Master Class: Audio Alchemy

Apr 1, 2008 12:00 PM, By Dennis Miller

Learn how to use convolution to transform your sound files.

BONUS MATERIAL
Additional Resources: Click for a list of good programs that support convolution.
Web Clips: Listen to audio examples that demonstrate the many things you can do with convolution.

Lurking inside your computer are dozens, perhaps even hundreds, of unique sound effects. No, I'm not referring to some random sample libraries that have mysteriously appeared on your desktop. Rather, I'm talking about all of the audio files you own, each of which can become a source for creative and one-of-a-kind effects processing if used as an impulse response with a convolution program.

Convolution is a powerful technique that multiplies the spectra of two files (one file is the source, or carrier, and the other is the impulse response) to produce a vast range of effects. If the impulse response (IR) is the “acoustic signature” of a real space — a large concert hall, for example — the result of the convolution process will be reverb, which many of the convolution programs currently on the market try to achieve (see “Trading Spaces” in the October 2004 issue of EM, available online at emusician.com). An endless number of filtering effects can also be created using very short, percussive IRs — such as a single clave or marimba hit — with the results sounding like lowpass, highpass, or even comb filtering. And you can create many types of exotic echoes and delays using different types of synthetic-waveform spectra.

You get many of the most unique and original sounds, however, when you use two standard audio files. This often results in a form of cross-synthesis, in which one file takes on some characteristic of the other: a sustained chord sung by a choir, for instance, could adopt the rhythmic pattern of a drum loop, or a stream of white noise could be molded into a 4-note seventh chord. Clearly, the sound-design potential is unlimited.

We've covered convolution in several previous articles (see “Sound Design Workshop: Convolution Reverb and Beyond” in the April 2005 issue and “Square One: Convolution Number Nine” in the June 1999 issue). But this time I'll go into a lot more depth about the creative uses of the technique. I'll discuss how to pick good files to produce interesting pairings and crossings, what tweaks you can make to your IRs to improve their potential, and how you might add convolution to your work flow. Many of the tips and tricks come from Virtuasonic's Alessandro Camnasio and Spirit Canyon Audio's Darrell Burgan, both of whom produce libraries of IRs intended specifically for sound-design purposes (see the sidebar “Dedicated Collections”). There are numerous programs that support convolution, so look around your desktop to find out what you might already own or see the online bonus material “Get in the Game” for a list of some of the best options.

Before discussing specific tips, both our experts stressed the importance of lowering the volume on your playback system as you conduct your convolution experiments. It's likely that you'll pair two files with strong peaks in the same region, which will produce resonances with very high amplitudes. At all times, be sure to moderate your listening level to avoid damage to your speakers and your ears (you have been warned!). Fortunately, many convolution programs offer gain adjustment in case you need to make a rapid move for the mouse.

Filter Fun

Getting started with convolution means finding IRs that you want to use and then deciding how to put the process into play. If you want to start simply and explore convolution's filtering potential, look for some short drum sounds, interesting sound effects (machines, clicks, gears, and such), or other broad-spectrum samples. On the PC, most convolution programs let you use any WAV file as an IR, so you should already have loads of material to work with.

On the Mac, different formats are supported by different programs: Apple Space Designer supports SD II, AIF, and WAV, among others, while Audio Ease Altiverb (also available on Windows) requires split-mono files. Check to see what the options are for loading IRs into the software you plan to use, then consider copying potential IRs to a dedicated folder so you won't waste time during a work session searching for them all over your drive.

Burgan describes what some of convolution's potential for filtering is and how to get good results. “The output of convolving an IR with a signal is the spectral overlap of the two signals. Another way of saying this is to say that only the frequencies that are present in both signals will end up in the convolved signal. Musicians can use this to filter signals arbitrarily,” he says. “If they want to have as much of the spectrum of the original signal as possible, then the IR should contain as much of the audible spectrum as possible — white noise, for example.”

Burgan continues, “Much fun can be had by using IRs that have a much smaller spectral range, however. You could ensure that an IR causes a highpass-filter effect simply by ensuring that the IR itself has no frequencies below the cutoff. Taken to the logical extreme, if an IR has a discernible pitch — in other words, only one dominant frequency (say, A 440) — then anything convolved with that IR is going to sound like it has that pitch. If the pitch matches the pitch of the song, very interesting things can result.” According to Camnasio, using multiple convolution passes on the same source and filtering each differently (highpass, lowpass, bandpass, and so on) gives you even more possibilities. This would allow you, for instance, to keep only some desired range of frequencies in the final output.

For examples of convolution as a filtering effect, see Web Clips 1, 2, 3, 4, 5, and 6. In these examples, I've used the same male vocal sample with different IRs, each of which produces a different result.

Moving On

To move past the filtering approach, there aren't too many hard-and-fast rules. Convolution is a technique that is well suited to experimentation — you can try convolving just about any two files and see what happens. You can also tweak the settings of many convolution programs to improve your initial results if they aren't totally satisfactory (see the section “Tweaking Time”). There are, however, a few guidelines to consider when matching up files for convolving.

For starters, the two files you choose to convolve should have some frequency components that overlap. That is because when the two spectra are multiplied, the regions that they have in common are emphasized, and the regions that have nothing in common produce only long strings of zeroes in the resulting audio file. Camnasio says, “When the spectra of the two files overlap, it is quite easy to obtain a well-balanced convolution, especially if your source has a wide spectrum. This makes drums and other forms of percussion sounds good candidates for fast and very easy convolution. You can often get away with little or no equalization, depending on your personal taste.” He also points out that vocal samples have a lot of potential for convolving.

