Does a "chirp"-like generalization of the Gabor or Morlet wavelets definitions exist in the lit somewhere?

60 Views Asked by At

Please forgive me for predicating this on the definition of the continuous Fourier Transform preferred by most electrical engineers:

$$ X(f) \triangleq \mathscr{F} \Big\{ x(t) \Big\} \triangleq \int\limits_{-\infty}^{+\infty} x(t) \, e^{-i 2 \pi f t} \ \mathrm{d}t $$

and inverse:

$$ x(t) \triangleq \mathscr{F}^{-1} \Big\{ X(f) \Big\} = \int\limits_{-\infty}^{+\infty} X(f) \, e^{+i 2 \pi f t} \ \mathrm{d}f $$

Even with different signs on $i$, the elegant symmetry between the forward transform and inverse should be clear. And it makes remembering the Duality property, Parseval's theorem easy:

If $X(f) = \mathscr{F} \Big\{ x(t) \Big\}$, then $x(-f) = \mathscr{F} \Big\{ X(t) \Big\}$.

$$ X(0) = \int\limits_{-\infty}^{+\infty} x(t) \ \mathrm{d}t $$

$$ x(0) = \int\limits_{-\infty}^{+\infty} X(f) \ \mathrm{d}f $$

$$ \int\limits_{-\infty}^{+\infty} \Big| x(t) \Big|^2 \ \mathrm{d}t = \int\limits_{-\infty}^{+\infty} \Big| X(f) \Big|^2 \ \mathrm{d}f$$

NO nasty asymmetrical scaling factors to worry about!! (Just remember the $2\pi$ in the exponent.) This is why EE's like this definition of the Fourier Transform.

Given this definition, then the Fourier transform of the gaussian function (exponent scaled as shown) is itself:

$$ \mathscr{F} \Big\{ e^{- \pi t^2} \Big\} = e^{- \pi f^2} $$


So, harmonizing the parameters and symbols, the normalized Gabor "mother wavelet" (with this parameter $\mathcal{F}$) is simply a complex sinusoid with a gaussian "window" or "envelope":

$$\begin{align} w(t) &\triangleq e^{-\pi t^2} \, e^{i 2 \pi \mathcal{F} t } \\ &= e^{\pi (i\mathcal{F})^2} \left(e^{-\pi t^2} \, e^{i 2 \pi \mathcal{F} t } e^{-\pi (i\mathcal{F})^2} \right) \\ &= e^{-\pi \mathcal{F}^2} e^{-\pi (t-i\mathcal{F})^2} \\ \end{align}$$

The Fourier Transform isn't too hard to get:

$$ W(f) = e^{- \pi (f-\mathcal{F})^2} $$

My question is, in the literature, has this been generalized a little more and does this have a name? (Like where can I read about it?)

$$\begin{align} w(t) &\triangleq e^{-\pi t^2} \, e^{i \pi \beta t^2} \, e^{i 2 \pi \mathcal{F} t } \\ &= e^{-\pi (1 - i\beta) t^2} e^{i 2 \pi \mathcal{F} t } \\ &= e^{-\pi (\sqrt{1 - i\beta} \, t)^2} e^{i 2 \pi \mathcal{F} t } \\ &= e^{-\pi \mathcal{F}^2} e^{-\pi (\sqrt{1 - i\beta} \, t - i\mathcal{F})^2} \\ \end{align}$$

This makes this windowed sinusoid, a windowed "chirp" function. We need both the $\beta$ parameter and the $\mathcal{F}$ parameter because the scaling on the width of the window is still normalized to 1.

I believe the Fourier Transform is

$$ W(f) = \frac{1}{\sqrt{1 - i\beta}} \, e^{- \pi (f/\sqrt{1 - i\beta} -\mathcal{F})^2} $$

This can be generalized one step further by putting in an exponential "ramp" parameter $\lambda$

$$\begin{align} w(t) &\triangleq e^{-\pi t^2} \, e^{i \pi \beta t^2} \, e^{i 2 \pi \mathcal{F} t } e^{2 \pi \lambda t } \\ &= e^{-\pi (1 - i\beta) t^2} e^{i 2 \pi (\mathcal{F}-i\lambda) t } \\ &= e^{-\pi (\sqrt{1 - i\beta} \, t)^2} e^{i 2 \pi (\mathcal{F}-i\lambda) t } \\ &= e^{-\pi \mathcal{F}^2} e^{-\pi (\sqrt{1 - i\beta} \, t - i(\mathcal{F}-i\lambda))^2} \\ &= e^{-\pi \mathcal{F}^2} e^{-\pi (\sqrt{1 - i\beta} \, t - i\mathcal{F}-\lambda))^2} \\ \end{align}$$

And it looks like the Fourier Transform is

$$ W(f) = \frac{1}{\sqrt{1 - i\beta}} \, e^{- \pi (f/\sqrt{1 - i\beta} -\mathcal{F} + i \lambda)^2} $$

Does this generalization exist in the lit somewhere and, if so, can I read about it without a pay-wall?


Also, it appears to me that, in general, we can say that

$$ \mathscr{F} \Big\{ e^{a t^2 + b t + c} \Big\} = e^{A f^2 + B f + C} $$

where the constants $A$, $B$, and $C$ can be explicitly mapped from $a$, $b$, and $c$. It appears to me that the mapping is:

$$\begin{align} A &= \frac{\pi^2}{a} \\ B &= i \frac{\pi b}{a} \\ C &= c - \frac{b^2}{4a} - \tfrac{1}{2}\log\left(-\frac{a}{\pi}\right) \\ \end{align}$$

and the inverse mapping (which should be self-similar) is:

$$\begin{align} a &= \frac{\pi^2}{A} \\ b &= -i \frac{\pi B}{A} \\ c &= C - \frac{B^2}{4A} - \tfrac{1}{2}\log\left(-\frac{A}{\pi}\right) \\ \end{align}$$

Looks like $\Re\{a\}<0$ and $\Re\{A\}<0$ for the integrals to converge and for the $\log(\cdot)$ to be real and finite in the mapping.

This appears to be true for quadratics in the exponent. Is it also true for higher-order polynomials in the exponent? Does this also exist in the lit somewhere?