So an equalizer manipulates the gain of certain frequencies by use of a variety of filters, as we all know. But is there a way to manipulate delay in a frequency-specific manner?
Above is a link to a picture of a parametric equalizer. What if the white line in the picture represented delay time instead of gain?
Is this possible? Does it exist? And if so, what is it called?
Yes it does exist and these are called "all-pass" filters in that over the band of interest the magnitude does not change (other than a possible fixed gain at all frequencies) but modify the phase response. The time delay at a specific frequency is the negative derivative of phase with respect to frequency, so the goal of these filters is to selectively change the slope of the phase according to the desired delay at each frequency.
The primary characteristic of a digital all-pass filter is that it will only have poles inside the unit circle on the z-plane and for each of those poles will have a conjugate reciprocal zero outside the unit circle, as depicted in the plot below.
