Vesa Välimäki, Roope Salmi, Stefan Bilbao, Sebastian J. Schlecht, David Zicarelli
Companion page for the paper submitted to DAFx25, Ancona, Italy.
Given the speedy computation of the FFT in current computer hardware, there are new possibilities for examining transformations for very long sounds. A zero-phase version of any audio signal can be obtained by zeroing the phase angle of its complex spectrum and taking the inverse FFT. This paper recommends additional processing steps, including zero-padding, transient suppression at the signal's start and end, and gain compensation, to enhance the resulting sound quality. As a result, a sound with the same spectral characteristics as the original one, but with different temporal events, is obtained. Repeating rhythm patterns are retained, however. Zero-phase sounds are palindromic in the sense that they are symmetric in time. A comparison of the zero-phase conversion to the autocorrelation function helps to understand its properties, such as why the rhythm of the original sound is emphasized. It is also argued that the zero-phase signal has the same autocorrelation function as the original sound. One exciting variation of the method is to apply the IFFT separately to the real and imaginary parts of the spectrum to produce a stereo effect. A frame-based technique enables the use of the zero-phase conversion in real-time audio processing. The zero-phase conversion is another member of the giant FFT toolset, allowing the modification of sampled sounds, such as drum loops or entire songs.
Original excerpt from Martin Arnold, contact;vault performed by Quatuor Bozzini.
String quartet, no zero padding
String quartet, P=2 zero padding
Original 130 Bpm Classic Techno Drums by BEATMATCH_info, CC BY 4.0
Techno drums, no zero padding
Techno drums, P=2 zero padding
Speech, P=2 zero padding
Singing 1, P=2 zero padding
Singing 2, P=2 zero padding
Singing 3, P=2 zero padding
Pink noise, P=2 zero padding
Drums, P=2 zero padding