This article discusses All-Pass Filters, taking a look at what their function is in the audio world and how we can use them in our own applications.
An All-Pass Filter is a signal-processing filter that allows all frequencies to pass through it equally (as opposed to a high-pass or low-pass filter, which only allows one or the other to pass through). Still, it changes the phase relationship between various frequencies. These filters do not affect the amplitude of the frequencies as they pass through, only the phase between each order of frequencies. All Pass filters are most commonly used to correct an undesired phasing issue in a mix, as they can be used to pinpoint and compensate for phasing issues in the signal. Beyond their main function, all pass filters may also be used to create effects within the mix as you see fit. For example, you can mix two versions of a signal, one dry signal and one running through an all-pass filter, in order to create a comb filter at the desired frequency. Each all-pass filter can be slightly different in response, and like with any filter or piece of gear that will affect the signal, be sure to take the time to run several types of signals through it in order to gain a better understanding of the phasing response at the different frequencies.
All Pass Filters are a great way to clean up phasing issues you may encounter in your mix, but you want to make sure you are minimizing phasing issues during the recording process and only use them when necessary.