Università degli Studi di Pavia

Centro Interdisciplinare di Bioacustica e Ricerche Ambientali

Digital Signal Processing Workstation (version 1990-1994)

Spectrographic representation of animal voices has been widely used since the first analogical analysis instruments (Koenig et al., 1946) were developed for military acoustic research. The development of digital signal processing techniques and high-speed hardware at relatively low-cost has actually made the visualization of acoustic signals an every-day invaluable tool for bioacoustic research and for educational purposes.
Real-time analysis is relevant in evaluating extended audio/video recordings and is strictly required in detecting and observing acoustic activity during behavioural experiments and observations; moreover, real-time techniques have proved to be relevant in acoustic monitoring and censusing methods in the field, both in air and underwater. The integration of high quality acquisition boards with DSP processors on a PC platform gives a flexibility greater then those given by dedicated instruments. Thus, a PC based Digital Signal Processing Workstation for bioacoustic research has been realized (Pavan, 1992, 1994) and is continuously upgraded to meet most of the requirements regarding the acquisition, filing, and analysis of signals in the lab as well as in the field.
It can be configured as a portable or desktop workstation, with or without real-time capabilities, and can be customized to suit specific requirements.

Low-cost sound acquisition

The basic version allows hard-disk recording/playback and high-resolution true real-time spectrographic analysis of sounds up to 22 kHz without the need of expensive DSP coprocessors or external anti-aliasing filters.
It is based on a 16 bit ISA board model Duetto made by Audiologic (Italy). The board is characterized by a high sound quality: two channels, simultaneous sampling, 16 bit resolution, line input with selectable gain (0, 3, 6, 9 dB), S/N ratio 90 dB, sampling rate selectable among 8000, 9600, 11025, 16000, 22050, 32000, 44100, 48000 s/sec, on board high-precision anti-aliasing filters (64x oversampling with digital filtering).
Hard-disk recording can be performed in mono or stereo, in full 16 bit linear format or with two selectable compression algorithms (compression ratios 2:1 and 4:1) to save disk space. In full 16 bit linear format this board allows direct acquisition, hard disk recording and playback of audio signals with CD or DAT quality (Pohlmann, 1989); the length of the recordable signal is limited only by the available storage space.

Digital I/O

Alternatively, the DSPW can be equipped with an Audiologic Audioboard Plus with digital I/O modules which allow a direct digital connection (SP-DIF) with DAT recorders. In this way, sounds are directly transferred from the DAT to the PC, and vice versa, without further Digital to Analog and Analog to Digital conversions, thus preserving the full quality of the recording.

Real-time analysis

A custom driver developed by Audiologic allows the continuous gap-free real-time DMA transfer of samples between the acquisition board(s) and the computer memory; by means of a specific software and a fast PC, recording/playback with real-time spectrographic analysis and display has been made possible at low cost on signals extending in frequency up to 22 kHz (48000 s/sec, single channel with an i486/66DX2 CPU, two channels with an Intel Pentium P90 CPU). The use of faster CPUs could further increase the performances of the DSPW, allowing, for instance, to increase the overlap factor, to enhance the detail and the smoothness of the display and to add more advanced analysis functions.

Advanced version for ultrasound analysis

The advanced version requires a DSP (Digital Signal Processor) based acquisition board (Microstar DAP 2400E/6, made in USA) with 16 analog inputs and 2 output channels, 12 bit resolution, variable gain, sampling rate up to 320000 s/sec on a single channel.
By means of the DSP, the board allows true real-time spectrographic analysis and display of audio signals, as well as ultrasonic signals up to 130 kHz. Spectrographic analysis (single channel) can be performed in true real-time up to 80 ks/sec, in pseudo real-time up to 300 ks/sec and in triggered mode up to 160 ks/sec (frequency ranges up to 35 kHz, 130 kHz and 70 kHz respectively). Hard disk direct recording can be performed up to 200 ks/sec, depending on the disk speed (up to 320 ks/sec on a RAM-disk). A PC with a Pentium 200 CPU and a fast hard disk allows real-time performances up to 320 ks/sec. The DSP board requires an external conditioning unit to provide amplification, high-pass and anti-aliasing filtering of analog signals on both input and output channels.

