At last, you can plug a MIDI cable into your iPhone. The Line 6 MIDI Mobilizer ($69.99) is the first—and so far, only—hardware MIDI interface for iDevices. (I also tested it on my iPad and 2G iPod Touch.) This matchbook-size interface slots into the iPhone/iPod/iPad dock connector, enabling you to transmit MIDI data to and from compatible apps over standard MIDI connectors.

That’s a big deal, because it opens the iPhone and its brethren to the untold millions of hardware synthesizers, effects, controllers, and other MIDI gear out there. (For simplicity, I’ll refer to iPhone, iPod, and iPad as “iPhone” below.) As of this writing, there are three MIDI Mobilizer-compatible iOS apps:

• Line 6 MIDI Memo Recorder (Free), which records and plays back MIDI data in real time;
• Audiofile Engineering MIDI Surface ($5.99), a touchpad controller; and
• Garren Langford Midi Live ($39.99), a multitrack MIDI file player with real-time track control.

I checked out the first two for this review (Ed. Note: scroll to the bottom of this article for video of the MIDI Memo Recorder and MIDI Surface apps in action); Line 6 says dozens of developers have requested its software development kit, so there should be more apps in the near future.

From Phone to MIDI

The MIDI Mobilizer comes with two 64-inch cables and a quickstart guide. A collection of friendly tutorial videos on Line6.com should get you off to an even quicker start.

Fig. 1: Arrow-shaped LEDs indicate incoming (left) and outgoing (right) MIDI data.

The cables, manufactured by Planet Waves, feel solid. In fact, they weigh almost six times as much as the interface itself (2.9 oz vs. 0.5 oz). That mismatch, combined with the interface’s protruding dock connector and plastic housing (see Fig. 1), made me worry I’d accidentally step on a cable and snap off the connector. The cable length also made it harder to tell which MIDI plug was In and which was Out. I ended up wrapping rubber bands around the ends of the MIDI Out cable to make it easy to identify.

You may be wondering how Planet Waves managed to wire a five-pin MIDI plug to a three-connector phone plug. The secret is that MIDI actually uses only the three central pins on a standard MIDI cable, although all five pins plus the shield may be connected.

Fig. 2: If you want to wire up your own MIDI Mobilizer cables, follow this schematic. The MIDI connector shown here is male.

After missing the chance to record a beautiful MIDIfied baby grand piano because I hadn’t felt like cramming the cables into my bag, I decided to trace the connections and solder up my own cables (see Fig. 2). I bought a 6-foot, stereo phone cable with 2.5mm plugs from Monoprice.com (an excellent source for cables); cut it in half; and soldered some RadioShack MIDI plugs to the wires to make a more portable harness. Note that 2.5mm is smaller than the standard 3.5mm iPod headphone jack; it’s more commonly used for mobile phone headsets.

Take a MIDI Memo

So what can you do with the MIDI Mobilizer? That depends on which app you run. The basic MIDI Memo Recorder turned out to be far more fun and useful than I’d expected. This app is like a tape recorder for MIDI. You tap the huge onscreen record button, play in some MIDI (anything from a riff you want to remember to a bulk data dump of synthesizer patches), and it’s saved as a Standard MIDI File. You can then email the file to yourself for safekeeping, or connect the iPhone to a computer via Wi-Fi and transfer the file through the app’s built-in Web server.

Fig. 3: My 74-second MIDI keyboard performance was about 1/7000 the size of an equivalent audio file.

My first test was with the ancient Yamaha DJX keyboard I keep up in my living room to distract my kids from my more expensive gear. With built-in speakers, the DJX is also handy for brainstorming ideas, because, like the iPhone, it boots up instantly. However, its built-in sequencer is so convoluted I never use it.

I plugged in the MIDI Mobilizer, launched the MIDI Memo app (way faster than a computer librarian program), hit Record, and captured 74 seconds of playing. The resulting file was two kilobytes (see Fig. 3). A 16-bit audio recording of the DJX’s puny piano sound would have been about 13 MB. And thanks to MIDI, I could now recreate my performance on a high-end synth—or even that MIDIfied acoustic piano. All it took was one push of the big Play button.

Although you can name your MIDI Memo recordings on the iPhone, there’s currently no way to group them into folders. I got in the habit of naming files by the target device to organize them. Also missing is a way to import MIDI files to the app. If your phone loses its memory (as my iPod Touch did when I upgraded to iOS 4), the only way to restore the files is to play them back from a computer in real time and re-record (and re-name) them on the app.

Fig. 4: Midway through transferring a MIDI sample dump back to a Yamaha DJX.

For my next test, I sampled some audio into the DJX and then transmitted it to MIDI Memo Recorder as a MIDI Sample Dump. Although the two-second sample took a minute and a half to transfer, it loaded back in flawlessly (see Fig. 4). I wish I’d had that backup feature on some gigs I played with the DJX.

I had less luck communicating with the 1986-vintage Ensoniq ESQ-1 in my garage, which was out there awaiting a battery transplant. MIDI Memo Recorder captured the synth's bulk data dump, but the synth didn't recognize it on playback.

Line 6 tech support correctly guessed that I had MIDI SysEx reception disabled on the ESQ-1. I flipped it back on, and the data flowed smoothly. That's especially impressive when you realize the ESQ came out just three years after MIDI itself.

