Vocoder-like effects and the Filter Palette. A variety of DSP techniques, in effect, impose the time-variant frequency spectrum of one sound file (typically speech) on another sound file (usually pitched or noise). MetaSynth provides four such processes on its Morph menu: Cross Convolve, Formants Filter, Osc Bank PhaseVocoder, and Convolve. You can also produce a wide variety of these kinds of effects by using filters in either the Image Synth or the Filter Palette. The use of groove filters, described previously, is one way to produce such an effect.

MetaSynth’s Filter Palette is similar to the Image Synth, except that it has fewer tools (for example, no Hot Filters), and the window size, Frequency Map, and tuning are fixed. The window size is 256 (horizontal) by 128 (vertical) pixels, and the Frequency Map is in semitones tuned to the pitch A1. Keeping these limits in mind, you can transfer pictures freely between the Image Synth and the Filter Palette. Just make sure that the Image Synth’s parameters are set to match the Filter Palette’s. The active filter library is shared between both windows so that when you open or change a filter file in one, the change is reflected in the other.

Using the Filter Palette for vocoding is a simple matter of transferring a sonogram of the source material to the Filter Palette and applying it to any sound file with sufficient harmonic content to produce intelligible results. As a quick example, set the Image Synth’s dimensions to match the Filter Palette, load a short speech sound file into the Sample Editor, fit the Image Synth duration to match the sound file, and analyze the sound by typing N. (The reason to analyze the sound file in the Image Synth is that analysis in the Filter Palette produces a reference template rather than a filter.) Now copy the sonogram to the Image Synth’s clipboard and paste it into the Filter Palette. With the sound file still in the Sample Editor, select “WhiteNoise” from the Sounds menu. This creates a noise sound file of the same length. Apply the filter by clicking on the Filter Palette’s Synthesize button (the Mac icon on the bottom border).

As with resynthesis, the ability to graphically manipulate the pictures used as filters distinguishes this process from run-of-the-mill vocoding. The five tools at the bottom left of the Filter Palette—scaling, rotation, offset, contrast, and displacement—are particularly effective. For example, the scaling tool used horizontally provides time compression and stretching without formant or pitch shifting; the offset tool provides pitch and formant shifting without affecting time; and the displacement tool provides a variety of effects from simple vibrato to a total mangling of the sound. Each of these can be used by clicking and dragging in one or two dimensions or by double-clicking to enter parameters numerically.

Stereo and color. The images shown here have all been black and white, which means the analyzed and rendered sound files have all been mono. MetaSynth can also analyze and render stereo files when the Image Synth is in stereo mode. (To switch between stereo and mono modes, click on the sample icon in the middle of the top border.) If you analyze a stereo sound file in mono mode, MetaSynth analyzes the left (red) channel.

Analyzing mono sound files is faster, and for things like groove analysis, it is generally a better choice. Once you have analyzed a file in mono, however, you may want to convert it to stereo in the Image Synth. This is handy for rendering stereo effects and creating templates in the silent blue channel.

When the Image Synth is in stereo mode, a blue-grid submenu appears on the top border. You can use this to create various horizontal and vertical grids quickly, and you can paste the clipboard’s contents here. In the case of a groove analysis, setting the grid size to 16 and typing G produces blue gridlines at eighth-note intervals. The gridlines give you a guide against which you can compare the groove to straight eighths. If you copy the groove analysis to the blue channel, you can utilize it as a painting template. You can also use the blue-grid submenu to apply the image on the blue channel as a filter.

BACK | NEXT

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