GB1427351A - Tuning aid for musical instruments and a method for tuning employing the same - Google Patents

Abstract

1427351 Tuning musical instruments INVENTRONICS Inc 3 May 1973 [3 May 1972] 21148/73 Heading G5J [Also in Divisions G1 and H3] Tuning of a musical instrument is assisted by a battery operated device providing a visual display of the drift of relative phase between the squared waveform of the fundamental or selected harmonic of the tunable instrumental note and a square wave at a frequency suitably subdivided from an adjustable master clock oscillator. The operator selects the pitch of the signal component to be examined by means of note selector 28 for the position within the octave and by octave selector 30, both modifying the tuning of active filter 22 to extract the desired component from the instrument note, and at the same time respectively modifying the high frequency of the master clock oscillator 26 (in semitone steps) and the number of stages of binary subdivision in divider 32 necessary to produce a reference frequency close to double that of the signal component. The master clock oscillator 26 also has fine and coarse frequency controls providing proportional adjustment consistent over its octave range. The square reference wave from divider 32, entering detector 36 feeds, together with its inverse, two pairs of cross coupled flip-flops which generate four half-frequency square waves in successive phase quadrature. The signal, squared at entry to detector 36, is compared, together with its inverse, by means of two exclusive OR circuits 60 each based on three NAND gates (62, 64, 66, 70, 72, 74) with the four quadrature waves. Four outputs of pulses at reference frequency, each having pulse width related to the phase difference between the signal component and the respective input, feed via low pass filers 40 a set of four pairs of diametrically opposed light sources in a ring of eight 42; non-linear circuits based on silicon diodes 93 ensure that only a phase match closer than 45 degrees will cause a pair of sources to glow; if they glow at all their strength is dependent on the outputs passing the lowpass filters. If the signal frequency is higher than half-reference frequency the display will appear as a bright bar rotating clockwise, if lower then anticlockwise. For frequency differences above 5 Hz the display appears persistent and the low-pass filters dim the glow; a tuner therefore adjusts the instrument to maximum brightness until rotation is visible, and then to zero rotation rate. To reduce excessive sensitivity on high notes, frequency dividers may be inserted in both reference and signal inputs to detector 36. A method of tuning an instrument, using the device comprises the tuning of one specified " temperament " octave of 12 notes to distribute evenly the " stretch " of the second partial of the lowest note of this octave above double the frequency of the lowest note, followed by stepping stages up and down to other octaves by matching partials from the temperament octave with fundamental or other partials from other octaves.