You can use an all-pass filter to match the phase shift of a dry signal with that of a filtered signal. This allows summing them without funny cancellation artifacts. Many filter types have phase shift that increases with frequency. When you add the filtered signal to the dry signal, the result is a vector sum, where the angle between the vectors is determined by the phase shift (assuming sine waves for simplicity). For example, a simple 4th-order filter (24 dB/8va) has 180° phase shift at the cutoff frequency, so addition at that frequency would actually be subtraction.
So, all-pass is useful when you have parallel signal paths, some filtered and some unfiltered.
Thanks I was fairly sleep deprived yesterday and couldn’t process anything. I gave it a try and I can see the slightest difference on the scope but I don't hear any difference. Though at the moment I'm just using ipad speakers so maybe that's the problem. Should the cutoff match the cutoff of the filtered sound?
Yes, the cutoff should match, so the phase curves will be similar. For a typical low-pass filter, the difference will be most significant for frequencies at or above the cutoff. And it may depend on the type of the main filter.
It finally dawned on me that it doesn't flip the phase it delays them just slightly within a specific frequency range. So in a sense an ap filter is micro delay effect. Am I getting this right?
Yes. Phase shift is similar to a delay; it shifts the start time of a sine wave. However, the amount of delay depends on frequency. When we said it affects the phase, we never said it applied a 180° phase shift (phase flip).
Comments
You can use an all-pass filter to match the phase shift of a dry signal with that of a filtered signal. This allows summing them without funny cancellation artifacts. Many filter types have phase shift that increases with frequency. When you add the filtered signal to the dry signal, the result is a vector sum, where the angle between the vectors is determined by the phase shift (assuming sine waves for simplicity). For example, a simple 4th-order filter (24 dB/8va) has 180° phase shift at the cutoff frequency, so addition at that frequency would actually be subtraction.
So, all-pass is useful when you have parallel signal paths, some filtered and some unfiltered.
I’m gonna sound dumb here but it doesn’t seem to filter at all, do I place it after a filtered signal before a mixer?
@uncleDave said it.
It's certainly helpful when you mix multiple signal paths on a track that have to be phase corrected.
And it's also a nice phase modulator: Try modulating the cutoff frequency by an oscillator.
Just to clarify, you put the all-pass in a path parallel to a similar filter, when the two signals will later be added.
All pass in a delay line can get you some mild dispersion, yeah?
Thanks I was fairly sleep deprived yesterday and couldn’t process anything. I gave it a try and I can see the slightest difference on the scope but I don't hear any difference. Though at the moment I'm just using ipad speakers so maybe that's the problem. Should the cutoff match the cutoff of the filtered sound?
Yes, the cutoff should match, so the phase curves will be similar. For a typical low-pass filter, the difference will be most significant for frequencies at or above the cutoff. And it may depend on the type of the main filter.
Another example
it sounds great but I don’t understand the patch. Why are there like 6 consecutive Ap Filters with the same settings?
It finally dawned on me that it doesn't flip the phase it delays them just slightly within a specific frequency range. So in a sense an ap filter is micro delay effect. Am I getting this right?
Yes. Phase shift is similar to a delay; it shifts the start time of a sine wave. However, the amount of delay depends on frequency. When we said it affects the phase, we never said it applied a 180° phase shift (phase flip).