The Kyma Language for Sound Design, Version 4.5

It looks very much like a series of impulse responses (as we saw in the earlier tutorial on filtering).
The FrequencyScale works best on sounds that can be modeled as a source of impulses hitting a resonant
filter, for example the trombone (flapping lips generate an impulse that is fed into the filter of the body of
the instrument) or the human voice (glottal pulses passing through the resonant cavity of the mouth and
lips). The FrequencyScaler looks for what it thinks are individual impulse responses in the waveform and
creates little grains by putting an amplitude envelope (shaped like the Window function) on each one.
Then, to lower the frequency, it stretches these grains further apart so they occur less often. And to raise
the frequency, it pushes the grains closer together so that they occur at a faster rate. This is the same as
decreasing or increasing the rate of the impulses without changing the impulse response of the filter. If the
impulse-into-filter model is a good match for the way the sound was originally produced, then the FrequencyScaler
will do a good job of changing the frequency without changing the duration or moving the
formants around. Why? Because the formant structure is inherent in the filter or the resonator and can be
inferred from the way it responds to a single impulse. Since this method does not change the shape of the
impulse response, it does not change the formants.
The FreqTracker input is used to get an idea of where to find the impulses in the Input. The Delay is
to compensate for the delay through the FrequencyTracker; it delays the Input by the same delay introduced
by the FrequencyTracker, so that the frequency estimate lines up with the current Input. The
minimum delay you should use is 256 samp and the maximum is 20 ms. Within those boundaries you
should use something close to the period of the lowest frequency you expect to see in the input. Remember
that to convert a frequency to a period, type the frequency in hertz followed by the word “inverse”,
for example
4 d sharp hz inverse
The quality of the frequency scaled sound depends on the quality of the frequency estimate, so it is important
to start with the best possible frequency estimate. Double-click the FrequencyTracker to see its
parameters. The most important parameters to adjust are MinFrequency (the lowest frequency you expect
to see in the Input) and MaxFrequency (the highest frequency). The narrower you can make this
range, the better your frequency tracking will be. In this case, we know that the original frequency of the
single trombone tone was 4 d sharp, so the range of 4 d to 4 f is pretty safe!
All of the other parameters are pretty nonlinear (in that tiny changes in these parameters can destroy the
frequency tracking), so it is recommended that you do not change these. You can experiment with more
or fewer Detectors, with the caveat that more is not necessarily better in terms of the effect this will
have on the tracking.
This technique has one more advantage (even beyond the fact that it does not affect the duration or the
formant structure), because it can work in real time on live input. For example, play the Sound called
scale w/Frequency and try various settings of the !Frequency fader. If you put !Frequency low
enough, you will hear the individual grains. Play it again, this time choosing live input in the Generic-
Source dialog, and try frequency scaling your own voice in real time. You may have to edit the Sound to
adjust the MinFrequency and MaxFrequency parameters of the FrequencyTracker to more closely
match your own range.
Wavetable Synthesis
If you model a sound as oscillators reading from wavetables, then you can select durations and frequencies
independently of one another; frequency is controlled by the size of the increment you use in
stepping through the wavetable, and duration is simply how long the oscillator is left on.
For example, wavetable frequency scale uses a GA resynthesis of three different trombone tones mapped
to two different ranges of keys. The lower range has 2 c at its low end, 3 c sharp at its high end, and fills
in the intermediate timbres by continuously morphing between those two endpoints. The higher range
has 3 c sharp as its low endpoint, 4 d sharp as its high end and, similarly, morphs between the two to
get the intermediate waveforms and envelopes.
See Wavetable Synthesis on page 185 to see how to create a GA analysis/resynthesis from the Tools
menu.
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