GB1011409A - Apparatus for furnishing information as to positioning of objects - Google Patents

Abstract

1,011,409. Acoustic ranging. NATIONAL RESEARCH DEVELOPMENT CORPORATION. March 19, 1964 [March 19, 1963], No. 10782/63. Heading H4D. The invention relates to an adaptation of the acoustic ranging apparatus for aiding blind persons, described in Specification 978,741, wherein the frequency modulation of the radiated signal has the form shown by the solid line in Fig. 1. The fly-back portion 2 of the modulation, which was substantially vertical in the above Specification, is now rarely steeper than the main modulation portions 1, with the fly-back period T<SP>1</SP> some five to fifteen times shorter than the period T of the main modulation portion. With an object at the maximum usable range R M , the echo signal has a frequency modulation shown by the chain dotted line in Fig. 1, and the beat frequency signal produced from mixing the echo signal with the contemporaneously radiated signal, is as shown by the chain-dotted line in Fig. 3, having a frequency fm during portion 1 and rising to a high value and then falling therefrom during the fly-back period. A rejector circuit is provided which rejects all beat frequencies above fm, such that the beat signal fed to the audio reproducer means has an amplitude varying as shown by the chain-dotted line in Fig. 4, with the signal dropping to zero during the fly-back portion. The operator then beams a high-frequency tone having the power density/frequency spectrum shown in Fig. 6. With a near object the beat frequency signal, shown by the plain dotted line in Fig. 3 has, in addition to the portion at a frequency f produced in the apparatus of the above Specification, and possibly below audible range, a portion rising to a higher frequency, and reaching a maximum frequency of fm for T<SP>1</SP> objects at a range Rm.- The beat signal has a T practically constant amplitude, as shown by the plain dotted line in Fig. 4, and a power density/ frequency spectrum as shown in Fig. 5, the portion at frequency f having a narrow bandwidth and the higher frequency portion having a much wider bandwidth. The latter portion will be audible as a click when the low-frequency portion is below the audible range or too faint. The relationship between T and T<SP>1</SP> is chosen so that an audible click will begin to be heard at the range at which the low-frequency portion becomes difficult to bear.