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ATPM 6.11
November 2000



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Beyond the Barline

by David Ozab,

My Summer at Camp—Part 3

Fading Memories

When I originally planned this column (sometime in May, when the MetaCamp was first planned), I had no idea that I would end up with so much material. Now, as I finish this series, the 100-degree heat of early August has given way to the inevitable autumn rain (not to be confused with the winter or spring rain) of late October. My impressions of the MetaCamp linger, and my notes are still at my side, but the details have faded somewhat. So, instead of attempting to report the specifics, I will cover the two topics of Sunday morning’s session: Metasynth’s phase vocoder and the Videodellic demo.

Separating Pitch from Time

As anyone who’s ever played with the playback speed of a tape deck or record player will recall, pitch and duration are intimately linked in a recorded medium. As speed increases, pitch rises and duration shortens, while as speed decreases, pitch lowers and duration lengthens. This same principal governs the keymapping of hardware and software samplers, but loops generally cover the duration changes.

But what if you want to change pitch without changing duration, or duration without changing pitch? Through analysis/resynthesis techniques, you can break any sound down into a set of frequencies (this is called Fourier Analysis, after the French mathematician Jean Baptiste Fourier who first theorized this in 1822), then resynthesize the sound with a bank of oscillators equal in number to the total number of frequencies (Additive or Fourier Synthesis).

Since the frequencies and durations of the oscillators can be changed independently, so can the overall pitch and duration. Metasynth achieves this through its phase vocoder. An in-depth discussion of phase vocoders, along with the related topic of fast Fourier transforms, is far beyond this column. Instead, I will cover some basic tips for applying the phase vocoder within Metasynth and refer the interested reader to The Computer Music Tutorial by Curtis Roads (MIT Press, 1996).


The Oscillator Bank Phase Vocoder

The Basics of Phase Vocoding

In the last issue, we used the image synth to analyze our Welcome to Metasynth sample and resynthesize it based on a custom scale derived from the Guitar Reverb sample. The analysis/resynthesis in the phase vocoder works in a similar fashion, but with a far larger—and therefore more accurate—oscillator bank. To apply the phase vocoder, open a sample, then select Osc Bank Phase Vocoder in the Morph menu. In this menu, you can change the fundamental (important for pitched samples), the length (either by percentage or number of samples), the pitch, and the window size. You can also apply a filter. In general, small changes sound the most realistic, while larger ones are progressively more bizarre.

The Filter/Effect Option

With this option you can filter out certain frequencies or apply inertia effects to your sample. “None,” of course, means no filtering, which is best for realistic results. The Noise Filter removes low amplitude data, which the system deems less important, often reducing noise but sometimes producing a slightly metallic result as well. The Harmonic Filter removes all frequencies that don’t correspond to the harmonic series of the fundamental. In this case, the choice of fundamental is critical. The Odd Harmonic Filter leaves only odd harmonics, producing a result similar in quality to a square wave, and the Octave Filter leaves only the fundamental and its octaves.

In addition to the filters, there are three inertia effects. FFT Amp Inertia averages original and resynthesized amplitudes, producing a pseudo-reverb. FFT Pitch Inertia averages original and resynthesized frequencies, producing a drone and micro-glissandi on attacks. FFT Inertia combines the two.

Window Size

Perhaps the most critical setting in any phase vocoder is the window size. This is due to the central paradox of analysis/resynthesis. Smaller windows are better at placing attack times, yet cannot capture enough cycles to track frequency; larger windows work well for frequency, but cannot accurately place attacks. In other words, the better the timing the worse the pitch, and vice-versa. In Metasynth, the default size of 1024 samples seems to work best, but I recommend adjustment if the sound just won’t come out right.


After a short break, Eric gave us a demo of Videodellic, which is now available on the U & I Web site. My initial impressions were quite good. Videodellic is a video editing program that seems to fill a void between iMovie 2 on the low end and Final Cut Pro on the high end. It has very sophisticated video synthesis functions and complex video effects and, most intriguingly, can operate in real-time mode synchronized to either MIDI or audio input. The interface is similar to Metatrack’s, with video tracks in place of audio tracks, and tracks can be overlapped or intercut in numerous ways. The result can be bounced down to QuickTime, or played full-screen in “overdrive” mode.What separates Videodellic from other video editing programs, though, is its potential as a real-time multimedia instrument. This characteristic places it in the same category as Imaga’s Bliss Paint, but with far greater control over all the parameters.

Final Thoughts

I remember leaving Ashland that afternoon filled with enthusiasm. I had met some interesting artists and musicians, learned a lot about Metasynth (and finally got the encouragement to buy it for myself), and had completed my first composition since May. Since my return, I’ve kept in touch with everyone through the U & I mailing list, and I’ve contributed to the growing collection of Single Preset Compositions (though I’ve been a bit busy lately to work on any more of them). Meanwhile, the premiere of The Blue Hole has been scheduled as part of the Future Music Oregon Concert, November 11th, in Eugene, Oregon.

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