You can probably determine whether there is any overlap just by listening carefully — no doubt you'll notice, for instance, that a cat's meow and a cymbal share spectral content (see Fig. 1 and Web Clip 7). But if you want a more accurate opinion, open the spectral-analysis window of your favorite audio application and compare the content of the two files. If you don't own a suitable program, check out Praat (www.praat.org) or Christoph Lauer's Sonogram (www.christoph-lauer.de), both of which are cross-platform. Or pick a more basic spectral-analysis tool from your favorite music-software download site.

If you want to use only a portion of the IR's spectrum (everything between, say, 100 and 1,500 Hz over the first 3 seconds, for example), then simply cut or trim away the part you don't need. You can easily do spectral-domain editing of this type with Mike Klingbeil's excellent cross-platform analysis-resynthesis tool, Spear (www.klingbeil.com/spear). Adobe Audition and Steinberg WaveLab 6 (among others) also have similar spectral-domain editing features (see Fig. 2).

The duration of the files you use shouldn't be a limiting factor, nor do the IR and the carrier need to be the same length — it doesn't hurt to try files of different lengths to see what type of results you get. But keep in mind that many convolution programs limit the length of the IR. Sony Acoustic Mirror, for instance, found in the Effects menu of Sound Forge (all recent versions), has a 12-second limit, and Waves IR-1 limits the IR to 6 seconds. But Altiverb and Cakewalk Sonar Perfect Space impose no limit (see Fig. 3). As Burgan notes, the duration of the IR correlates directly with the amount of “smear” that happens when that IR is convolved with an audio signal. You can either shorten the IR directly by using your audio editor or use the convolution engine's envelope function to limit its duration. (Voxengo's Pristine Space, Christian Knufinke's SIR2, and others offer IR envelopes, which I'll cover in a moment.)

Choosing files with the same tempo can also produce good results. Burgan explains, “Let's say you have a song that is 100 bpm. Let's also say you have an IR that has some kind of modulation or motion in it that comes and goes in a tempo that matches 100 bpm. Also assume that the IR and the signal have the same sample rate. Now, if you convolve a 100 bpm signal with this 100 bpm IR, the beats will line up and very interesting things happen, particularly if you run something percussive through the IR, like a drum loop. Our IR collection Kaleidoskopy provides a large number of bpm-synced IRs for precisely this purpose. Run a 100 bpm loop through one of the 100 bpm IRs, and you can get some truly fun tempo-synced drum madness.” Web Clip 8 illustrates this approach. (You can find other examples on the Spirit Canyon Audio Web site at www.spiritcanyonaudio.com.)

Time and Again

Once you've identified a pair of files you want to convolve, there are many ways to put them to use. In most cases, using your convolution software as a plug-in effect on an audio track is the most efficient way to try out different IRs on a source. For instance, maybe you're doing vocal effects for a video game and have to create numerous variations on the voice of a character. Using the vocal sample on a track in your digital audio sequencer or editor, you could apply the convolution engine as an insert effect. This works well with programs like Altiverb and SIR2, both of which run as a VST (or other format) plug-in effect.

Another option, suggested by Camnasio, is to place the convolution plug-in on the audio output track of a soft synth. Set the wet to 100 percent and the dry to 0 percent, and you'll have endless new variations on your synth sound set. Consider using short IRs in this context so that you can switch IRs in real time without a glitch from overloading your computer's processor. And be sure to disable any effects processing on the synth patch — too much reverb, for example, can lead to a muddy sound very quickly (see Web Clip 9).

You can also use multiple instances of your convolution engine in a single session and process your source file, whether audio or the output of a synth, using serially arranged instances: the output of the first becomes the source for the second, the output of the second becomes the source for the third, and so on. Another option is to use the convolution engine as the input to a vocoder — try it as both carrier and modulator to see what happens — or dial in just a bit of convolution using a send slot to add a little spice to your audio track.

Burgan recommends using a different convolution instance in the left channel than in the right, each with a unique IR (see Web Clip 10). He says, “There is nothing in the world that says the L and R channels of an IR must have any relationship. Musicians can put different waveforms into the L and R channels, and the convolution engine will produce the corresponding different L and R output, which can be useful for some things. For example, let's say you convert an audio signal to mid-side (M-S) encoding. Then, when you convolve that signal with a stereo IR, effectively the convolution engine will convolve the M with the IR's L channel, and the S with the IR's R channel. When you convert the signal from M-S back to L/R after the convolution engine, you will have different apparent spectral signatures for the center of the panning zone versus the sides of the panning zone. Done carefully, you can apply convolution only to the mid or to the side, which can result in spectacular effects.”

He continues, “Imagine convolving a signal that has a lush saw-waveform pad in it, such that the mid channel remains untouched but the side channels are mostly convolved signal. You can get very unique evolving pads this way. Some downstream delay or reverb can give the pad even more motion.” To hear this effect, see Web Clip 11. (Note that SIR2 has a utility that will build a stereo IR file using two different mono files as input. It then adds the new file to its IR list automatically.)

Burgan also suggests freezing tracks if you plan to use multiple convolution instances simultaneously. As you'll soon discover, convolution is one of the more CPU-intensive effects. Finally, don't forget the option of processing a live audio signal. That can be a great approach in a live performance or just for experimenting with your voice or some other acoustic source. If your audio software doesn't support live processing, then consider a program like Tobybear's MiniHost (Win; www.tobybear.de/p_minihost.html), a standalone VST host that can load a convolution plug-in and process any incoming ASIO audio input signal.

Click to continue reading "Audio Alchemy."

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