Vector Drive: Quick Start Guide

The Vector Drive is host to a digital playground that gives you the ability to craft a diverse range of unique tones; anything from crushing distortion to sweet attack-sensitive overdrive, and fuzz which tears at the seams.

Drive Mode

The Vector Drive’s main mode of operation gives you access to everything you could expect from a drive pedal: gain, volume and a three band EQ. On top of this is there is a mid EQ frequency knob, the Shape knob, which can be programmed as a mid EQ width adjustment, an extra “high mids” EQ channel, or a fuzz shape adjustment, and the Boost switch which provides extra gain and presence to give any passage that extra oomph.

Advanced Tuning

The Vector Drive also allows you to get under the hood and modify a series of advanced tuning adjustments not normally found in drive pedals.

These features are accessible through two advanced tuning modes which temporarily change the function of the front panel controls.

Drive Tuning

Drive Tuning mode allows you to adjust the overall flavour of the drive tone. This is achieved by filtering the guitar signal prior to clipping.

‘Input High Pass Filter’

This filter adjusts how much bass to remove before the clipping stage, allowing you to either tighten up the low end of the distortion by removing more bass, or get more crackle from your fuzz tones by allowing the bass frequencies to thicken your drive tone.

‘Input Low Pass Filter’

Conversely, the low pass filter allows removal of excessive treble prior to clipping. This lets you retain tonal brightness or tame harsh high frequencies and hiss from ultra high-gain distortion.

Drive Tuning is accessed by pressing both of the footswitches simultaneously while the effect is enabled. While in Drive Tuning mode, the LED will periodically emit a single flash.

Clean Mix

A whole new library of tones are available using the Vector Drive by utilising the Clean Mix mode to create a custom blend of the driven and clean signal.

Mix

This control adjusts the ratio of clean signal to drive signal at the output of the pedal. This can be used to dial in subtle cleans alongside your drive or create unique blends of

Clean EQ

In Clean Mix mode, the three band EQ, with parametric mid, is applied to the clean signal prior to mixing. This allows you to make adjustments to the tone of the clean signal without affecting the drive tone. With this feature, completely unique sounds can be invented with combinations such as distorted bass frequencies with clean treble, or mixing clean bass into a treble boosted distortion.

Clean Mix is accessed by pressing both of the footswitches simultaneously while in Drive Tuning mode. While in Clean Mix mode, the LED will periodically emit a double flash. Pressing both, or either, of the footswitches will return to the main drive mode.

Internal Memory

The Vector Drive will retain the Drive Tuning and Clean Mix settings when it is switched off. They are written to memory when you return to main drive mode. When the Shape knob is moved its setting is also written to internal memory allowing you to keep any adjustments made after switching its function.

September 29, 2017January 30, 2018

The Vector Drive’s Audio Signal Chain

Recently we recorded a video which demonstrates many of the Vector Drive’s capabilities. It’s a great resource if you’re keen to see a mix of distortion, overdrive and fuzz tones created by the pedal.

But how are all these tones made? To learn about the Vector Drive’s audio processing capabilities we’ve sketched up the following diagram:

Rendered by QuickLaTeX.com

The squares (and gain triangle) represent processing blocks while the ellipses are settings you have control over. The output volume, bass/mid/treble EQ and gain controls are omitted for clarity but, as a guitarist, hopefully you have a feel for what those knobs do.

Distortion Block

In the diagram’s top half you can see the effects main purpose: gain and saturation (the ‘s’ shaped block). This is modelling amplifier clipping and is what creates distortion; everything else in the processing chain shapes the tone to your taste. The “gain boost” kicks in when you hit the boost switch and, along with a bit of tone tweaking, makes your playing jump out from the rest of the band.

Before the distortion we’ve placed a high pass filter, this removes bass from the audio before it gets distorted. Exactly how much bass is controlled by a frequency cut-off parameter. Notes below this frequency are removed and the rest allowed the pass.

A low cut-off frequency keeps the bass, allowing creation of a classic 70s fuzz tone, while increasing the cut-off frequency removes it and makes the output more “scratchy”. The cut-off can go all the way down to 20Hz so the Vector Drive is suitable for bass guitar too!

There are two symmetry parameters, one of which is discussed at length in the fuzz article and another which clips the top half of the wave more than the bottom, leading to an overdrive-like tone. We’ll be writing more about that one in a future article.

Clean Mix

Above the distortion section is a 3 channel EQ which is applied to the clean signal. This EQ’ed clean audio is then mixed in with the distorted signal to create overdrive effects. This is discussed at length in our overdrive article but in short it mimics the sound of classic overdrive pedals.

