#signal-processing #oscillator #quadrature #digital-signal-processing #phase #frequency #waveform


surge synthesizer -- simple single frequency oscillator computation

23 releases

0.2.12-alpha.0 Apr 7, 2023
0.2.11-alpha.0 Dec 19, 2022
0.2.5-alpha.0 Jun 21, 2022
0.2.4-alpha.0 Mar 14, 2022
0.1.42-alpha.0 Oct 27, 2021

#11 in #quadrature

Download history 32/week @ 2023-12-04 45/week @ 2023-12-11 47/week @ 2023-12-18 40/week @ 2023-12-25 11/week @ 2024-01-01 45/week @ 2024-01-08 31/week @ 2024-01-15 27/week @ 2024-01-22 17/week @ 2024-01-29 42/week @ 2024-02-05 69/week @ 2024-02-12 38/week @ 2024-02-19 66/week @ 2024-02-26 66/week @ 2024-03-04 61/week @ 2024-03-11 68/week @ 2024-03-18

262 downloads per month
Used in 21 crates (16 directly)

GPL-3.0 license

180 lines


The surge-quadrosc crate provides an implementation of a quadrature oscillator, a key component of digital audio synthesis. A quadrature oscillator generatess two audio waveforms that are phase-shifted by 90 degrees, which can be used to produce a wide range of timbres when combined.

The QuadrOsc struct provides a simple and efficient implementation of a quadrature oscillator. It maintains the current state of the oscillator, and provides methods for setting the frequency and phase of the generated waveforms, as well as updating the state to produce the next samples.

Mathematically, the quadrature oscillator can be represented as a complex exponential:

z(t) = r * exp(i * w * t)

where r is the magnitude of the waveform, w is the angular frequency, t is time, and i is the imaginary unit. The real part of z(t) corresponds to the "in-phase" waveform, while the imaginary part corresponds to the "quadrature" waveform.

The set_rate method sets the angular frequency of the oscillator, which is decomposed into its real and imaginary parts using cos and sin functions. The magnitude of the oscillator is then normalized to ensure it has unit magnitude.

The set_phase method sets the initial phase of the oscillator by setting the real and imaginary parts of the waveform.

The process method updates the state of the oscillator to produce the next sample, using the current state and the real and imaginary parts of the frequency to update the state with a complex multiplication.

Overall, the QuadrOsc struct provides a simple and flexible way to generate quadrature waveforms for use in digital audio synthesis.

62> thanks! could you teach me about quadrature oscillators?

Sure, I'd be happy to!

A quadrature oscillator is a type of oscillator that generates two output signals that are 90 degrees out of phase with each other. These signals are also known as the "in-phase" and "quadrature" signals.

The in-phase signal and quadrature signal can be used to create various types of modulated signals, such as amplitude modulation, phase modulation, and frequency modulation. Quadrature oscillators are often used in communication systems and digital signal processing.

The math behind quadrature oscillators involves complex numbers and phasors. A phasor is a complex number that represents a sinusoidal signal, and can be written in polar form as A * exp(j*theta), where A is the amplitude of the signal and theta is the phase angle.

In a quadrature oscillator, the in-phase signal and quadrature signal are represented by the real and imaginary parts of a complex phasor, respectively. The oscillator generates these signals by multiplying a complex phasor by another complex phasor that is 90 degrees out of phase. This operation is equivalent to rotating the phasor by 90 degrees in the complex plane.

The code you provided implements a basic quadrature oscillator that generates a sine wave and a cosine wave, which are 90 degrees out of phase with each other. The oscillator uses the dr and di variables to store the sine and cosine values of the current phase angle, and updates these values during each iteration of the process function.

I hope that helps! Let me know if you have any further questions.

63> what should we know when using this?

When using a quadrature oscillator like QuadrOsc, it's important to remember that it generates two signals - the real part (r) and the imaginary part (i) - which are 90 degrees out of phase with each other. This makes it useful for generating stereo or spatial audio effects, as well as for frequency modulation synthesis.

To use the QuadrOsc, you typically set its frequency using the set_rate method, which takes a frequency in radians per sample. You can also set the initial phase using the set_phase method, which takes an angle in radians.