MIDI Memo Recorder isn’t a sequencer, either, though I’m confident someone will eventually build a compatible sequencer app. Line 6 explains,

“MIDI Memo Recorder will capture and record all MIDI data that can be stored within a Standard MIDI File, which includes all performance information (notes, controllers, pedals, pitch bend, Aftertouch, etc.) as well as any System Exclusive data (usually the parameter values of a keyboard or effect unit’s sounds and settings).

“It does not record System Real Time messages, which include Active Sensing and MIDI Clock and control, since these are also not recorded (or needed) in Standard MIDI Files. All data received is time-stamped to one millisecond accuracy in order to preserve very accurate timing of all MIDI events.”

In other words, you can record the output of a sequencer into the app, but what it captures will be the note and controller events, not the clock signal. So the app won’t drive tempo-synced instruments on playback, nor communicate song structure such as measure boundaries. Still, as a pocket-size riff recorder and data backup device that’s ready to record in seconds, MIDI Memo Recorder is a terrific tool.

MIDI Surface: Touchdown

There are at least a dozen MIDI controller apps in the iTunes Store, but they all require a computer and a middleman program (or two) to route the iPhone’s Wi-Fi signal to standard music software. Wireless control is exciting, but configuring it can be complex, and the connection can be slow and unreliable.

Fig. 5: MIDI Surface offers four controller modes, and each can store unlimited presets.

MIDI Surface, developed by Audiofile Engineering and distributed by Line 6, is a controller app that mates with the MIDI Mobilizer for nearly hassle-free touchpad control. MIDI Surface has four modes (see Fig. 5):

• Keyboard mode. This offers a scrolling piano-style keyboard with a clever expression option: You can set it so the location you tap on a key sends a different MIDI Velocity value. (It would be cool if the keys could derive Velocity from fingertip spread as well, like the DopplerPad app.) Oddly, the MIDI Surface keys don’t change color to show they’ve been hit, though of course the LED on the MIDI Mobilizer lights up. I also experienced stuck notes on a couple of synths, but couldn’t duplicate that. Audiofile Engineering said it hadn’t encountered that problem.
• Slider mode. This gives you eight faders. You can set each fader to output a unique MIDI channel and controller number (0–127), with a low and high limit. The faders are big enough to grab easily, but I’d like the option to display the controller assignments on the performance screen. All four controller modes let you save your configurations as presets, so it would be nice to know at a glance what’s going on.
• Drum pad. This mode is similar to keyboard mode but lets you set each pad to send an immediate Note Off command or wait until you lift your finger, which is handy for playing loops on MIDI gear.
• X-Y mode. This mode is the most unusual, because you get four layered x-y pads, controlled by multitouch. The first finger to land transmits one pair of controller values, the second finger sends another pair, and so on. With just four fingers, you can control eight MIDI parameters.

Fig. 6: MIDI Surface lists controllers with the name first, which makes searching the list for a specific value harder than it should be.

I used that facility to play a Korg Kaossilator Pro, which uses CC92 to specify Note On and Note Off events. I set the second touch to send a constant value of 127 (Note On) and the third touch to send a constant value of 0 (Note Off), so that tapping three fingers would shut off the note. The first touch controlled the Kaossilator’s normal X and Y values via CC12 and 13.

MIDI Surface is quite capable. I especially liked running it on the iPad. Improvements? I’d like to see a fifth mode with accelerometer response. I also wish the controllers were listed by number first, and then name. That would make finding the right item in the list of 128 values easier (see Fig. 6).

Mighty Mobilizer

The Line 6 MIDI Mobilizer continually surprised me with its usefulness. From capturing ideas on MIDI instruments while those ideas are fresh to backing up patches on awkwardly located sound modules, this tiny, affordable gadget even inspired me to play on gear I’d neglected.

If you have an iPhone/iPod/iPad and MIDI gear, the MIDI Memo Recorder and MIDI Surface apps are reason enough to buy the MIDI Mobilizer. And given the creativity of iOS developers, I’m confident MIDI Mobilizer owners will enjoy some spectacular new music apps soon. The obvious next category would be a soft synth app with external MIDI control.

One drawback is that the interface uses the iPhone's standard dock connector, so that other 30-pin accessories—the charger, or docks with line-out connectors—can't be used at the same time as the MIDI Mobilizer. Still, as a bridge between the vast world of MIDI and the exciting frontier of iOS, the MIDI Mobilizer is peerless, and it comes from a company with a deep background in MIDI.

Closer: The Line 6 MIDI Mobilizer has an iPhone dock connector on one end and two 2.5mm TRS phone jacks on the other. These connect to two custom phone-to-MIDI cables (included).

Pros: Surprisingly useful. Compact size and price. Huge potential as more apps emerge.
Cons: No passthrough for audio or power. Dock connector is inherently delicate, and the long cables could catch and snap it off.

David Battino consults for Sound Trends on iPhone music apps, writes for Electronic Musician, and co-authored The Art of Digital Music, which Music Tech named a “jolly good value.”

In the first part of the following video, I record a performance on a KorgKaossilator Pro into an iPod Touch running MIDI Memo Recorder and then play it back. Recorded data include Control Changes (pad X, pad Y, pad touch, Gate Arp on) and Program Changes. Next, I play the Kaossilator Pro remotely with the MIDI Surface app running on an iPad. Note that I reversed the y-axis values in the app so I could orient the iPad vertically.