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Electro-Harmonix Q-Tron and Bassballs Pedals as a
RackAFX Plug-In
Kerline Moncy
University of Miami
Music Engineering
MMI503
Project 3
Q-Tron Pedal
Fig. 1. Q-Tron pedal
The Q-Tron is an auto-wah guitar pedal by Electro-Harmonix. Its sound resembles a
standard wah-wah pedal except that the wah effect is automatically controlled by the user’s
player dynamics over time rather than a foot pedal. The Q-Tron’s effect comes from a filter
whose center frequency is swept according to the signal’s envelope at a given time. The pedal
includes many controls that allow the player to tweak various characteristics of the filter’s
movement.
Fig. . The basic wah effect is achieved by sweeping a filter’s center frequency
“K-Tron” Plug-In Algorithm
Fig. 2. “K-Tron” plug-in filter block diagram
The “K-Tron” plug-in’s signal flow and controls are based on those of the Q-Tron pedal.
The Q-Tron’s most basic mode of operation features the input signal being fed into both a second
order filter (high-pass, low-pass, or band-pass) and a gained version of the signal into an
envelope detector. The envelope detector directly controls the center frequency of the filter by
mapping the detected amplitude value to a frequency range. The signal flow ends at the output
of the filter. The Mix switch changes the filter mode to band-pass and adds a scaled-down
version of the dry input to the output to give the filtered version a greater presence. The Boost
switch allows a gained version of the input signal (same as the envelope detector’s input) into the
filter to allow for distortion at the output. A Bypass switch offers the option of listening to the
processed signal or the dry input.
“K-Tron” Plug-In Controls
Fig. 3. Custom “K-Tron” GUI
The “K-Tron” plug-in includes controls similar to the Q-Tron pedal. The user may
switch among different filter modes (LP = low-pass, BP = band-pass, HP = high-pass, MIX =
band-pass with dry input). The Sweep dictates whether the filter’s center frequency is swept up
or down as the input’s envelope increases. Range sets the limits that the filter’s center frequency
can sweep (HI = 2k to 5kHz, LO = 100 to 2.5kHz). The LO setting emphasizes vowel-like
sounds and overtones on the HI setting. Peak in this plug-in is simply another name for the Q
(.5 to 20), which adds more resonance to the sweeping filter. Gain (0 to 20dB) controls the gain
of the input signal to the envelope detector and, when Boost mode is enabled, to the filter. The
Overload LED is lit when the envelope detector reaches its maximum amplitude. The stomp
switch functions as a Bypass on/off switch and the LED above serves as a status indicator (LED
lit = effect on).
Bassballs Pedal
Fig. 4. Bassballs pedal
The Bassballs pedal has many similarities with the Q-Tron. The Bassballs is also an
auto-wah pedal with the exception that instead of one filter there are two band-pass filters
present whose center frequencies are swept by the same envelope detector. The filters have a
high Q value and are separated by an octave at all times. The pedal also has a distortion switch
that accentuates harmonics.
Fig. . Twin sweeping band-pass filters in the Bassballs pedal
Bassballs Plug-In Algorithm
Fig. 5. Bassballs plug-in filter block diagram
The default operation of the Bassballs plug-in features the input signal being fed into two
band-pass filters. The envelope detector controls the movement of top band-pass filter’s center
frequency, which is an octave above that of the bottom filter at all times. The outputs of the
filters are added in parallel to produce the output of the plug-in. The Dist. switch routes the dry
input to a wave shaper before it is filtered to emulate the original pedal’s distortion. The
waveshaper’s function is:
ATAN: y =
.
The Bypass switch offers the option of listening to the processed signal or the dry input.
Fig. 6. The Dist. wave shaper function of the Bassballs plug-in
Bassballs Plug-In Controls
Fig. 6. Custom Bassballs GUI
The Bassballs plug-in has a much simpler control set than the “K-Tron.” The Dist.
switch adds distortion by enabling a waveshaped version of the input signal to feed both bandpass filters. The Response set the high limit of the range the top filter’s center frequency is
allowed to sweep. Like the “K-Tron,” the stomp switch is a Bypass on/off switch and the LED
is a status indicator (LED lit = effect on).
The Combined Plug-In
Fig. 7. Combined plug-in GUI
The entire plug-in can be thought of as both the “K-Tron” and the Bassballs in series.
With bypass switches present on both components, one can experiment with either or both
effects enabled.