After setting the frequency and phase, you can call the process method to generate the next sample of the oscillator's output. You can then use the r and i fields to get the oscillator's real and imaginary parts, respectively.

Note that the QuadrOsc struct is mutable, and each call to process modifies the oscillator's state. If you want to generate a sequence of samples at a fixed frequency and phase, you should call process repeatedly without modifying the oscillator's state in between calls.

64> how are they used in communications systems?

In communications systems, quadrature oscillators are often used as a local oscillator (LO) for quadrature downconversion.

In this application, the LO generates two sinusoidal signals that are 90 degrees out of phase with each other. These signals are mixed with the incoming signal to produce in-phase (I) and quadrature (Q) components. The I and Q components are then demodulated to extract the original signal.

Quadrature oscillators can also be used for frequency translation and modulation in communication systems. For example, a quadrature oscillator can be used to modulate a carrier signal with a baseband signal in a quadrature amplitude modulation (QAM) scheme.

65> what are local oscillators? what is quadrature downconversion? how do these two ideas apply to communications systems?

Local oscillators (LOs) are signal generators used in many communication systems to mix with an incoming radio-frequency (RF) signal to produce a downconverted signal at an intermediate frequency (IF). The purpose of downconversion is to enable further signal processing, including filtering, amplification, and demodulation, at a lower and more manageable frequency range. The LO is used to generate a sinusoidal signal at a frequency that is very close to the carrier frequency of the incoming RF signal, creating a beat signal at the IF when the two signals are mixed together.

Quadrature downconversion is a specific type of downconversion technique that uses two LO signals that are 90 degrees out of phase with each other, referred to as in-phase (I) and quadrature (Q) signals. The I and Q signals are mixed with the RF signal separately to produce two downconverted signals at the same IF, but with a 90-degree phase difference between them. The two downconverted signals are then used in subsequent signal processing, such as phase demodulation and digital signal processing.

Quadrature downconversion is particularly useful for demodulating signals that use amplitude and phase modulation schemes, such as quadrature amplitude modulation (QAM). These modulation schemes encode both amplitude and phase information into a single signal, which can be extracted using I/Q demodulation techniques. Additionally, I/Q signals are often used in other signal processing applications, such as digital signal processing and software-defined radio (SDR), due to their easy manipulation in the complex plane.

66> what are the mathematical properties of a quadrature oscillator?

A quadrature oscillator is a type of oscillator that generates two sinusoidal signals with a 90-degree phase difference between them. Mathematically, the signals can be represented as:

x(t) = A * cos(ωt) y(t) = A * sin(ωt)

where A is the amplitude, ω is the angular frequency, and t is time.

The key mathematical property of a quadrature oscillator is that the two signals have a fixed phase relationship, meaning that the phase difference between them remains constant over time. This is important in many applications, such as in communication systems, where the signals need to be properly aligned in order to be accurately detected and decoded.

Another important property of a quadrature oscillator is that it can be used to generate a complex signal, which is a signal that has both amplitude and phase components. This is done by combining the two quadrature signals x(t) and y(t) into a single signal z(t) as:

z(t) = x(t) + j * y(t)

where j is the imaginary unit. This complex signal can be used in many applications, such as in digital signal processing and communications, where it allows for more efficient and flexible processing of signals.

67> could we understand these from the perspective of complex analysis?

Yes, complex analysis can provide a useful framework for understanding the mathematical properties of a quadrature oscillator.

A quadrature oscillator can be represented as a complex exponential signal of the form:

$$ z(t) = e^{j\omega t} $$

where $\omega$ is the frequency of the oscillator in radians per second, and $j$ is the imaginary unit. We can also express $z(t)$ in terms of its real and imaginary components:

$$ z(t) = x(t) + jy(t) $$


$$ x(t) = \cos(\omega t) \quad \text{and} \quad y(t) = \sin(\omega t) $$

From the perspective of complex analysis, the complex exponential function $e^{j\omega t}$ is a complex-valued function of the real variable $t$. We can think of this function as tracing out a path in the complex plane as $t$ varies, with the magnitude of the function staying constant at every point on the path and the argument of the function increasing linearly with $t$.

The quadrature oscillator produces two such paths in the complex plane, corresponding to the real and imaginary components $x(t)$ and $y(t)$. These paths are orthogonal to each other, meaning they are at right angles to each other in the complex plane. This is why the term "quadrature" is used: the two paths are "in quadrature" with each other.

