Five years ago, Moog Music proved you could use the iPad as a real musical instrument when it released Animoog, a polyphonic synthesizer app that made full use of the tablet's touchscreen. You could prop your tablet up on a stand and go at it with four, five, or six fingers to channel your inner Keith Emerson.

Although Animoog offered a healthy dose of the classic Moog analog DNA, it was mostly a new take on what a synth looked and felt like. This time around the company goes full retro, dusting off some old blueprints to rebuild one of yesteryear's most beloved instruments.

The Moog Model 15 Synthesizer app is an iOS-powered recreation of the iconic Model 15 modular synth from 1973. You can download it now for $30. If you find that steep, consider two things. One, this is a pro-grade instrument that plays and sounds like the business. And two, a real Model 15 is the size of a suitcase and tops $10,000; the iPad version delivers 90 percent of the goods in something easily carried in your backpack.

A Knob-Twiddler's Delight

You use two-finger gestures to zoom and slide around on a console filled with knobs, switches, and buttons. You move the colorful patch cables to manipulate sounds, just like the real deal. And you'll find all the same modules you'd see on a full-size Model 15: a bank of 921-series oscillators, low-pass and high-pass filters, the 907 fixed filter module, a ping-pong delay unit, a looping sound-on-sound recorder, and inputs for other instruments. (The original had analog inputs, but the iPad app does MIDI.)

The instrument features dozens of presets, from old-school monophonic Moog sounds to all manner of goofy, hypersonic polyphony. You get drippy bass, buzzy brass, sizzling leads, percussive thwacks, windy swooshes, echoey space radiation waves, and a didgeridoo emulator that instantly turns you into an eager Aphex Twin wannabe. Absolutely every sound is tweakable. "Tutorial" presets break down complex sounds into smaller pieces so you can learn what different modules do. Helpful, considering it looks like a bowl of psychedelic spaghetti when you first open it.

The controls are satisfyingly smooth. To twist a knob, just touch it on the screen and drag your finger up or down or sideways. The response rivals that of real controls. Sure, absolutely nothing about the iPad's glass screen exactly replicates the feel of physical knobs, but you can dial in positions with the accuracy of real potentiometers. The synth offers four touch interfaces, too: piano-style keys, an arpeggiator, a ribbon controller, and the dynamic slider keys of the Animoog. And like iOS's other MIDI-powered softsynths, you can connect a keyboard or knob controller to your iPad.

Forged From Metal

Moog designed the app using Metal, Apple's new graphics optimization framework, so the pinching, scrolling, and twisting feels snappy even when you've got a complex sound moving through multiple modules. I tested the app on the new iPad Pro, but it runs on an iPad Air or later as long as it's running iOS 9.3. It also works on iPhones later than the 5s, and any iPod Touch from the last two years. But you definitely want a bigger screen. The iPad Pro's vast display makes the Model 15 a truly formidable instrument. It helps that the Pro sports four speakers, making the app just loud and rich enough for jamming with other musicians. Plug it into an amp and your hair stands on end.

In the video below, you can see synth pioneer Suzanne Ciani playing the app at the Moog factory. She also designed one of the preset packs, and she wrote a song with it which she uploaded to Soundcloud.

https://www.youtube.com/watch?v=gGCg6M-yxmU