I think Bob Moog would have liked Filtatron, Moog Music’s first app for the Apple iPhone and iPod touch. Back in 1964, when Herb Deutsch suggested that he add a keyboard to his first modular synthesizer, Moog was skeptical at first. He felt that musicians would be constrained by the confines of a musical keyboard and might prefer something like the ribbon controller. Fortunately for history, Deutsch convinced him otherwise.

Filtatron ($4.99), however, has no keyboard. Instead, it takes advantage of Apple’s multi-touch technology to control live editing and performance. Moog Music describes it as a real-time audio effects suite with a Moog ladder filter at its heart. But the filter is just one of several circuits you’d typically find in a real analog synthesizer, and Filtatron models several of them. It also has an oscillator, an LFO, an envelope follower, a mixer, and distortion and delay effects (see Fig. 1). You can dynamically control all the parameters on the fly, using your fingertips to turn onscreen knobs, touch virtual buttons, or tap and stroke rectangular pads. Filtatron also gives you an onboard sampler that can sample its own audio output and import audio clips from your computer. And unlike many iOS apps, it has onboard documentation built right in, including a glossary of electronic music terms.

FIG. 1: Most synthesis parameters are on Filtatron’s Main page.

Proto Synthesis
Although all of Filtatron’s functions are DSP-generated, the oscillator is labeled VCO (voltage-controlled oscillator) and the filter is labeled VCF (voltage-controlled filter)—Moog synthesizers have always labeled them Oscillator and Filter—in order to save space onscreen. The oscillator is continuously variable between sawtooth and square waves, and its frequency range reaches down into LFO territory, making it uniquely suitable for simulating Geiger counters and other clicking sound sources.

Although Filtatron has no traditional envelope generators, the oscillator section does offer a release stage that affects amplitude. Whenever you touch the Release knob to adjust its duration, a tone plays so you can hear its effect. Think of it as a one-stage envelope, with a fast attack and a user-definable release. An Enabled button toggles the release stage on and off. When release is disabled, the tone drones continuously. In future versions, I hope Moog can find room for an Attack knob as well.

The 4-pole resonant filter is continuously variable from lowpass to highpass, which means that it can attenuate high frequencies, low frequencies, or anything in between. Like the Moog Voyager’s Dual Lowpass mode, Filtatron lets you assign different filter cutoff frequencies to each side of a stereo mix using the Separation knob. Separation is a nice touch, but it would be even better if you could modulate it with an LFO, as you can on the Voyager.

FIG. 2: Filtatron’s Effects include overdrive (labeled Amp) and a delay that can sync to the filter’s LFO.

The LFO is hardwired to modulate filter cutoff. It offers five continuously variable waveshapes, including sample-and-hold. You enter the LFO rate by either turning a knob or tapping a tempo button. An envelope follower uses a sound’s amplitude contour to control filter cutoff, and a Speed knob controls how quickly it reacts. With the filter set for lowpass and the Cutoff knob turned down, then, louder sounds become brighter, and quieter sounds are darker. The envelope follower is most effective when you’re playing samples with lots of dynamics.

Filtatron has two onboard effects processors: amp and delay (see Fig. 2). Amp is actually a distortion effect with two intensity controls. The Drive knob imparts warm distortion, and the Feedback knob generates a harsher clipping effect. Turning them both all the way up causes a self-oscillating feedback loop. Controls for the delay are surprisingly comprehensive and enable an enormous range of effects, from gentle chorus and metallic flanging to discrete echoes. Syncing delay time to the LFO is great for echoing, bubbling filter effects.

The Presets page presents a list of 33 factory patches alphabetically; tapping on any one of them loads it instantly (see Fig. 3). You can save any new patch you create—either from scratch or by modifying an existing patch—and name it on the same page. An especially thoughtful touch is that you can send a patch directly from your iPhone, iPod touch, or iPad to anyone’s email address. If the recipient opens the preset on a similar device with Filtatron installed, he or she can save it into Filtatron, which means you can share your patches freely.

FIG. 3: The Presets page lets you load and save patches.

