It’s small, but I think it’ll be pretty huge. We’ve seen plenty of guitar audio interface adapters out there. Here’s the dirty little secret: they’re pretty awful. Because they connect to the audio jack of the iPhone or iPad, there are issues with impedance matching and noise / signal-to-noise. (Read: they don’t sound great. I’ve tried them, and since they all use the same in/out jack, they all sound more or less the same.)

The Mobile In from Line 6 has three things going for it. First, it works with Line 6’s POD effects and guitar amp modeling, which is already popular with guitarists. Second, the app that it works with the audio interface is free. No paid app, no add-on fee for presets – you buy the hardware and get all of the software for free.

Third, and most importantly, the Mobile In connects to the proprietary Apple 30-pin connector, so you get digital audio I/O rather than relying on the jack. That means the quality of the Mobile In is, well, the quality of the Mobile In. Without a test unit, it’s impossible to say for sure, but Line 6 at least claims a 110 dB dynamic range on the guitar input and a 98 dB range on the stereo line input. There’s up to 24-bit/48 kHz support, but I’m happy just to get a portable device with line and guitar ins. They’ve even tossed in a 6-foot guitar cable. Total price: US$79.

The Mobile POD app, while free, is no slouch, either: you get 10,000 presets, a built-in tuner, 64 amps and effects and more, even with a neat iTunes play-along mode. And it’s free for everyone, whether or not you have the hardware. And if you want to use a different mobile app, of course, you can do that, too, if you do use the hardware.

With specs like that, this looks like a must-have for the iPhone and iPad, even if you use them exclusively for practicing guitar. Now, I just wish the beautiful New Signal Process accessories, which turn iOS gadgets into stompboxes, had a MIDI-only version. Maybe there’s a MIDI-compatible controller that could eventually be compatible with these. (Line 6, you listening?)

There’s only one catch. Stereo line in. Guitar in. There’s something missing here – output. That means that the Camera Connection Kit with a USB audio interface could be a better option if you have an iPad. You’re still limited by the headphone output jack here – no other output is available. That makes this fine for practice, but people wanting more flexible ins and outs (well, outs period, beyond the headphone connection) will want to consider a USB audio interface. (Of course, the free app is still cool.)

My other wish? It’d be nice to see the Android platform compete with, well, any of this. Theoretically, there may be a way to do digital audio I/O on that platform, too; sounds like a research question. We’ll see if, Android or otherwise, another tablet/mobile platform starts to evolve these sort of features.

Any mobile guitarists who want to test this out for CDM, and share your music in the process? Let us know.