All-pass filters provide delay without altering the spectrum but with frequency-dependent phase response

An all-pass filter has a flat frequency response (amplitude unchanged) but a non-flat phase response. Near the cutoff frequency, the phase shifts steeply, producing a local maximum in group delay. This frequency-dependent delay causes dispersion: different frequency components travel at different speeds, which can distort waveforms with multiple harmonics but has no effect on pure sine waves. All-pass filters can substitute for explicit delay lines in physical models, using less DSP at the cost of frequency-dependent delay. In the Nord Modular, an all-pass can be constructed by summing the lowpass and highpass outputs of a multimode filter. The delay is greatest near the LP/HP cutoff frequency.

Examples

Slide flute patch: all-pass filter (LP+HP summed) replaces one delay line. Uses ~25% less DSP than the delay-based version but with non-ideal delay response.

Assessment

Explain why an all-pass filter’s flat frequency response does not mean it has no effect on a complex waveform. When would you prefer an all-pass delay over a true delay line?