Integration with video recordings

The workstation can be further expanded and customized to satisfy particular requirements; by means of an external broadcast-quality video encoder, computer generated spectrographic images can be converted in a standard video signal (PAL or NTSC) to be seen on conventional TV equipments, recorded on a video-tape recorder, or mixed/inserted on live video recordings. This broadcast-quality extension can be positively used to incorporate spectrographic images of sounds in animal behaviour documentary films.

Portable DSPW

A portable version of the DSPW is actually based on a 16 MHz 80386SX/80387 laptop PC with a 120 Mb Hard Disk, a VGA LCD display, two 8 bit ISA slots holding both acquisition boards, internal battery and external 12 Vcc powering with photovoltaic panels for continuous operation and battery recharge. Due to the low processing speed available on the portable unit, real-time processing is available only throughout the DSP-based acquisition board. A faster portable unit incorporating both standard ISA and PCMCIA slots could dramatically improve the application of digital processing of signals in the field and the integration with other data acquisition tasks such as GPS-based data logging and tracking. Unfortunately such type of instrument is at the moment unavailable on the market and is too expensive to be realized as a custom project.

Analysis software

The DSPW is equipped with its own DOS custom software designed to acquire, store, edit, analyze and replay signals as well as analyze them in real-time. Non-compressed digital signal files can be edited, analyzed and/or played back to perform play-back experiments (Pavan, 1992, 1994); also, signal files can be easily red and processed from within the Matlab environment.
All the spectrograms shown on our www pages have been realized with the custom software we developed. Some of them have been edited and labeled with standard software tools.

Specific software was also developed to perform some particular analysis tasks, such a two channel envelope display, event triggering and direction finding when searching for sperm whales with the two-channel towed array.
A software capable of approximating the direction of an impulsive sound source by measuring the arrival time differences on two transducers, was developed to acoustically find and track sperm whales. The software actually operates in a triggered mode on two channels and requires a good Signal to Noise Ratio to accurately detect and continuously process sperm whale clicks in real-time. The detection algorithm can be accurately tuned in relation to the transducer distance, the assessed sound speed, and the environmental background noise; also it can be programmed to reject triggers within a specific delay from a previously triggered event.
In spite of the left-right ambiguity due to the two transducer design, we are able to approximate the location of submerged animals and to discriminate and count clicks emitted at a different bearing by two or more individuals.

Selected bibliographic references

KOENIG W., DUNN H.K., LACY L.Y., 1946. The sound spectrograph. J.Acoust.Soc.Am., 18 (1): 19-49.
STRONG W.J., PALMER E.P., 1975. Computer-based sound spectrograph system. J.Acoust.Soc.Am., 58 (4): 899-904.
POHLMANN K.C., 1989. Principles of Digital Audio. Howard W.Sams & Company, Indiana, USA: 1-474.
STODDARD P.K., 1990. Audio computers. Theory of operation and guidelines for selection of systems and components. Bioacoustics, 2 (3): 217-239.
WILLIAMS J.M., SLATER P.J.B., 1991. Computer analysis of bird sounds: a guide to current methods. Bioacoustics, 3 (2): 121-128.
PAVAN G., 1992. A portable DSP workstation for real-time analysis of cetacean sounds in the field. European Research on Cetaceans, Cambridge (UK), 6: 165-169.
PAVAN G., 1992. A portable PC-based DSP workstation for bioacoustical research. Bioacoustics, 4 (1): 65-66.
PAVAN G., 1994. A digital signal processing workstation for bioacoustical research. Atti 6 Conv. Ital. Ornit., Torino, 1991: 227-234.
PAVAN G., MANGHI M., 1994. Real-time processing and visualization of animal sounds. Boll. Zool. Suppl., 61: 98.

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