The clean/distorted mix is smoothly adjustable for maximum control. You can, for example, remove all the bass from the distortion with the high pass filter then mix in clean bass by adjustment of the clean EQ. This can avoid the distortion sounding “muddy”, which can happen if too much bass goes through the clipping section, but keeps bass in the output.

Post-Processing

Output Equalizer

In the diagram’s lower half we start with another 3 channel EQ but this time the mid frequency is adjustable. Setting the mid frequency to around 700Hz lets you create a brutal scooped mids tone or it can be turned up and boosted to really cut through the mix for a shredding solo.

Boost Filter

Next comes the boost filter. Activating boost not only gives you more gain but also increases the presence of your sound. This is a switchable increase of the upper-mids which makes your tone clearer and more “present”.

Speak Cabinet Emulation

After the boost filter comes the cab model which is detailed in an earlier article. This is a filter which emulates the tone of a real guitar speaker cabinet, allowing the Vector Drive to sound like a real amp when practicing with headphones, recording or even playing at a live gig.

Output Limiter

Finally there’s an output limiter. This stops the output from hard clipping when turned up too loud. When the limiter is engaged the indicator LED flashes rapidly. If the limiter is activated occasionally it isn’t a major issue but the output volume should be dropped if it is always on.

Final Words

If you haven’t seen the video yet you can check it out at the top of this post or here on our Facebook. The Vector Drive is scheduled for release in late 2017 or early 2018, we’ll keep you updated via the Facebook page.

September 12, 2017September 20, 2017

Overdrive: What is it and how is it implemented in the Vector Drive? Part One

Introduction

Overdrive is at the heart of modern guitar playing. We all know what it sounds like; from the hint of breakup as tubes begin saturating, to the mighty crunch and roar of an amp being pushed by a boost pedal. Whether you play blues, rock, psych, metal or industrial-post-noise-core, you’ll need an overdrive which can give you what you’re looking for.

Understanding the causes and characteristics of overdrive in amplifier electronics is crucial to making a quality pedal that delivers versatile overdrive. In this article, we show you some of our research and inspiration for development of our algorithms.

The first section will show analysis of a classic solid state overdrive circuit and how the Vector Drive distortion pedal models this effect. The second half of the post will look at measurements from a tube amplifier pushed into saturation along with the Vector Drive’s wave shaping feature which can mimic this.

Classic Overdrive Pedal Analysis

Lets begin this section by looking at the core of traditional overdrive pedal circuits. Below is a typical overdrive clipping circuit, such as that in the Tube Screamer, built in LTSpice. The circuit has been simplified by removing all frequency dependent components and using ideal diodes and op-amp. As with our fuzz analysis we’re stripping everything back to its core to understand the essence of an effect, not copy an existing product.

NB: Non-ideal (ie: real) diodes produce a much smoother clipping than is presented here but ideal diodes make the circuit’s underlying behaviour easier to see.

The circuit above is based around an op-amp in a so-called non-inverting topology which, without the diodes, has a gain of:

(1) $\begin{equation*}Gain=1 + \frac{R_f}{R_g}.\end{equation*}$

The resistor $R_f$ is what is adjusted when turning the gain knob on an overdrive pedal.

If the diodes are ignored and the typical 50k to 550k resistance range is used the circuit above has a gain of between 9.5 and 118, or approximately 20dB to 41dB. With this much gain a typical 300mV guitar signal would be amplified to between 3V and 30V, enough for the op-amp’s output to hard clip near its power supply voltage (typically 0-9V). We will see later that situation is avoided by the diodes.

For the following analysis we will assume the diodes are ideal, this means that if their forward voltage is below some threshold, we chose 0.6V, their resistance is infinite and if it is above 0.6V their resistance becomes zero.

The diodes avoid hard clipping by reducing the circuit’s gain when the output reaches a certain amplitude. The gain reduction occurs because when one of them conducts the effective value of $R_f$ drops to zero, causing unity gain as per the equation above.

Given that the diodes conduct when their voltage exceeds 0.6V we need a way of finding their terminal voltage as a function of $V_{in}$ . The voltage across the diodes can by calculated by noting the characteristics of ideal op-amps and concluding that the inverting (-) and non-inverting (+) inputs of the op-amp will always be the same voltage. Therefore we claim that the op-amp’s inputs will both be equal to $V_{in}$ (the input voltage) and, considering the case where $V_{in} > 0$ the diode D9 will conduct when:

(2) $\begin{align*} V_{out} -V_{in} &\geq 0.6 \\ V_{out} &\geq 0.6 + V_{in}.\end{align*}$

The same logic can be used to derive the equation for D10’s conduction threshold as $V_{out} \leq 0.6 - V_{in}$ but the proof is left as an exercise for the reader.

