The Sonic Pathfinder

Historical Background

The Sonic Pathfinder has evolved out of the work of the Blind Mobility Research Unit at Nottingham University, England. It is the product of a research program dedicated to achieving an understanding of mobility and the information requirements of the independent blind traveler. The Sonic Pathfinder embodies many of the findings of the Nottingham program; the emphasis is on achieving a simple display. For instance, the device does not attempt to provide the user with a sonic picture of the visual world. Instead it is selective; it displays only that information which is of immediate practical interest to the moving pedestrian. The device uses an auditory display but one which has been chosen so that it does not mask the user's ability to hear, and make use of, environmental sounds. The display comprises the notes of a musical scale, which make a familiar tonal progression as the user approaches an object.

A Technical description

The Sonic Pathfinder is a head-mounted pulse-echo sonar system controlled by a microcomputer. The five ultra-sonic transducers which are mounted on the head-band comprise three receivers, one pointing left, one right and one straight ahead, and two transmitters. The two transmitters are angled so as to irradiate the user's pathway with ultra-sonic energy. Echoes from objects lying within this pathway are received by the three receiving transducers and this information is processed by the on-board computer. The output of the device, the display, is fed to one or other of the two ear pieces depending on whether the object is on the left or the right of the user or to both ear pieces if the object lies directly in the travel path.

The Sonic Pathfinder does not give information about the surface texture of the objects which produce the echo signal. This is in line with the findings of the work carried out at Nottingham which suggests that such information is of secondary importance. Another simplification is that the display is restricted to the nearest object only. The exception to this simple rule is that priority is given to objects which lie in the centre, so that, for example, the presence of a shoreline at 1 metre (3ft.) will not prevent the user being given advance warning of a centrally placed object at 2 metres (6ft.).

A Functional description

The function of the device is most easily described by cataloging a number of imaginary situations.

Imagine yourself in an open space some 4 metres (12ft.) away from a wall. If you were to turn to face the wall and start walking, suddenly you would hear, in both ears, the notes of the musical scale descending in order. Each note represents a distance of approximately 0.3 metres (1ft.) of travel. If you were to stop when the tonic is reached; you would be able to reach forward and touch the wall with the outstretched hand.

To illustrate a slightly more complicated example, let us suppose we wish to explore the side beams and the priority given to objects which lie straight ahead. For this exercise one would need a long stretch of featureless shoreline which is suddenly terminated by an object which sticks out from the shore line; this might well be another wall at right angles or a buttress. Imagine following the shore line and keeping a constant parallel line. You would hear an intermittent note in one ear, the pitch of which would remain constant. Should the pitch become lower you would know that you had veered closer to the wall. If the pitch becomes higher you would have veered away from the wall. When you reach a point some 3 metres (9 ft.) from the buttress the device will stop displaying the side information and indicate the straight ahead object. But now the display is in both ears and at four times the repetition rate, thus grabbing your attention and warning you that evasive action must be taken.

Its artificial intelligence

The computer program which runs the Sonic Pathfinder includes, within the software, a number of sophistries which give the device a fairly high level of 'artificial intelligence'. The massive amount of information collected by the receivers is digitized and analyzed by a hierarchy of decision making algorithms which purport to select only that information relevant to the user's immediate needs. This selected information alone is displayed, thus, avoiding the informational overload commonly experienced by the users of some earlier devices.

The basic and simplest algorithm to understand is the one that gives priority to central objects. For example an object some 2 metres (6ft) away but in the direct line of travel would be displayed in preference to a shore line only one metre (3ft) to the side.

An early evaluation of the device showed that this algorithm alone did not reduce the displayed information sufficiently and clients were still finding difficulty when trying to negotiate narrow gaps. Careful observations of subjects using the device led to the hypothesis that much of the unwanted information would be eliminated if the device were made to have an overall range which changed automatically depending upon user need. In particular the overall range should be dependent on walking speed. The software algorithms to achieve this are complex. However, the new Sonic Pathfinder, instead of having a range of 2.5 metres (8 ft), has a range of 2 seconds! This is an unusual and odd concept. However, given the information processing demands inherent in independent blind travel and the moment to moment problem-solving nature of blind travel, it is highly desirable to have a device which is limited to providing information solely about those objects which would be encountered during the next 2 seconds of travel. What is more, the user is completely unaware of the massive amount of computation which is being perform on his or her behalf; the user hears a very simple display but one which is relevant to current needs.

Considerable effort has been put into developing a training course suitable for teaching Mobility Instructors how to train clients in the use of the Sonic Pathfinder. Ideally Instructors attend a three day training course. To benefit from the course Instructors should be physically fit, experienced in the teaching of primary mobility devices and capable of independent travel under blind-fold. The course includes both theoretical and practical sessions. The Instructor Training Course manual is in the public domain. It is written in English and is 46 pages long in Word for Windows. To download a Zipped up version (approx. 560Kbytes) click on: Manual . Alternatively, a text version of the manual may be found on Tom Brennan's Page devoted to Electronic Travel Aids. The manual has been translated into German, French, Italian and Dutch.

In the light of the experience gained in developing and running the Instrucor Training Course a new revised Distance Learning Package has been developed for use my Mobility Instructors unable to attend a formal course.

Instructor training courses have been run in Australia, Indonesia, Germany, the UK and the USA. The device has been evaluated and approved by the US Department of Veterans' Affairs. There are now distributors and Accredited Sonic Pathfinder Instructors in Australia , UK, Europe , Indonesia and the USA .

Finally, to see some comments from a user; see what Graeme has to say.