INFRASONICON: Time-Compressing Infrasonic Recordings to Discover New Sounds, by Clark Huckaby
Please refer to Figure 2 for a block diagram of my infrasonic recording channel (the "Infrasonicon"). Naturally, the signal path starts with a transducer. The terms transducer, pickup, probe, detector, and sensor are equivalent in this project. The analog output of these devices is a linear measurement of a specific physical parameter. I've used two classes of transducer, based on their output mode: frequency or voltage.
With frequency-output transducers, the infrasonic signal modulates (determines the instantaneous frequency of) a higher frequency carrier. This is like FM radio, only the carrier frequency is lower. It offers transmission noise immunity and the option to store the FM signal in an audio recorder. I've tried two transducers of this class: an "Optical Probe" for light intensity and a "Water Theremin" for water surface waves. Voltage-mode transducers simply output an AC analog of the infrasonic signal. This class includes my "Audio Amplitude Detector." I will separately describe each transducer in subsequent parts of this Overview.
Most of the other blocks in Figure 2 deal with either signal processing or storage. The "Interface Unit" (Figure 3 is its photo) is only used with frequency-output transducers. It scales the carrier (using a choice of division by 1, 2, 4, 8 ... 128) to a nominal 1000 Hz and routes the signal to an audio recorder and/or the "Data Converter" (and also headphones, for a useful system check). Without re-scaling the carrier, the interface is also used when playing audio-recorded FM signals into the Data Converter. The interface has a built-in binary counter, which is handy for calculating exact carrier frequencies and check-sums for audio record/playback fidelity tests. A separate page contains a technical description of the Interface Unit, including its schematic diagram.
|Figure 3 (left): Interface Unit. Figure 4 (right): Data Converter.|
Figure 4 is a photo of the Data Converter. As Figure 2 shows, its first stage is a demodulator, which is a phase-locked loop (PLL) converting FM signals to a voltage analog. Its watch-dog "lock fail detector" drives LEDs indicating demodulation errors. Signals from voltage-output transducers bypass the demodulator and go directly to the Data Converter's next stage: a voltage-mode analog signal processor. This "analog block" has variable gain and low-pass filtration. The filter is needed to reduce or prevent aliasing at the Data Converter's next stage, which is a 12-bit analog-to-digital converter (ADC). The "Basic Stamp" is a 16-bit microcontroller module from Parallax, Inc. (BS-2; Ref. 3); I've programmed it to drive the ADC at user-chosen sampling frequencies up to 268 Hz and stream the data to a PC. The page containing a technical description of the Data Converter has more detailed information, including schematics and links to the source code.
The PC runs either a metering or a file storage program, for "setting a level" and recording, respectively. I coded these in MS-DOS QuickBasic; they are available for download at the Data Converter's technical page. Time compression is determined by formatting the signal as a WAV file (Ref. 4) with a user-specified sampling frequency for playback. I typically used the audio CD standard, 44.1 KHz. If the original infrasonic signal was sampled at 268 Hz, time compression is 44,100 divided by 268, which equals 165-fold. In this case, a 1-Hz infrasonic signal plays back at an audible 165 Hz. Total playback time scales down by the same factor, of course (165 seconds of original time plays in one second).
Overview continues in Part 3==>
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References Cited in Overview Part 2:
3. Basic Stamp BS-2:
4. WAV File Format:
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