Given equation 2 and the fact that a conducting diode drops $R_f$ to zero (and the gain to 1) we can conclude that if D9 is conducting then $V_{out}$ is, in fact, equal to $0.6 + V_{in}$ . ie: the output is clamped by the diode’s conduction threshold.

The diode’s conduction requirement can be written in terms of $V_{in}$ by observing that:

(3) $\begin{align*}Gain &= \frac{V_{out}}{V_{in}} = 1 + \frac{R_f}{R_g} \\ &\Rightarrow V_{out} = V_{in}\left(1 + \frac{R_f}{R_g}\right)\end{align*}$

and substituting this expression for $V_{out}$ into equation 2:

(4) $\begin{align*} V_{out} &\geq 0.6 + V_{in} \\ V_{in}\left(1 + \frac{R_f}{R_g}\right) &\geq 0.6 + V_{in} \\ V_{in}\left(1 + \frac{R_f}{R_g}\right) - V_{in} &\geq 0.6 \\ V_{in} \frac{R_f}{R_g} &\geq 0.6 \\ V_{in} &\geq 0.6 \frac{R_g}{R_f} \end{align*}$

We can now write a complete set of equations for the overdrive circuit:

(5) $\begin{equation*} V_{out} = \begin{cases} \left( 1 + \frac{R_f}{R_g} \right)V_{in} &, V_{in} < 0.6 \frac{R_g}{R_f} \\ 0.6 + V_{in} &,V_{in} \geq 0.6 \frac{R_g}{R_f} \end{cases} \end{equation*}$

So, if the input is driven with a sine wave the general shape of the output is shown below:

It can be seen that between approximately -0.6V and 0.6V the output is a sine wave which has been amplified with high gain. However, as soon as the output exceeds $0.6+V_{in}$ the diodes conduct and the gain drops to 1. This causes the output to follow the input, shifted by +/- 0.6V. This creates smooth peaks instead of a hard clipped square wave.

Another way of visualising the circuit’s behaviour is by plotting the input signal’s amplitude on the x-axis and the corresponding output amplitude on the y-axis to create a graph of the circuit’s static non-linearity. The plot below shows this for three different values of $R_f$ :

Observe that the “knee point” is at $(V_{in}+0.6)~V$ . The gain of a circuit is equal to the gradient of the above plot and the two gain “regions” can clearly be seen. The gain is high when the output is between $\left(0.6 + V_{in}\right)~V$ and $(-0.6 + V_{in})~V$ then suddenly drops to 1 once one of the diodes conducts.

The broad effect of this circuit is a type of smooth clipping where the “smoothness” comes from the clean input being mixed in with the distorted output. This is, in fact, the core feature of overdrive: it is a saturated signal with some clean input mixed back in.

This can be seen from the circuit’s gain equation:

(6) $\begin{align*}Gain &= \frac{V_{out}}{V_{in}} = 1 + \frac{R_f}{R_g} \\ V_{out} &= V_{in} \times \left(1 + \frac{R_f}{R_g}\right) \\ &= V_{in} + V_{in} \times \frac{R_f}{R_g}\end{align*}$

One way of looking at the circuit’s two operating regions is to imagine the value of $R_f$ varying with the input voltage. This equation then informally shows that the output, $V_{out}$ is equal to the clean input, $V_{in}$ plus a high gain copy of it where the high gain signal gets saturated (clipped) at the diode’s forward voltage. This supports the statement above that the overdrive effect is a saturated version of the input with some of the clean input mixed over the top.

The Vector Drive’s Overdrive Implementation

The basic signal chain of the Vector Drive’s overdrive effect is shown below:

The full signal chain contains several filter blocks (such as the main 3 channel parametric tone controls) but these have been omitted for clarity.

In traditional overdrive pedals the distortion gain level, set by $R_f$ , is adjustable. In the Vector Drive, however, the versatility of DSP allows for both the distortion gain and clean gain to be set by the player.

In our DSP code the saturation function is the smooth clipping equation:

(7) $\begin{equation*}y = \cfrac{x}{1 + \left| x \right|}\end{equation*}$

which, when mixed with the clean signal, results in waveforms such as the one below; a beautifully smooth clipped sine wave:

So lets look at the effect of varying the distortion and clean gains. If we plot the above waveform shaping as a static non-linearity and vary the distortion gain we get the following plots:

The distortion gain adjusts the underlying tonal mix of the output, increasing this gain creates higher frequency harmonics leading to a more crunchy sound.

If, instead, we vary the clean mix the static non-linearity changes as follows:

With this adjustment the output can be varied from totally saturated hard core distortion to super subtle overdrive.