.nf
	kr	\fBport\fR	ksig, ihtim[, isig]
	ar	\fBtone\fR	asig, khp[, istor]
	ar	\fBatone\fR	asig, khp[, istor]
	ar	\fBreson\fR	asig, kcf, kbw[, iscl, istor]
	ar	\fBareson\fR	asig, kcf, kbw[, iscl, istor]
.fi

A control or audio signal is modified by a low- or band-pass recursive filter
with variable frequency response.


INITIALIZATION

\fIisig\fR - initial (i.e. previous) value for internal feedback.
The default value is 0.

\fIistor\fR - initial disposition of internal data space.  Since filtering
incorporates a feedback loop of previous output, the initial status of the
storage space used is significant.  A zero value will clear the space; a
non-zero value will allow previous information to remain.  The default
value is 0.

\fIiscl\fR - coded scaling factor for resonators.  A value of 1 signifies
a peak response factor of 1, i.e.  all frequencies other than \fIkcf\fR
are attenuated in accordance with the (normalized) response curve.
A value of 2 raises the response factor so that its overall RMS value equals 1.
(This intended equalization of input and ouput power assumes all frequencies
are physically present; hence it is most applicable to white noise.)
A zero value signifies no scaling of the signal, leaving that to some
later adjustment (e.g. see \fBbalance\fR).  The default value is 0.


PERFORMANCE

\fBport\fR applies portamento to a step-valued control signal.  At each
new step value, \fIksig\fR is low-pass filtered to move towards
that value at a rate determined by \fIihtim.
ihtim\fR is the "half-time" of the function (in seconds),
during which the curve will traverse half the distance towards the 
new value, then half as much again, etc., theoretically never reaching
its asymptote.

\fBtone\fR implements a first-order recursive low-pass filter in which the
variable \fIkhp\fR (in c.p.s.) determines the response curve's half-power
point.  Half power is defined as peak power / root 2.

\fBreson\fR is a second-order filter in which \fIkcf\fR controls the center
frequency, or cps position of the peak response, and \fIkbw\fR controls its
bandwidth (the cps difference between the upper and lower half-power points).

\fBatone, areson\fR are filters whose transfer functions are the
complements of \fBtone\fR and \fBreson\fR.  \fBatone\fR is thus a form of
high-pass filter and \fBareson\fR a notch filter whose transfer functions
represent the "filtered out" apsects of their complements.  Note, however,
that power scaling is not normalized in \fBatone, areson\fR, but remains
the true complement of the corresponding unit.  Thus an audio signal,
filtered by parallel matching \fBreson\fR and \fBareson\fR units, would
under addition simply reconstruct the original spectrum.  This property is
particularly useful for controlled mixing of different sources (e.g., see
\fBlpreson\fR).

Complex response curves such as those with multiple peaks can be obtained
by using a bank of suitable filters in series.  (The resultant response is
the product of the component responses.)  In such cases, the combined
attenuation may result in a serious loss of signal power, but this can be
regained by the use of \fBbalance\fR.
.bp