Allpassphase

Digital reverbs, such as Schroeder or FDN (Feedback Delay Networks) reverbs, use allpass filters to increase the echo density without introducing the unnatural frequency coloration caused by low-pass or high-pass filters. D. Time-Alignment

Where α is related to the center frequency and Q of the filter.

// Update delay lines x2 = x1; x1 = x0; y2 = y1; y1 = y0; allpassphase

The holy grail of reverb algorithms (like Schroeder reverbs) relies entirely on .

Are you designing an or digital (z-domain) filter? Digital reverbs, such as Schroeder or FDN (Feedback

Mathematically, a first-order all-pass filter is defined by the transfer function:

Sometimes, flipping the polarity switch (the "Ø" button) isn't enough. Flipping polarity is a binary switch—it inverts the wave entirely. But if a waveform is only partially out of phase, flipping it might make it worse. An Allpass filter allows you to dial in the exact phase relationship needed to restore the signal to full strength. // Update delay lines x2 = x1; x1

Engineers often apply an Allpass filter to the Mid channel or the Side channel. This phase shift can alter how the stereo image is perceived, making the center feel more solid or widening the sides, all without changing the EQ balance.

are the "unseen architects" of the audio world. Unlike most filters that cut out bass or treble, an all-pass filter lets every frequency through at the same volume, but it manipulates their timing (or phase ) as they pass through [1, 2].

When measuring a room’s impulse response, engineers use a sinusoidal sweep (e.g., a logarithmic chirp). The recorded response is convolved with the inverse allpass filter of the original sweep. The resulting relies entirely on the known allpassphase of the sweep signal to extract the true room response from background noise.

output[i] = (x0 + y0 * factor) * 0.5; // Variable from bandpass to bandreject