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I have a system where I cross correlate recorded audio signals with several smaller generated signals to extract feature vectors that later I use in a kNN matching. The system works the way I want it but I feel like I am doing things inefficiently and repetitively. I was hoping to get a simplification of what I am doing.

Let's say I have a recorded audio signal $S_k$ at a certain time. I am calculating the feature vector as:

$$ F_{ki} = \max\left(\left\lvert S_k \circledast H_i\right\rvert\right), \quad i \in {1,\ldots,N} $$

where $\circledast$ represents cross correlation, and the signals $H_{1..N}$ are just sinusoidal signals of the form $H_i = \sin(2\pi f_it)$ where $f_1$ is 18 kHz, $f_2$ 18.1 kHz, $f_3$ 18.2 kHz ... etc. every signal has a single fixed frequency value and my bandwidth is between 18 kHz and 22 kHz. The $H_i$ signal is 1/10 the length of $S_K$

My whole intention is trying to see which frequency values are strong and to what level by considering discrete frequency values like 18.1 kHz, 18.2 kHz etc.

I feel like I am doing some things unnecessarily. Given that cross correlation involves doing fft of both sides and inverse fft after their complex multiplication. I think I can just take the fft of the audio signal and finish the feature extraction there. But I don't know how to get as good relevant feature vector with lower dimension as the cross correlation would give me.

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Your notations is not entirely clear, but it looks to me that you are simply calculating the magnitude of the Fourier Transform (FT) of $s_k(t)$ at $f_i$. The cross correlation of a signal with a sine wave is a sine wave at the same frequency with the magnitude of the signal's FT.

Let's assume we have a singnal $s_k(t) \leftrightarrow S_k(\omega)$ and a complex sine $h_i(t) = e^{-j\omega_i t }$.

The cross correlation is

$$ r_{sh}(t) = \int s(\tau) \cdot h(t+\tau) d\tau = \int s(\tau) \cdot e^{-j\omega_i (t +\tau) } d\tau = \int s(\tau) \cdot e^{-j\omega_i \tau } \cdot e^{-j\omega_i t} d\tau = $$ $$ e^{-j\omega_i t} \cdot \int s(\tau) \cdot e^{-j\omega_i \tau } d\tau = e^{-j\omega_i t} \cdot S_k(\omega_i) $$

Note that the last integral in the last equation is simply the Fourier Transform. Since $|e^{-j\omega_i t}| = 1$ we simply get

$$ \max(|r_{s_kh_i}(t)|) = |S_k(\omega_i)| $$

There are probably some details that need to be nailed down (sine vs complex sine, boundary effects, spectral smearing, linear vs circular cross correlation), but that could be a very efficient approach.

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