Even More Tools

FIG. 6: The CrossConvolve window offers separate envelopes for the sound in the Sample Editor (green envelope), a sound file on disk (red envelope), and the cross convolution of the two (blue envelope).

MetaSynth provides a broad range of DSP tools. Some of these are found on the Transform and Morph menus, and among these some have their own unique graphic interfaces. A typical example is the CrossConvolve window shown in Fig. 6.

Cross convolution is one of the vocoder-like effects mentioned earlier. It operates on two sound files by performing an FFT analysis of each and multiplying the resulting spectra. The effect is similar to analyzing both sound files in Image Synth, then using one image to filter the other. However, cross convolution uses much higher resolution. Because image filtering is simply pixel-by-pixel multiplication, it is symmetric. It makes no difference which picture filters the other. The same is true of cross convolution.

MetaSynth’s approach to cross convolution provides independent envelopes for mixing each of the source sound files with the cross convolution. Some familiar graphic tools are provided across the bottom of the envelope window, and you will find wave-shaping tools along the right edge (see Fig. 6). In the figure, the sound file in the Sample Editor is faded in (green envelope), while the sound file on disk is faded out (red envelope). The cross convolution fades in and out with a bell-shaped curve (blue envelope). To hear the result of cross convolution by itself, flatten the red and green envelopes at the bottom and flatten the blue envelope at the top.

Loop Bassix. Here’s an example of how you might apply cross convolution along with the MetaSynth processes described above to produce variations of a bass loop. First load a bass loop into the Sample Editor, fit the Image Synth duration to it, and analyze the loop to make a groove filter (see the section “Stealing the groove”). Save the groove filter in the Filter library.

With the bass loop still in the Sample Editor, select a small clip with as much harmonic content as possible and make an Instant Spectrum from the clip. Then select Custom Scale as the Frequency Map and import the spectrum as a scale. (To have the spectrum recognized as a scale file, hold down the Option key when clicking on the import-scale button—the disk icon with an upward-pointing triangle.)

Next clear the Image Synth and paint or recall a picture suitable for additive synthesis (see the section “Add ’em up”). Ensure that the Image Synth’s duration still matches the bass loop’s length and that its Frequency Map is set to the spectrum scale from the previous step. Then either render the picture and filter the resulting sound file in the Filter Palette or graphically filter the picture before rendering. Save this sound file to disk, so you can recall it for each application of cross convolution.

The final step is to cross-convolve the sound file with the original bass loop a number of times using different envelopes. To keep the various convolutions consistent and have the original bass loop high in the mix, set the red envelope to a horizontal line fairly high in the window and modify only the green and blue envelopes for each variation.

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Reprinted with permission from Magazine, February, 2001
© 2001, Intertec Publishing, A Primedia Company All Rights Reserved