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"All pass" filters and phase rotation in ReaEq

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"All pass" filters and phase rotation in ReaEq

Postby manwilde » Tue Mar 02, 2021 4:10 pm

Hi all. According to this article by Sam Inglis (https://www.soundonsound.com/techniques/making-multi-mic-recordings-work, see the "Phase rotation" box at the end of the article), which of course I don´t question, an all pass filter is meant to change the phase relationships of a soundwave at a given frequency. I reckon this is more evident when mixing a live session, with different amounts of spill on some or all the tracks.
Anyway, I´d like to know whether any of you use this tool, when and how, specifically using Reaper´s ReaEq. Does adjusting the gain of the chosen "all pass" filter has any effect?. Is it just a matter of sweeping the band across the spectrum to find what sounds best?. I´ve done my experimentations but am a bit confused, and afraid to complicate things unnecessarily...

Thanks in advance for your help, as always.
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Re: "All pass" filters and phase rotation in ReaEq

Postby Tomás Mulcahy » Tue Mar 02, 2021 8:08 pm

Wow. I didn't know it had an All Pass filter. Just tried it now, can't change the gain?
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Re: "All pass" filters and phase rotation in ReaEq

Postby manwilde » Tue Mar 02, 2021 8:30 pm

Yes, sorry, the gain is not adjustable when in "all pass" mode... :oops: :headbang:
At the momet I was speaking from memory and probably thinking about different things at the same time, sorry for that.
So, do you find it useful? Is it a better way to deal with phase issues than playing with sample delays?. I know there´s no cure for a badly set microphone, but I wonder if there is something to be gained with this tool. I think I´m gonna have a closer look, but any help would be greatly appreciated.
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Re: "All pass" filters and phase rotation in ReaEq

Postby Mike Senior » Thu Mar 04, 2021 6:28 pm

manwilde wrote:So, do you find it useful? Is it a better way to deal with phase issues than playing with sample delays?

For me: yes, and 'kind of'. I think a better way of thinking about it is that microdelays change phase relationships at different frequencies in one set of ways, while all-pass filters change phase relationships at different frequencies in another set of ways. They're both to a certain extent 'suck it and see' processes, although of course there is an element of 'time-aligning' sometimes with microdelays. In practice, though, I use Reaper's all-pass filter in the same way I'd use a phase rotator -- in other words, not often, and usually only in situations where two mics are pointing at the same stationary sound source (or with bass amp+DI recordings).

I actually wrote about it a bit in Reaper Notes a while back, come to think of it, so maybe check that out too:
https://www.soundonsound.com/techniques/master-eq-plug
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Re: "All pass" filters and phase rotation in ReaEq

Postby manwilde » Thu Mar 04, 2021 11:56 pm

Thanks for that link, the "flip all bands" trick is very intetesting! :thumbup:
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