Effect Pedals for the Electrosmith Daisy
OverviewEffects pedals (and audio signal processing techniques in general) have a jargon. Before looking at the pedals themselves, here’s a brief glossary of some terms commonly found in effect pedal documentation; many others will be introduced when talking about the pedals themselves:
Analog Electronics - refers to electronic circuits that work - at least primarily - with continuous (as opposed to discrete) signal levels. More broadly, an analog signal can be thought of as any representation of a phenemenon (e.g. a sound pressure wave) that is analogous to the phenomenon itself; this is in contrast to a digital signal, where the signal is quantized and encoded into numerical values that are represented in electronic systems through discrete (typically binary - on or off) signals.
Analog to Digital Converter (ADC or A/D) - circuitry that takes a continuous (analog) signal and converts it into a discrete, numerical sample encoded as a digital signal. The frequency at which the conversion of the signal occurs is called the sampling rate; the resolution (range of values) of each sample is called the bit depth. Different media have different common techniques for analog-to-digital conversion, including Pulse Code Modulation (PCM) and Sigma-Delta (Σ-Δ).
Bit Depth - the number of bits allocated for each sample in a digital audio signal; in PCM-encoded audio, this value directly correlates to the resolution of how the signal’s amplitude is recorded. Each additional bit of resolution doubles the range of possible values for the sample, giving approximately a 6dB boost to the signal-to-noise ratio. CD-quality audio has a 16-bit resolution (with sample values in the range of −32,768 to +32,767); modern professional-quality audio systems typically have a 24-bit resolution or higher.
Bucket-Brigade Device (BBD) - a circuit that allows for the delay of analog signals using an array of capacitors where each capacitor discharges into the next in the array (the “bucket brigade”). In analog audio effects design, this circuit allows for a delay line that can be used to create modulation and echo effects. Although a BBD stores continuous voltages in each capacitor, it is a discrete-time device, so its frequency response is subject to the Nyquist-Shannon theorem.
Buffer - a circuit that electrically isolates two parts of a system. In audio devices, buffer circuits are typically put at the inputs, outputs, or both of a circuit that needs to be treated as a distinct part of a system (e.g. an individual module in a modular synthesizer or a pedal in an effects chain). Different buffer amplifier circuits “copy” the electrical signal across the buffer, while guaranteeing that the circuit load on one side of the buffer doesn’t change the behavior of the circuit on the other side. Simple buffer circuits can be constructed using op-amps with the output of the amplifier connected directly to the inverting input (full negative feedback). Buffer circuits are often used to optimize the impedance of an audio line, e.g. as part of a direct box. N.B. in some analog audio circuitry (e.g. classic fuzz circuits designed for electric guitars), buffers are undesirable as the effects circuit is intended to interact with the electronics of the instrument.
Bypass - a bypass system in an audio effects context will route the input signal to the output unchanged, bypassing the effect. In effect pedals and other electronic audio equipment, this can be a switch that, when engaged, routes the signal directly from the input jack to the output jack (“true bypass”); it can also be a switch that routes around all the circuitry except the buffering amplifier(s) in the circuit (“buffered bypass”). In digital systems, the bypass is sometimes implemented in software, so that the digital audio stream is simply copied from input to output without modification.
Capacitor - a passive, two-terminal electronic component that stores electric charge. Capacitors are used ubuquitously in analog audio circuits in circuits such as filters and electronic oscillators, as well as for simple electrical tasks such as transforming and decoupling voltages.
Clipping - the effect of a signal exceeding the threshold of its possible output range; in analog circuits, this can be caused by pushing an amplifying circuit beyond its maximum gain; signals can also be clipped with other techniques, such as intentionally causing clipping with diodes. In a digital system, a signal may be clipped if it exceeds the mathematical range of its bit depth. Clipping is undesirable in many circumstances but is also a core technique in effects such as overdrive.
