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State-Space Analysis of Waveguide Oscillator

We will now use state-space analysisC.15[452] to determine Equations (C.130-C.133).

From Equations (C.125-C.129),

$\displaystyle x_1(n+1) = c[g x_1(n) + x_2(n)] - x_2(n) = c\,g x_1(n) + (c-1) x_2(n)
$

and

$\displaystyle x_2(n+1) = g x_1(n) + c[g x_1(n) + x_2(n)] = (1+c) g x_1(n) + c\,x_2(n)
$

In matrix form, the state time-update can be written

$\displaystyle \left[\begin{array}{c} x_1(n+1) \\ [2pt] x_2(n+1) \end{array}\right] = \underbrace{\left[\begin{array}{cc} gc & c-1 \\ [2pt] g(c+1) & c \end{array}\right]}_\mathbf{A} \left[\begin{array}{c} x_1(n) \\ [2pt] x_2(n) \end{array}\right] \protect$ (C.133)

or, in vector notation,
$\displaystyle \underline{x}(n+1)$ $\displaystyle =$ $\displaystyle \mathbf{A}\underline{x}(n) + \mathbf{B}u(n)$ (C.134)
$\displaystyle y(n)$ $\displaystyle =$ $\displaystyle \mathbf{C}\underline{x}(n)$ (C.135)

where we have introduced an input signal $ u(n)$ , which sums into the state vector via the $ 2\times 1$ (or $ 2\times N_u$ ) vector $ \mathbf{B}$ . The output signal is defined as the $ 1\times 2$ vector $ \mathbf{C}$ times the state vector $ \underline{x}(n)$ . Multiple outputs may be defined by choosing $ \mathbf{C}$ to be an $ N_y \times 2$ matrix.

A basic fact from linear algebra is that the determinant of a matrix is equal to the product of its eigenvalues. As a quick check, we find that the determinant of $ A$ is

$\displaystyle \det{\mathbf{A}} = c^2g - g(c+1)(c-1) = c^2g - g(c^2-1) = c^2g - gc^2+g = g. \protect$ (C.136)

When the eigenvalues $ {\lambda_i}$ of $ \mathbf{A}$ (system poles) are complex, then they must form a complex conjugate pair (since $ \mathbf{A}$ is real), and we have $ \det{\mathbf{A}} = \vert{\lambda_i}\vert^2 = g$ . Therefore, the system is stable if and only if $ \vert g\vert<1$ . When making a digital sinusoidal oscillator from the system impulse response, we have $ \vert g\vert=1$ , and the system can be said to be ``marginally stable''. Since an undriven sinusoidal oscillator must not lose energy, and since every lossless state-space system has unit-modulus eigenvalues (consider the modal representation), we expect $ \left\vert\det{A}\right\vert=1$ , which occurs for $ g=1$ .

Note that $ \underline{x}(n) = \mathbf{A}^n\underline{x}(0)$ . If we diagonalize this system to obtain $ \tilde{\mathbf{A}}= \mathbf{E}^{-1}\mathbf{A}\mathbf{E}$ , where $ \tilde{\mathbf{A}}=$   diag$ [\lambda_1,\lambda_2]$ , and $ \mathbf{E}$ is the matrix of eigenvectors of $ \mathbf{A}$ , then we have

$\displaystyle \tilde{\underline{x}}(n) = \tilde{A}^n\,\tilde{\underline{x}}(0) = \left[\begin{array}{cc} \lambda_1^n & 0 \\ [2pt] 0 & \lambda_2^n \end{array}\right]\left[\begin{array}{c} \tilde{x}_1(0) \\ [2pt] \tilde{x}_2(0) \end{array}\right]
$

where $ \tilde{\underline{x}}(n) \isdef \mathbf{E}^{-1}\underline{x}(n)$ denotes the state vector in these new ``modal coordinates''. Since $ \tilde{A}$ is diagonal, the modes are decoupled, and we can write

\begin{eqnarray*}
\tilde{x}_1(n) &=& \lambda_1^n\,\tilde{x}_1(0)\\
\tilde{x}_2(n) &=& \lambda_2^n\,\tilde{x}_2(0)
\end{eqnarray*}

If this system is to generate a real sampled sinusoid at radian frequency $ \omega $ , the eigenvalues $ \lambda_1$ and $ \lambda_2$ must be of the form

\begin{eqnarray*}
\lambda_1 &=& e^{j\omega T}\\
\lambda_2 &=& e^{-j\omega T},
\end{eqnarray*}

(in either order) where $ \omega $ is real, and $ T$ denotes the sampling interval in seconds.

Thus, we can determine the frequency of oscillation $ \omega $ (and verify that the system actually oscillates) by determining the eigenvalues $ \lambda_i$ of $ A$ . Note that, as a prerequisite, it will also be necessary to find two linearly independent eigenvectors of $ A$ (columns of $ \mathbf{E}$ ).


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``Physical Audio Signal Processing'', by Julius O. Smith III, W3K Publishing, 2010, ISBN 978-0-9745607-2-4.
Copyright © 2014-03-23 by Julius O. Smith III
Center for Computer Research in Music and Acoustics (CCRMA),   Stanford University
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