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I want to shift the response phase of an audio signal (float array, length N, value range -1..1). I'm about using Fast Hartley Transform to transform it to the frequency-domain, process phase shift, then convert back to the time-domain and output the result.
I'm now can do the Hartley transform, and I also can calculate the phase of kth bin:

real(k) = (x(k) + x(-k))/2
imag(k) = (x(k) - x(-k))/2
phase(k) = atan2(imag(k), real(k))

I want to shift the phase(k) by an α degree. What should I do?

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I think your notation is a bit fishy but I'll use it. Assuming that your $\text{phase}(k)$ calculation is correct, do a complex multiplication of $\text{real}(k) + i\ \text{imag}(k)$ by $e^{i\alpha} = \cos(alpha) + i\sin(alpha)$. That can be done using four real multiplications and a few add/sub. Then you need to reverse the process of how you obtained $\text{real}(k)$ and $\text{imag}(k)$:

$$x(k) = \text{real}(k) + \text{imag}(k)$$ $$x(-k) = \text{real}(k) - \text{imag}(k)$$

Finally retransform.

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  • $\begingroup$ Ah, x(-k) is actually x(N-k), and the formula is from a document, I'm not sure about it much. Anyway, thanks for your help, I've successfully shifted the phase. I test the result by substract the phase value before and after, and it's alpha. $\endgroup$ – MPhuc Jun 25 '15 at 9:45

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