FEW GIFTED individuals have "perfect pitch" which allows them to recognise by ear when the note of a musical instrument is accurately tuned (within 1 musical cent, or 1/100th of a semitone). However, the vast majority, including many musicians, simply don’t have this ability or anything like it. For most of us, the only way of tuning an instrument is by comparing its notes with those from tuning forks or some other source of accurately known sound frequencies.

Fig.1: block diagram of the Musical Instrument Tuning Aid. It's based on a PIC microcontroller (IC1) and a 16MHz crystal frequency reference. The PIC divides down the frequency reference and drives a 5-bit DAC (digital-to-analog converter). This in turn feeds audio amplifier IC2 to deliver the selected tone (set by switches S1-S4). IC3a, IC3b, IC4 and LEDs 22-29 form a simple stroboscope beat indicator, to enable precise "visual" tuning.

Until about 1970, tuning forks were really the only option. The standard method was to use a single tuning fork at one standard note frequency or "pitch" (usually A = 440.00Hz). The corresponding note of the instrument was first tuned against this frequency, then the other notes of the octave were tuned against this note using the technique of "beats" or heterodynes.

This technique involved tuning each note high or low until the audible difference frequency between one of its harmonics and a harmonic of the reference note was correct (for that particular note). Once the notes in the middle octave had been tuned in this way, the corresponding notes in the other octaves could be tuned against them by adjusting for a zero beat. It was a pretty tedious business and required plenty of patience, as well as a good ear.

Instrument tuning became a lot easier in the 1970s when electronic musical tuning aids appeared. In most cases, these aids were based on special ICs known as "top octave synthesiser" or TOS chips, which had been developed mainly for the second generation of electronic organs.

Inside a TOS chip were 12 or 13 digital frequency dividers, each of which produced one note of the top octave for the organ by dividing down from a shared crystal oscillator (usually around 2MHz). So by combining a TOS chip with a multi-stage binary divider, it was quite easy to produce a device which could generate virtually any note in any octave, all accurate enough to be used as a tuning reference.