The ZVEX Ringtone is a unique ring modulation guitar effect. What is ring
modulation? In essence, the term describes multiplying two electrical
signals together. Multiplication of electrical signals produces
harmonics known to nerds as side bands. As the frequency or timbre of
either signal changes, so do the resultant harmonics. These harmonics can
be interesting for musicians eager to experiment with sound, or to people
who like to annoy their neighbors and loved ones. Commercially available
ring modulation devices typically multiply an incoming audio signal with
another signal generated within the device itself. The self-generated
signal can vary wildly with frequency depending on how one sets the
controls for the device. What makes the ZVEX Ringtone unique is that it
can sequentially step through eight different multiplying frequencies,
either automatically or at the push of a foot switch. This opens up a
whole new realm for artistic expression.
A friend of mine bought one of these effects, but was unsatisfied with it.
He wanted to be able to cycle though the eight different modulations by
hitting his guitar strings hard instead of using his foot to push a button
or letting the effect step through the eight modulations by itself. This
is where I came in.
Part II: How? An Overview
After making a few measurements, I found that the Ringtone's "next
step" input accepts a simple 5V positive or negative edge as a trigger.
While I had the Ringtone open, I also found that there would be no way to
squeeze additional circuits in the existing enclosure. This meant that
any modifications to the effect would need to live in a separate enclosure
and be attached somehow to the Ringtone. At a high level, I settled on
the block diagram shown in Figure 1 below.
Figure 1. A block diagram of the Amplitude-Controlled ZVEX Ringtone system
The amplitude control circuit would take the guitar signal in and pass
that to the Ringtone's normal guitar-signal input jack. Power and ground
for the amplitude control circuit would come from the Ringtone to
eliminate the requirement for a new power supply. The amplitude control
circuit would also pass the control signal to the ZVEX Ringtone on the
same cable as the power and ground that comes from it. Finally, the
guitar signal leaves the Ringtone and goes on to the next item in the
player's signal chain.
The next step was to decide what should be inside the "amplitude control
circuit" box. How can one measure the volume of an audio signal and
convert that information into a binary "loud" or "not loud" signal that
controls the ZVEX Ringtone? Thankfully, this is a well-explored problem
space with a lot of good solutions. I settled on the design below.
Figure 2. A block diagram of the add-on circuits needed to create the
Amplitude-Controlled ZVEX Ringtone
Broadly speaking, the circuits work like this:
The guitar audio is buffered and sent to "guitar audio out," as well
as the input of the envelope follower circuit.
The level of the audio input is detected by the envelope-follower
circuit. This level, at any point in time, is represented by an
analog voltage VPEAK.
The level of the audio signal is compared to a fixed DC voltage
VTHRESH with a basic comparator circuit. if VPEAK is greater than
VTHRESH, its output is 5V. If VPEAK is less than VTHRESH, then its
output is 0V.
A flip flop circuit converts the pulse coming from the comparator
circuit into an alternating positive and negative edge that's more
appropriate to trigger the Ringtone.
Part III: How? The Circuits
Let's start with the envelope follower. Most popular and simple
envelope-follower circuits are terrible. Even circuits from famous guitar
audio effects like the Electro-Harmonix Doctor Q or the DOD 440 have poor
repeatability and are noisy. This is too bad, as it's not very
challenging or costly to make a good envelope-follower circuit.
Figure 3. The envelope-follower circuit. VREF is about 2.9V.
This circuit is a basic absolute-value circuit driving a capacitor and a
large-value resistor in parallel. The absolute-value circuit is able to
increase the voltage across the resistor and capacitor (VPEAK), but it
cannot decrease that same voltage because of D2. The only way for VPEAK
to fall is for charge from the capacitor C4 to flow through the resistor
R8 to ground. By choosing the right values for R8 and C4, VPEAK will
nicely follow the envelope of any audio signal.
R7 controls the sensitivity or gain of this circuit. Having some
adjustment available makes the circuit adaptable to different signal
levels. Guitars with active pickups that generate relatively large
peak-to-peak signal voltages would need much less gain than guitars with
more traditional passive single-coil pickups.
The envelope-follower circuit presented here is certainly more complex
than the aforementioned popular implementations, but not by a huge margin.
For a few extra components, you gain significantly better performance.
Figure 4. The comparator circuit. The LM311 was chosen simply because I
had several on hand. VREF is about 2.9V as before.
The comparator circuit shown in Figure 4 takes its input from the
envelope follower. Like all comparator circuits, its output voltage will
go high when the + input is greater than the - input and vice versa. This
particular circuit implements some hysteresis via R9 that prevents
undesirable behavior when the voltage VPEAK is nearly equal to VTHRESH.
