turned on by pressing for 2” the “ON OFF” button, the display and all internal circuitry is turned on and a startup message is displayed. By pressing 2” the same button turns off the device.
Turning the encoder allow navigation between functions, you need only a pressure for entering in the following functions:
- Heterodyne
- Frequency shift
- Gain
- Filter mode
- Display contrast
- Backlight regulation
- Informations
Heterodyne
Before entering in this function the display shows the actual most powerful frequency captured by the integrated microphone and this frequency is the starting point in heterodyne. This function is turned on pressing the encoder and the band of frequency is changed rotating it.
The frequency range is dictated by “FILTER MODE”.
Frequency shift
In this function we have a full band of ultrasound frequencies, a reduced wave in factor of 10 is generated and the frequency range is dictated by “FILTER MODE”.
Gain
The gain is the amplification of the incoming signal from the preamplifier and before the Analog to Digital convertion.
It's very important to set the right gain! A wrong value can lead to distorced samples and bad interpretation both in heterodine and frequency shift. Low gain can produce a reduction of signal to noise ratio.
The range of gain can be changed from 1 x to 32 x.
Filter mode
Filter mode has two ranges:
10 ~ 125 KHz
20 ~ 125 Khz
The first mode can be used in noisy environment where crickets or other unwanted noise mislead DSP interpretation.
The second selection allows a wider frequency range.
Display contrast
Display contrast can be changed by rotating the encoder, a press sets temporary the value selected.
Usually it's not necessary to change LCD contrast.
Display Backlight
The backlight is changed in order to reduce power comsumption and the range is from 0 to 8.
Informations
Here are some informations such as firmware release and data is visualized.
Saving setup
In order to save all settings is necessary to press both volume buttons for a while, a message will be displayed and setup data is saved. Even removing batteries after long time setup data are retained in internal memory.
Tips & Tricks
Dodoultra had been developed in order to guarantee an high sensitivity. This may lead a feedback between the loudspeaker and the microphone producing so unwanted distorced sounds (Larsen effect).
If you need an high sensitivity at high volumes you can increase the gain and volume using headphones. However the best way to hear bat is achieved with a gain of 10 and a volume in the middle range.
Technical specifications
- Decoding technology: Digital Signal Processing
- Ultrasound deconding: Etherodyne – Frequency shift
- Sampling rate 250,000 Samples/Second – 12 bits
- Frequency range: 10 KHz ~ 125 Khz
- Processor speed 40 Mhz = 40 MIPS (million instruction per second)
- Battery 9 volts standard
- LCD graphic display 128 x 64 dots
- Size: 89 x 147 x 25 mm
- Weight:160 g (battery not included)
- Output: Headphone out 3,5 mm Jack
Sources:
-Wikipedia http://en.wikipedia.org/wiki/Bat_detector
-Jeanette A. Thomas, Cynthia F. Moss, Marianne Vater. 2004. Echolocations in bats and dolphins.
-Ahlén, I. & Baagøe, H. 1999. Use of ultrasound detectors for bat studies in Europe -- experiences from field identification, surveys and monitoring. Acta Chiropterologica, 1:137-150.
