[LAD] Phase rotation

Hi all, During the last days, I looked into phase-ration: components of a signal are delayed differently depending on their frequency. A good write up on the subject can be found at [1], and a commercial tool is available from [2]. The interesting aspect is that phase rotation does not alter the sound of the signal nor the loudness. However changing the phase vs. frequency relationship between lower and upper harmonics changes the waveform and can affect where the digital peak occurs. For this reason phase rotation is commonly used by radio stations to reduce the signal peak, and make the signal more symmetrical. Phase rotation circuits are also used during mastering to increase headroom. To implementing this, the following operations are performed: * FFT * rotate phase, retain magnitude * inverse FFT This works well, except for the first FFT bin: 0 Hz, DC offset. If the phase-shift changes the average DC level of the signal there is a discontinuity. I could use some of the collective expertise of this list. Has anyone looked into this before? Is it even possible with piecewise block processing? Is there a way to calculate the DC offset? Since the overall magnitude does not change simply summing up the bins results in the same 0th bin. I considered to low-pass filter the DC offset, or simply remove the DC offset using a high-pass before the FFT to mitigate the issue. But either introduces different artifacts. If I understand correctly, a Hilbert transform as mentioned at [1] has the same issue. I've condensed the DSP into a simple test tool to toy around with the issue: https://gist.github.com/17fd61b04d4c4939727dfdccd79f53a5 with low frequencies, the differences are obvious: * https://i2.paste.pics/cc32f6c5d622e31ab0e830ab1ce205e9.png (2.5 * FFT's base-freq) * https://i2.paste.pics/3e6eb827148b90800569843c11da2e48.png (0.37 * FFT's base-freq) I'd appreciate any insights. -- robin [1] https://forum.juce.com/t/how-to-rotate-the-phase-of-an-audio-signal/39072/10 [2] https://www.izotope.com/en/products/rx/features/phase.html