Check Out My Pad
Controlling Filtatron with the pads is just plain fun. Each of the two pads lets you manipulate two parameters simultaneously, with a choice of three virtual circuits for each pad (see Fig. 4). The top pad controls filter cutoff and resonance, LFO rate and amount, or delay time and feedback. The bottom pad controls the oscillator’s frequency and level, the envelope follower’s speed and amount, or the amp effect’s drive and feedback. When controlling oscillator frequency, be careful to lift one finger before pressing down another for the pitch to change.

Even though it’s an iPhone app, Filtatron ran just fine on my iPad. To fill the screen, I simply pressed the 2x button; occasionally, though, I touched the button accidently, reducing everything to iPhone size. The only other disadvantages it has over a real iPad app is that the graphics are a little fuzzy and it has no landscape orientation, only portrait. The one stumbling block I encountered was that I was unable to download the app directly to my iPad, as I could with most apps. I had to download it to my computer and then sync the iPad to that to install it. A continuing problem is that using Filtatron disables my iPad’s auto-lock. Though this feature may be convenient, it could drain your iOS device’s battery if you leave it unattended.

Free Samples
Breaking free of the analog-synth interface, the Sampler page lets you resample the Filtatron’s output and access 11 included sample loops (see Fig. 5). Even though there’s no noise generator, one of the included samples is white noise, and you can use that for your noise source.

FIG. 4: Two onscreen pads give you access to six different functions for real-time performance.

To record a sample, just press the Record button once to initiate sampling and again to stop it. While recording, the sample length and remaining time are displayed; file size is limited to 10MB, which translates to about two minutes per sample. When complete, Filtatron prompts you to either give your new sample a name or discard it. (I had hoped I could sample from my iPad’s built-in microphone or through its audio input, but no such luck.) Once you capture a sample, you can set its start and end points for looping. Unfortunately, you can’t truncate your sample to recover memory. You can copy and paste samples, but it copies the entire recording, not just data that’s within your start and end points.

A Rate knob lets you increase sample playback as much as twice its original rate, or slow it down so much that it almost (but not quite) freezes. The knob is bipolar, so that turning it counterclockwise plays the sample backward at the selected rate. Being able to record a sample and almost instantly play it in reverse, at any available speed, delivers a lot of instant gratification.

You can’t delete a sample file directly from the Filtatron once you’ve saved it. However, you can transfer samples and patch presets to your computer by putting Filtatron in FTP (file transfer protocol) mode and entering the address it displays into your computer’s browser or FTP client. Then you can use your computer to delete the file from your iOS device, if you like, or even upload 16-bit mono or stereo WAV files from your computer to Filtatron. That means you can load a sample from your computer, process it with Filtatron, resample the results, and then send it back to your computer. If you have a Mac, you can save time by typing Command+K to open the Connect to Server window, but that doesn’t allow you to upload samples to Filtatron. Obviously, transferring files is somewhat cumbersome, but being able to load your own samples could open up a whole new world to novice synthesists.

FIG. 5: The Sampler page takes Filtatron beyond what most synthesizers can do.

In the Palm of Your Hand
The best thing about Filtatron is that it’s so much fun to use, even before you learn your way around. I was able to create all kinds of classic sci-fi sound effects I might never have programmed on a real synth. Just processing the included patches with effects could yield hours of sonic exploration.

Filtatron was bound to get some attention when word got out that you could purchase Moog Music’s first soft synth for just $4.99. After all, Moog instruments are known for having a premium price tag to go with their premium sound. As an inexpensive means for anyone to generate sophisticated electronic sounds, Filtatron has few equals. In fact, I dare you to duplicate some of the sounds you produce on Filtatron using any other software or device. At this price, Filtatron is a no-brainer if you own an iPhone, iPod touch, or iPad. Download it today and start making some noise.

Pros: An incredible bargain. Very good sound. Versatile sampling functionality. Imports audio clips. High fun factor.

Cons: No envelope generator. No onscreen keyboard. Can’t truncate samples to recover memory.

Geary Yelton has been writing books and articles about music technology for over half his life. He’s the former senior editor of Electronic Musician, and he lives in Asheville, North Carolina.