Current - when referred to as a unit of measurement, the current of a circuit is the net rate of electric charge flowing through it at a given point; this charge is measured in amperes. In the hydraulic analogy of electricity, the current of a circuit maps to the flow rate of a water system.
Delay - the act of shifting a signal backward in time. Delays are used in signal processing in both analog and digital contexts for a wide variety of effects, ranging from filters (made with short delays) to echo and reverberation effects. Originally, long delays were implemented in audio studios using loops of reel-to-reel tape; later analog circuits used capacitors (see Bucket-Brigade Device, above), while delay lines in digital signal processing tend to be arrays of sample memory that are being continuously written into, with the delay being calculated by an offset between the read and write position (similar, in many ways, to analog tape delays).
Diode - a diode is a passive, two-terminal electronic component that, by design, allows current to flow in only direction; diodes come in a wide range of flavors - some are doped with chemicals to be used for illumination (a light-emitting diode), while others are designed to restrict voltages above a certain threshold in their forward direction (a clipping diode). Still other types of diodes allow current to flow in the reverse direction over a certain voltage threshold (a Zener diode). Diodes are used in signal processing for distortion, generating noise signals, and - using LEDs - as a component in resistive opto-isolators (see Vactrol, below).
Distortion / Fuzz / Overdrive - a variety of techniques to shape an input signal so that higher overtones are emphasized in a nonlinear or expressive way (compary Waveshaping, below). These techniques are most often associated with the amplified electric guitar, with “fuzz” being the result of a damaged speaker, “overdrive” being the result of an amplifier receiving a hot input signal, and “distortion” being the result of the amplifier itself being asked to deliver too much gain. With solid-state analog electronics, these effects can be simulated with diodes, transistors, and other components to clip and shape the waveform. Digital algorithms can distort signals using a variety of mathematical operations ranging from simple clipping to complex transfer functions.
Digital Signal Processing (DSP) - transforming signals using computers, microcontrollers, or other digital devices that treat the signals as streams of discrete numeric values that can be modified using mathematical transformations.
Digital to Analog Converter (DAC) - the inverse of an ADC, a DAC takes the numerical values of a digital signal and converts it to analog continuous voltage. Like ADCs, there are a variety of techniques and canonical circuit designs for DACs.
Dynamics Processing - signal processing techniques that operate primarily on the amplitude (dynamics) of the input signal without coloring the frequency or harmonic content; these include compressors, limiters, expanders, and noise gates. Dynamics effects often rely on generating a key signal using an envelope follower to get a sense of the overall volume of the input signal; some dynamic effects allow a second signal (called a sidechain) to serve as the key signal, allowing, e.g. the volume of a backing track to be ducked in response to a vocal performace.
Equalization (EQ) - processing techniques that adjust (or “equalize”) different bands of frequencies in an input signal. In traditional analog circuits (and time-domain DSP), equalization is accomplished with filters. More modern digital EQs may use frequency-domain techniques after transforming the input signal with an FFT. Equalization circuits range from simple tone controls on consumer stereo equipment to graphic equalizers with many bands that are a staple of professional recording and reproduction systems.
Envelope Follower - a circuit that transforms a fast-moving, alternating current signal into a slower-moving (though accurate) DC signal that provides an overall sense of the envelope (or amplitude) of the input signal. Envelope followers are used to generate the key signals in dynamics processors as well as in many other applications.
Expression Pedal - a pedal that can provide a system with a continuous range of foot presure (as opposed to a switch, such as the sustain pedal on the piano). Expression pedals are common on organs, and are used in volume and Wah-Wah pedals as well as other effects where one parameter is continously adjusted by a performer.
Feedback - the act of routing part (or all) of the output of a circuit or signal processing algorithm back into itself. Feedback at the micro level is key to the design of many common audio circuits, such as op amps and IIR filters; when combined with delay, feedback can be used to provide signal regeneration to make for more realistic echo and reverberation. In digital systems, feedback can be used in conjunction with delay lines for many of the same techniques.
