Apologies in advance for my ignorance on this topic, but I’m honestly trying to wrap my head around this. I watched
@amirm ’s video response to Paul and by the end I had come to believe that phase distortion doesn’t matter. Then I read 30+ pages about rephase, linear phase filters, minimum phase filters, etc and it seems that there are a lot of folks going to a lot of effort to correct phase without over correcting phase etc. If it’s inaudible why correct it? If it’s inaudible, why worry about over correcting it? Why are we using FIR filters at all? If it’s inaudible, why aren’t FIR filters considered a solution in search of a problem at the expense of additional processing power and potentially needing to manage significant latency? I realize I’m missing something. Can someone possibly shed some light on what I’m missing? Please and thank you.
Some types of phase are audible.
If the phase rotation is the same between left and right = not audible.
On the other hand, if there are phase differences between left and right, this is audible, depending on how much phase rotation there is, and at what frequency. If it is 180deg out of phase, it will cause cancellation. If it is misaligned, it will cause a difference in the ITD (interaural time difference), because phase = time. The law of the first wavefront says that the image will be pulled towards the side of the earlier arrival - "phantom centre image drift". This is where the phantom image seems to be pulled left or right depending on frequency.
I think a lot of confusion arises from the fact that the terms sound so similar yet have different meanings depending on context. For e.g:
- Minimum phase (in context of loudspeakers and rooms) - this is the only version of the measurement which is correctible by inversion, which corrects both magnitude and phase at the same time. Loudspeakers are generally considered min phase systems, although there may be regions within a loudspeaker which are non-min phase.
- Minimum phase (in context of DSP) - this is the minimum amount of delay encountered by a signal when it passes through a filter.
- Linear phase (in context of DSP) - every frequency encounters the same delay when passing through a filter. Suppose this is 1ms (1/1000s). This is 10 full sine waves for a 10kHz signal, 1 sine wave for a 1kHz signal, and 1/10 of a sine wave for a 100Hz signal - i.e. there is frequency dependent phase rotation.
- Excess phase (in context of loudspeakers and rooms) - non-minimum phase behaviour introduced by the room, e.g. furnishings, room shape, boundaries, etc. The measured phase as captured by the microphone contains the loudspeaker's min phase response + excess phase. The EP is normally removed and discarded, as it can not be corrected (strictly speaking, not true - it can be partially corrected).
- Mixed phase (in context of DSP) - a DSP product that contains both IIR and FIR filters, i.e. both lin phase and min phase filters.
- Mixed phase (in context of rooms) - another term for measured phase, i.e. loudspeaker's minimum phase response + excess phase.
As alluded to earlier, there is also absolute phase and relative phase. Absolute phase = where phase integrity is maintained between left and right. For e.g. if you flip the polarity of both speakers, you won't hear it. Relative phase = where the phase of left and right is misaligned, for e.g. if you flip the polarity of one speaker you will definitely hear it. Sound seems to be coming from inside your head. If you do a sweep of both speakers playing together, you will see comb filtering. Having said that, phase and polarity are not the same thing, but this post is getting too long already.
It took me months to understand all this confusing terminology. It is misleading to say that
all phase rotations are inaudible. You have to refer to exactly what type you are talking about.