Amidio makes some heavy-duty musical apps for the iPhone and iPod Touch; I was particularly impressed with StarGuitar, which gives you a virtual guitar with a bunch of preset rhythms, letting songwriters create quick sketches of ideas when they're nowhere near a guitar.
On Tuesday, Apple approved a new Amidio app, called TouchDJ, for the iPhone and iPod Touch, and it's both very impressive from a technical standpoint and a heck of a lot of fun. The iPhone can only play one audio track at a time, but TouchDJ essentially fools it into placing two MP3s side by side for simultaneous, real-time manipulation and playback. It's like a two-track digital DJ setup right on your iPhone.
You get a crossfader to control the balance between the two tracks, plus individual controls for each track's volume, pitch/speed (which aren't independent from one another, unfortunately), equalization (three bands), and effects (the built-in real-time effect sounds like a kind of flanger, and there are several lame samples of a low-pitched robot voice, but you can upload your own). Each track is represented by simple waveform images that use a different color for the bass, which helps you match beats more effectively. A tempobend effect, which lets you quickly bend the speed up or down on either track, also helps you get in sync.
The looping functions were most impressive--you can create a cue and loop mark at any point in either track, then return to the cue with the rewind button, move to the loop mark with the fast forward button, or create an endless loop between the two points. All of this is in real time. If you've got an audio splitter, you can even create a separate cue track for your headphones--for example, to set up a loop in your second track while the first one is playing, without exposing your experimentation to your audience--although this requires some serious processing power, and is recommended only for an iPhone 3GS.
There are a couple caveats. First, TouchDJ cannot use songs directly from the iTunes library on your device. This isn't by choice; it's a technical limitation in the iPhone and iPod Touch. This means you have to download and run a separate Amidio Sync app on your computer (Windows XP or later; or Mac OS 10.5 or later), import MP3 files from your hard drive into the app, and connect your sync cable to get them into the TouchDJ app on your iPhone. There's another method, but I wouldn't recommend it. Basically, you can upload files directly from your PC to your device via the Wi-Fi connection. But it's very slow, taking several minutes per file. That's because there's a type of conversion that must happen for MP3s to work with TouchDJ, and with the wireless method, this conversion takes place on the device, which has a much slower processor than your computer. Either way, it's an unfortunate kludge.
Second, as you might expect from a real-time two-track audio app, TouchDJ takes quite a bit of processing power. So while the app is technically capable of working on all iPhones and iPod Touches, Amidio recommends an iPhone 3GS or latest-generation iPod Touch 16GB or 32GB for best results. I tried it on my relatively new 32GB Touch and noticed no lags or glitches.
Finally, the price: $19.99. That may sound a bit expensive, but Amidio claims that TouchDJ can replace DJ equipment costing up to $1,000. I haven't done any professional DJ'ing, so I can't say if this is true or not, but it's not perfect--a BPM (beats per minute) counter would have been great, for example. But it certainly seems adequate for parties and casual gatherings. And for audio nerds, it's absolutely gripping. I loaded up the new Beach House song, "Norway," created a loop of the repeated vocal breaths near the beginning, then interspersed it with Pink Floyd's "Astronomy Domine." It sounded pretty good for five minutes of work by a rank amateur. The experience was so fun, it reminded me of the first time I set up a digital delay pedal for my guitar, or discovered GarageBand.