Filter - circuits or algorithms which shape the frequency content of a signal. Filters in analog circuits are typically created using networks of resistors and capacitors (RC filters), often with active amplification; in DSP, filters are created by working with short delays of the input (and previously output) signal. Filters can be characterized by the number of stages (“poles” and “zeros”) they contain - a 4-pole filter may consist of 4 analog RC filters in series or use 4 samples of digital delay; by their design - a finite-impulse response (FIR) filter works only by shaping the input signal, whereas an infinite-impulse response (IIR) filter uses feedback, recycling the output signal back into the input to create sharper filters with fewer stages; and by their typology, either in scientifc terms (e.g. Butterworth versus Chebyshev) or effect on the signal (e.g. lowpass, highpass, bandpass, notch, etc.). Filters that are neutral and don’t effect frequency content but create complex phase adjustments on the input signal are called allpass filters and are important in designing reverberation algorithms.
Fourier Theorem / Fast Fourier Transform / Frequency Domain - named after French Mathematician Joseph Fourier, the Fourier theorem states that any signal can be decomposed into constituent sine waves of varying frequencies, amplitudes, and phases. The Fast Fourier Transform (FFT, and its reverse, the Inverse Fast Fourier Transform, or IFFT) are the algorithms most commonly used in DSP to transform an audio signal from the time domain (samples of amplitude unfolding over time) to the frequency domain (frames of complex numbers representing the amplitudes and phases of frequencies). DSP in the frequency domain, while once computationally impactical to implement in real time, is increasingly used in audio effect design for effects such as microphone / amplifier modelling, convolution reverb, and time-stretching / pitch-shifting.
Impedance - the resistance of a component or circuit to an alternating current signal. Impedance effects both amplitude and phase and is an important consideration in the design of circuits using analog electronics.
Low Frequency Oscillator (LFO) - a time-varying, periodic signal too slow to be heard directly, LFOs are used frequently in effects processing to shift parameters such as the delay time in modulation effects, the filter sweep in “auto-wah” style filters, or the volume attenuation in tremolo units. LFOs, like audio range oscillators, have a frequency, an amplitude, a phase, and a shape, which is often a mathematically simple waveform such as a sine, square, triangle, or sawtooth wave.
MIDI - developed by Dave Smith (Sequential Circuits) and Ikutaro Kakehashi (Roland) with the backing of their companies within a consortium of Japanese and US-based synthesizer manufacturers, the Musical Instrument Digital Interface (MIDI) standard has been used since 1982 as a digital communications protocol for professional audio equipment, most notably synthesizers, drum machines, samplers, and computers. Effects systems use MIDI as well to transmit and receive parametric changes, patch / program changes, and other functions. MIDI, it is important to note, does not transmit digital audio - rather it sends different types of schematic messages (such as packets of numbers that represent musical “notes” as played on a controller).
Modulation - effects which use a time-varying signal to modulate the processing are called modulation effects; these commonly include effects that use LFOs to shift delay lines such as flangers, chorus, and phase shifters, as well as effects such as tremolo.
Nyquist-Shannon theorem - the principle in signal processing that dictates the relationship between sampling rate and frequency response of a discrete representation of a time-domain signal. The theorem dictates that for any signal represented with sampling rate x, the highest frequency that can be represented in that signal is x/2 (the Nyquist frequency of the signal).
Operational Amplifier (Op Amp) - a simple and ubiquitous analog circuit that allows for the amplification of an input signal with the gain set by simple external components. Op Amps are used in analog circuits for amplification of signals, and in digital circuits for designing comparators or inverting buffers in digital logic.
Pickup - an electromagnetic transducer commonly used on instruments such as electric guitars and organs. Typically a coil of copper wire wound in a bobbin around one or more magnets, a pickup serves as an inductor, generating a low voltage signal in relation to disturbances in the magnetic field around it that are caused by the string or tone wheel of the instrument.
