The Alden Non-Linear VCO –

The Proper Choice for Audibly Presenting Single-Value Graphics

In 1975, we were asked to design an audible system to present seismograph information to a blind geologist who was studying foreshocks of earthquakes. Other inquiries about graphic presentations soon followed; considerable interest in electrophysiology, such as electrocardiography, led to other requests.

We soon discovered that, just using a linear voltage-controlled oscillator (VCO) to indicate amplitude of a signal, had the effect that excursions in the upper range of the signal were perceived as "compressed". In other words, the apparent change in pitch for a given delta V was not as significant at higher pitches as it was for an equivalent delta V positioned where the VCO produced a low pitch. This effect was immediately corrected when we inserted a so-called "antilog function IC" ahead of the linear VCO.

Thus, the proper approach is to use a VCO whose response to a given delta V corresponds to a given ratio of pitches. In the Smith-Kettlewell Auditory Oscilloscope, for example, a vertical excursion of two divisions on the graticule is represented by a one-octave difference in pitch of the VCO; this will be true near the bottom, about the middle, and near the top of the screen.

Graphic-interpretation experiments, using our first oscilloscope prototype, (Gilden, 1979) showed that common wave forms were identifiable by naive subjects using the nonlinear VCO to represent them. In 2003, Joshua Miele, Ph.D., released a software sonification toolbox for blind and visually impaired users of MATLAB(TM) -- a popular programming environment used in industry and research. Dr. Miele's software also uses the antilog function when using frequency to represent Y values (Miele, 2003).

In 1982, Albert Alden built a self-contained auditory oscilloscope; a Tektronix single-trace scope has a Braille-calibrated control used to scan the X axis, while an internal speaker presents the vertical deflection by way of a VCO with nonlinear response. This design was published in SKTF, Vol. 5, No. 3, 1984. A commercialized version by Oehm Electronics (Richard Oehm, 408-971-6250) was written up in SKTF, Vol. 9, No. 2, 1988.

Mr. Alden's design used a VCO whose nonlinear function is achieved with a diode in the VCO's input amplifier; the result was considerably less costly than the function-generator IC's used in our earlier VCO experiments. Over a range of four octaves, it is perfectly adequate to present signal amplitudes as pitches.

The Alden VCO is presented here, separated from the adapter to oscilloscopes, because it can find use in a wide range of audible-graphics applications.