If I'm understanding the example, on each period, you want
to traverse
half wave table linearly within some fraction dist of the
total period,
and then traverse the rest of the table linearly for the
remainder of
the period (please correct me if I'm wrong).
This is a special case of FM: you are modulating the
frequency up and
down with a rectangular wave. Ignoring rounding error, you
could do this
with Nyquist's FMOSC, but then you are left with the problem
of creating
the modulator and I suspect that the phase would drift due
to rounding
error whereas it seems like this algorithm assumes the
modulator is
synchronized to waveform traversal (the up/down frequency
modulation
occurs at the same phase every period).
Another possibility is to use SND-COMPOSE, which computes
the functional
composition of two signals: f(g(t)). You'd want to make f()
be just the
output of an oscillator with a nice integer number of
samples per
period. Then g() is the modulator: a monotonically
increasing piece-wise
linear function. You'd have to construct this by splicing
together
linear segments, probably using something like (PWLV start
dur end)
inside a seqrep().
SND-COMPOSE uses linear interpolation, so the quality won't
be really
high, but then the modulator is not band-limited so there
will be
aliasing in any case. At least if you're careful you can get
a subsample
accurate g() which will help make slow modulation of the
distortion
parameter result in a slowly changing waveform as opposed to
one where
changes are quantized to sample boundaries.
This is definitely the high-level view. Let me know if you
need more
detailed help.
Also, in the example code, the use of SEQ is not correct (I
can see how
it might work due to some implementation short-cuts in
Nyquist). The
main problem is this will build a huge tree of
unevaluated/suspended
SOUNDS, probably a couple of kbytes per period.
The "right" way to do this would look something
like
(seqrep (i n) (let (...) ... (seq (compute-a)
(compute-b))))
or in SAL, it's more like
begin
with ... initialize variables ...
return seqrep(i, n, seq(compute-a(), compute-b()))
end
-Roger
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