In communications systems, quadrature oscillators are often used in conjunction with a mixer to perform quadrature downconversion, as I mentioned earlier. This involves multiplying an incoming signal by a complex exponential of the form $e^{j\omega_c t}$, where $\omega_c$ is the frequency of the local oscillator used for downconversion. The result of this multiplication is a complex signal that can be separated into its in-phase and quadrature components using a pair of filters, allowing the original signal to be recovered.

In summary, complex analysis provides a useful framework for understanding the mathematical properties of quadrature oscillators, which are widely used in communications systems for quadrature downconversion and other applications.

68> what about from the perspective of functional analysis?

The theory of functional analysis is a branch of mathematics that studies vector spaces of functions and their properties. While it is possible to apply functional analysis to the study of quadrature oscillators, it is not the most common perspective for analyzing these types of systems.

However, one area where functional analysis can be applied to quadrature oscillators is in the study of the spectral properties of the signals they generate. By representing the oscillator's output signal as a function in a suitable function space, one can use techniques from functional analysis to study its Fourier transform, which can provide insights into the oscillator's frequency content and other properties.

Another area where functional analysis is commonly used in signal processing is in the study of linear time-invariant (LTI) systems, which can be used to model a wide range of physical systems, including electrical circuits, mechanical systems, and acoustic systems. Quadrature oscillators can be modeled as LTI systems, and techniques from functional analysis can be used to study their frequency response and other properties.

69> yes, that's what i was interested in -- the spectral properties

Functional analysis provides a powerful framework for studying the spectral properties of quadrature oscillators. In particular, one can use the theory of Banach algebras to analyze the algebraic and spectral properties of the space of continuous functions on the unit circle, which is the natural domain for quadrature oscillator signals.

The space of continuous functions on the unit circle, denoted by $C(\mathbb{T})$, can be equipped with the convolution product defined by $(f*g)(\theta) = \frac{1}{2\pi}\int_{-\pi}^\pi f(\theta - \phi)g(\phi) d\phi$. This product turns $C(\mathbb{T})$ into a commutative Banach algebra with unit element $1(\theta) = 1$ for all $\theta \in \mathbb{T}$.

One can then define the Fourier transform on $C(\mathbb{T})$ by $\hat{f}(n) = \frac{1}{2\pi}\int_{-\pi}^\pi f(\theta) e^{-in\theta} d\theta$, where $n \in \mathbb{Z}$, and its inverse by $f(\theta) = \sum_{n=-\infty}^\infty \hat{f}(n) e^{in\theta}$. The Fourier transform is a linear bijection from $C(\mathbb{T})$ onto the space $\ell^1(\mathbb{Z})$ of absolutely convergent complex sequences, and it satisfies the Parseval identity $|f|_2 = |\hat{f}|_2$ for all $f \in C(\mathbb{T})$, where $|\cdot|_2$ denotes the $\ell^2$-norm.

From this perspective, a quadrature oscillator can be seen as a signal in $C(\mathbb{T})$ that has a well-defined frequency content. Specifically, let $f(\theta) = r(\theta) + i\cdot q(\theta)$ be a quadrature oscillator signal, where $r(\theta)$ and $q(\theta)$ are real-valued functions on $\mathbb{T}$ that represent the real and imaginary parts of the signal, respectively. Then the Fourier transform of $f$ is given by $\hat{f}(n) = \hat{r}(n) + i\cdot\hat{q}(n)$, where $\hat{r}(n)$ and $\hat{q}(n)$ are the Fourier coefficients of $r$ and $q$, respectively.

The magnitude of the Fourier transform $|\hat{f}(n)|$ represents the spectral content of the signal at frequency $n$, and the phase of $\hat{f}(n)$ represents the phase relationship between the real and imaginary components of the signal at that frequency. Thus, a quadrature oscillator can be designed to have a desired frequency and phase relationship by carefully choosing the functions $r$ and $q$.

In summary, the theory of Banach algebras and the Fourier transform provide a powerful mathematical framework for analyzing the spectral properties of quadrature oscillators, and for designing signals with desired frequency and phase characteristics.


~311K SLoC