Apple Logic Express 7 - Manual (Page 44)

44 Chapter 6 Filter
The Resonance knob emphasizes the frequency range surrounding the cutoff
frequency. When you turn the Resonance up sufficiently, the filter itself begins
oscillating (at the cutoff frequency). Self-oscillation is initiated before you max out the
Resonance parameter, just like the filters on the legendary Minimoog. When working
with Resonance, the manner in which the lowpass filter allows frequencies to pass
changes: higher Resonance values cause the filter to cut the bottom end, making the
signal sound thinner. The Fatness parameter compensates for this audio artefact. When
you turn Fatness up to its maximum value, the Resonance setting has no effect on the
response of the frequencies below the cutoff frequency.
The Slope buttons determine the steepness of the lowpass filter: frequencies above the
cutoff frequency are dampened by 6, 12, 18, or 24 dB per octave (in audio jargon, these
are called filters of the 1st, 2nd, 3rd, and 4th order). Even though the 24 dB filter is
largely the component of choice for synthesizer designers, be sure to experiment with
the other options, as they can also deliver pretty hip results. The Distortion Input and
Output parameters allow you to individually control each of the two distortion units—
one pre-input and the other post-output. Although the two distortion modules are
identical, their respective positions in the signal chain—before and after the filter,
respectively—enable them to generate remarkably different sounds.
All other AutoFilter parameters are used to dynamically modulate the cutoff frequency.
These fall into two sections: Envelope (ADSR, Envelope Generator) and LFO (Low
Frequency Oscillator, Modulation Generator).
The Threshold parameter applies to both sections, and analyzes the level of the input
signal. If the input signal level exceeds that of the variable Threshold level, the
envelope and LFO are retriggered. The Modulation slider of each section determines
the intensity of the control signal’s effect on the cutoff frequency.
Envelope: when the Threshold level is exceeded, the control signal is triggered at the
minimum value. Following a variable interval, the length of which is determined by the
Attack parameter, the signal reaches its maximum value. It drops in level during the
interval defined by the Decay value, and ends up at the Sustain value. Once the signal
level drops below the Threshold value, it falls all the way to its minimum value over the
time determined by the Release parameter. If the input signal falls below the Threshold
level before the control signal has reached the Sustain level, the Release phase is
triggered. The Dynamic Modulation parameter lets you modulate the peak value of the
Envelope section, by using the level of the input signal.