In Reply to: Analog Violin/Guitar String Tuner posted by Failop Chu on 01/30/02 at 3:11 PM:
One approach would be to use a 'top octave generator' I.C. This produces the 12 notes found within an octave span from a higher reference frequency (e.g. a crystal oscillator), and is used in electronic keyboards and pianos.
Then use the phase/frequency comparator section of a phase locked loop I.C. (such as the Cmos 4046) to compare the frequency of the string being tuned with the appropriate output from the top octave generator.
You might have difficulty if the string produces strong harmonic frequencies - which then fools your circuit because it ends up trying to compare a harmonic with the note from the top octave generator. If that happens, you might have to include a filter to reject the harmonic. I mention this potential 'gotcha' so that if it happens, you won't be at a complete loss.
Another technique for comparing two frequencies that is worth looking into is to mix the note from the string with the note from the top octave generator, and build a detector circuit that looks for the beat (or difference) frequency. When the note is in tune, this becomes very close to zero.
A limitation with the top octave generator is: it is likely to be designed around the equal-tempered scale, rather than the 'perfect' scale mentioned by John Dunn. Also, the top octave generator will not be able to produce perfect equal-tempered intervals, but will get as close as it can based on integer ratios. You'll need to study the data sheet for the device to know what the frequency error will be. Obviously, it is small enough, or the device would be useless for the musical application it is designed for! But some devices might be better than others.