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Why not just put an mp3 that plays a long A440? That would act pretty much the same as a tuner, except you need to listen to the earphones.
The work in this task has been committed in r22663, r22685 and r22753.
The Rockbox guitar tuner does the frequency analysis for you and tells you when you are correct.
I've tried it. It looks very good, but it appears to be not much accurate...
The best way to tune an instrument still is by doing it by ear, and don't let any of these technological things mess everything up.
I haven't tried it yet myself. Some guitar tuners work better if you sound the 2nd harmonic of the string you're trying to tune (i.e. touch the string at the 12th fret to force a node there). This tends to kill off a lot of extraneous overtones and gives the tuner a purer tone to work with. (No idea if you tried this or if you're using a guitar or not, just throwing this out there.)
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