Pitch Tracker - a circuit that attempts to accurately follow the fundamental frequency or pitch of the input signal in order to generate a value or signal that can control parameters of the signal processing. Pitch tracking is important in pitch correction algorithms but it is also used in a simpler way in effects such as octave dividers.
Potentiometer - a potentiometer as a passive electronic component that either sends a proportion of the input voltage to one of two outputs (a voltage divider) or adds a variable amount of resistance to a signal (a rheostat). Potentiometers are notable for being variable - controlled either by a physical affordance such as a dial or slider, or via digital control.
Resistor - a simple, two-terminal passive electronic component that adds resistance (reduces the voltage) on a signal. Resistor values are measured in ohms, and are the most common passive component (along with capacitors) used in analog circuit design.
Sample - the smallest unit of data in a time-domain digital audio signal or file. Each sample represents the amplitude of a specific point of time in the sound.
Sample and Hold (S&H) - a technique where an input voltage is sampled by a clock signal and then held as a continuous voltage output until the next clock signal. S&H circuits appear in many synthesizers and are used to create interesting voltage patterns. Sample-and-hold circuits are also infrastructural in many types of analog-to-digital converters.
Sampling Rate (SR) - the speed at which an audio signal is sampled to create its digital representation; the faster the sampling rate, the better the frequency resolution of the signal (and higher the Nyquist frequency). CD-quality audio has a sampling rate of 44,100 samples per second (44.1 kHz), resulting in an upper frequency limit of 22,050 Hz; modern professional-quality audio systems typically have a higher sampling rate of 48, 96, or 192 kHz. In some literature, the sampling rate may be referred to as the sampling frequency (appreviated Fs).
Signal-to-noise Ratio (SNR) - this number represents the power of the actual signal contrasted to background noise in a signal. This is often expressed in decibels. In digital signals, the SNR is a function of the bit depth of the signal.
Switch - a component in an eletrical system that connects and disconnecs a specific path. Switches are characterized by the number of poles (parallel signal lines that are controlled by the switch) and throws (number of options for the switch).
Time Domain - the most common representational schema for digital audio signals, with a stream of amplitude values unfolding over time at a constant rate.
Transistor - an active electronic component, consisting of a semiconductor and at least three terminals. The presence of current at one of the terminals (often called the base or the gate) allows a much stronger voltage to flow between the other two terminals (the collector / source and the emitter / drain). In analog electronics, transistors are the basic component that allows for solid-state amplification of a signal; in digital electronics, transistors are used to construct the gates and switches that form the digital logic of the system.
Waveshaping - a signal processing technique where an input signal is shaped into an output signal via circuitry (or a digital algorithm) that performs a transfer function. Waveshaping is often used in synthesizer design to create different oscillator waveforms from a single core shape. Certain transfer functions, called Chebyshev polynomials, have specific effects of spectrum transfer on an incoming signal, allowing, e.g. harmonics to be added to a sine wave.
Vactrol - a generic trademark name often used in audio engineering to refer to a photoresistive opto-isolator; these devices consist of an LED and a photoresistor physically coupled to one another but (typically) electrically isolated from one another. Voltage through the LED lights the light, which drops the resistance in the photoresistor’s circuit. Vactrols can be used in a pinch as drop-in components in place of rheostates to add a form of voltage control; they are also commonly used to generate key signals in dynamics effects. The unusual response curve of the device (between the LED and the photoresistor) gives them a characteristic ‘curve’ that is musically useful in many circumstances.
Voltage - when referred to as a unit of measurement, the voltage of a circuit is the potential difference (or pressure) of electric charge between two points; this potential is measured in volts. In the hydraulic analogy of electricity, the voltage of a circuit maps to the flow pressure of a water system.
Wattage - the quantity of electrical energy transferred at a given rate, calculated by multiplying the current of the system by its voltage. In the hydraulic analogy of electricity, the wattage of an electrical system maps to the quantity of water flowing through a point in the system in a given amount of time.