The DSi wasn't intended just for games; part of the meaning of the "i" is in reference to using it as a "personal tool," according to some Nintendo rep who may or may not have been full of it. We've already seen some fairly useless applications hit the service already, lending credence to this, so it makes a weird sort of sense for Nintendo to roll out applications for the musical crowd with Instrument Tuner and Metronome. And what do you know, they work well enough to possibly come in handy – if you carry your DSi with you everywhere you play, that is.
Instrument Tuner does exactly what it says and does it in two ways, depending on how you prefer to tune your gear. Start off by choosing your desired frequency in the range of 435-445 Hz and then start sending sound its way. On the touchscreen is a small scale keyboard for tuning by ear, and you can either tap the keys or opt to hold a note indefinitely. If you'd like visual feedback then the top screen displays the note picked up by the microphone — either alphabetic (C, D, E) or syllabic (Do, Re, Mi) — with a meter displaying how tuned your instrument is, just like any other tuner you've ever seen.
We tested the microphone uptake using an already tuned bass guitar as well as a software keyboard. In both instances the app was fairly accurate in interpreting pitch, able to differentiate sharps and flats from wholes. As long as your audio source is sufficiently loud then it shouldn't have any trouble picking up the correct note.
Being a first-party application, there are some Nintendo touches to the whole thing like a Game & Watch theme and a neat minigame called Tuner Fight, which is set in the Balloon Fight "universe," if you will. Balloons with notes float up out of pipes on the bottom screen and you need to play the correct note written on each one to pop them. It turns out to be a good way to practice scales, but since there aren't any options or difficulties it's not something you'd return to very frequently once you've managed to pop the two dozen or so balloons it sends your way each time.
Needless to say, those without instruments will find nothing useful here, and those who are musical most likely already have a tuner kicking around somewhere. Apart from the Nintendo touches, there isn't anything here that you can't get for free either online or on a smartphone, and splitting the metronome and tuner into two separate applications seems silly. What it does, it does well – probably better than expected, in fact – so if you happen to be in the super-niche market for an inexpensive DSiWare Nintendo instrument tuner then look no further.