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Minimum phase... How does it affect the sound of an IR?

DLC86

Power User
Hi all, these days I've experimented a bit with cab-lab, min phase and some non-MPT IRs I have. When comparing an MPT IR with the unprocessed version of it I've had mixed results.
For some of them the difference is minimal, almost non-hearable, for some it is quite evident and for some others it's pretty big (especially room IRs). In all cases anyway I thought the unprocessed IR sounded more "authentic", but it could very well be biasing on my part.

I've read a bit about minimum phase transform and Hilbert but I don't have enough math knowledge to understand how this translates in practical terms.
I mean, what kind of information is lost when applying the MPT to a cab IR?
Could someone who knows explain it in simple terms please?

Thanks in advance
 

FractalAudio

Administrator
Fractal Audio Systems
Moderator
If it's a cab IR the difference is basically nil because a speaker is a minimum-phase device. All minimum-phase does in this case is automatically remove the leading silence.

A room IR is not minimum phase so you should not use MPT when processing a room IR.
 

DLC86

Power User
Thanks Cliff!
That seems coherent with my empirical tests.

Anyway a few days ago I searched a bit on this subject and found some posts on TGP by Jay Mitchell saying that guitar cabs are not minimum phase systems.

From what I understand after reading some papers, a cab with a single speaker is indeed a MP system, while a cab with multiple speakers is not cuz there will be different time of flights for the various speakers reproducing the impulse, depending on the mic position and this has some effect even when close-mic'ing one of them.
It seems that also the electronics inside the mic-pre used to capture the IR could be non-MP.

Do you think the difference is negligible in these cases too?
 

yek

Moderator
Moderator
If it's a cab IR the difference is basically nil because a speaker is a minimum-phase device. All minimum-phase does in this case is automatically remove the leading silence.
Are all stock IRs in the III Min Phase?
 

ML SOUND LAB

Cab Pack Wizard
Vendor
But! You should note that when mixing two IR's within the unit or in Cab-Lab you will get audibly different results depending on if you're using MPT or not.
 

DLC86

Power User
But! You should note that when mixing two IR's within the unit or in Cab-Lab you will get audibly different results depending on if you're using MPT or not.
It depends on if they're time-aligned or not. Usually if they're from the same producer they are and there's virtually no difference (except the ones we discussed above inside the single IR)
 

ML SOUND LAB

Cab Pack Wizard
Vendor
Actually no. Even if you time align two non-MPT'd IR's and compare that to a mix of the same IR's MPT'd you will get different results. This is one of the main differences in the type of IR mixes my Cab Packs have included. You will hear a BIG difference especially in the high end.

Sometimes the MPT mix actually sounds better but when you're placing mics without MPT that's where I feel the magic happens. Real life mic phase and all that. :)
 
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