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Mostly just playing with my a/v setup, but I thought I might try to produce something useful at the same time....
I give a brief comparison of 3 instrument tuners: Cecilio 92-D, Sabine Metrotune MT9000, and the gStrings tuner app for Android. I was surprised at the quality of the Cecilio tuner that came packaged with my Mendini violin. The gStrings app is also quite capable of serving in a pinch. The Sabine is a tuner I've had for some time and use it here as a "control".
All three tuners will generate a tone and tune to the specified frequency (440 A is standard, I think). The android app isn't a metronome, but the other two are.
If you've never used a tuner before, you basically have an analog meter that registers in megahertz, with 440Mhz producing an A (2nd string on violin). If the frequency is too high or low, the meter moves to the right or left, respectively, and will light up to the right or left in orange/red signalling that you're out of tune in that direction. When the meter is pointing straight up and the light is green, you've hit the right note.
In metronome mode, you can set the time signature (4/4, 3/4, etc.) and the beats per minute (on sheet music, it's identified at the top left, and usually accompanied by a fancy italian word like allegro or andante). This will help you keep time while practicing.