Hysteresis requires positive feedback, so the input from the envelope
follower is taken from the comparator's - input in this case. Q1 is used
to invert the output signal so that the output voltage VPULS is high when
the VPEAK is greater than VTHRESH.
The comparator's threshold is controlled via R11. VREF is set at about
1/3 of +9V. Turning the potentiometer's wiper closer to +9V means that
louder signals will be needed to "trigger" the Ringtone. Turning it the
opposite way means that even quiet signals will trigger the Ringtone.
Following the comparator is a simple and very necessary element of this
design: a D-type flip flop. The Ringtone will advance to the next step
when it sees either a positive or negative edge on its control input. If
we just plugged the output of the comparator circuit into the Ringtone's
control input, the Ringtone would advance two steps for each time the
guitar signal exceeded the threshold. Figure 5, below, shows the D-type
flip flop's role nicely.
Figure 5. A three-section plot showing why the D-type flip-flop is
necessary in this circuit. The top section shows the envelope follower
output signal VPEAK crossing the comparator's threshold voltage. The
middle section shows the comparator circuit's resultant output VPULS, and
the bottom section shows the output of the following D-type flip-flop
circuit.
Figure 5 shows how the circuits work together to create a control signal
that transitions from high to low or vice versa each time the input
volume transitions from "not loud" to "loud." If the flip-flop were not
present, the Ringtone would advance to the next step when the input volume
transitions from "loud" to "not loud" as well. This would cause the
Ringtone to advance two steps instead of one for each time the player
plucked her guitar string(s) strongly.
Figure 6. The simple D-type flip-flop circuit used in this design. The
specific IC used is the SN74LS74A since I already had some on hand.
There are other circuits in this design -- power circuits and voltage
buffers, but their operation is much less interesting and are therefore
omitted from this discussion. A complete schematic of this design is
given at the bottom of this page for those interested in all of the gritty
details.
Part IV: Integration
Recalling Figure 1, the design intent is to source both power and ground
from the ZVEX Ringtone as well as send it a control signal. Guitar pedals
don't typically have power output and control signal in jacks, which means
that I had to drill a hole for a 1/8" TRS jack in the side of the
Ringtone.
Figure 7. Inside the modified ZVEX Ringtone. Note that the small jack
on the right-hand side of the chassis is the added 1/8" TRS jack. The
new wires are pointed out in yellow: The red wire for +9V, the black
wire for ground, and the green wire for the control signal to the
Ringtone.
Figure 7 shows a few added wires needed to steal power from the
Ringtone and to splice in the control signal. It was a tight squeeze
getting everything out on to the TRS jack, but it is possible. Adding a
jack like this means the modification can effectively be undone by
simply pulling the cable out of the jack. The jack itself is quite small
and is on the side of the pedal instead of the face.
Figure 8. The lot hooked up and ready to go. A simple headphone extension
cable connects power, ground, and control signal between the ZVEX Ringtone
and the new circuit.
Part V: Audio Example of the Amplitude Controlled ZVEX Ringtone
Below is an audio clip recorded to show off the modification.
This clip was recorded with a Fender Jaguar Baritone Custom plugged
directly into the Amplitude Controlled ZVEX Ringtone system. The output
of the effect was plugged directly into a KMI K-MIX feeding REAPER. A
small amount of reverb from the Alesis MIDIVERB II was added, as well
as a little compression from REAPER.
Part VI: Closing Thoughts on the Design
Part of me wonders if the envelope follower circuit was even needed at
all. It has its own rise and fall times that prevent frantic advancement
of the Ringtone's step, but I never tested the system without it. It's
totally possible that this part is unnecessary.
I also wonder if not connecting the D input / Q' output to ground with a
100KOhm resistor was a bad idea. Testing didn't show that this was a
problem, but my intuition tells me that not having some kind of ground tie
may cause the circuit to get stuck in a weird state upon startup.
Part VII: Fabrication Files and Schematics
Below are links to the files needed to build the PCB I designed for this
project, the bill of materials, and the schematic. Armed with these items,
anyone could build the circuits described here. Please note that the bill
of materials does not include things like jacks, and the potentiometers
for the controls. Those items vary wildly in availability based on
country, so I left those to be sourced by the enterprising reader.
I also neglected to share the details of the enclosure for the amplitude
control circuits. Since making this only requires drilling a few holes in
a run-of-the-mill metal enclosure, I don't feel the need to explain that.
