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Discussion Starter · #22 ·
I've continued to follow Nick's advice and ignore the Helix phase slider.

However I have been playing with AP filters more, and his other statement is starting to make more sense to me "That phase adjustment is a second order all pass filter that uses degrees relative to the crossover point" (which I understand to mean the xover pint being the starting frequency of the AP filter). I understand "degrees" is supposed to refer to the degrees of phase change, but GotFrog's last statement says "whether the change says its in degrees or frequency you are simply changing the frequency of the filter".

So is this a correct understanding
"if your HP xover is 80Hz, adjusting the phase slider is moving a 2nd order all pass filter from 80Hz up to ?? (some frequency at which the effect of the filter has come around 360 degrees from where you started at 80Hz and will behave the same as if you still had the filter at 80Hz)"

Is the following a decent practical approach to using an AP filter ?

I play each mid-range, they look good and delays are properly set. I play them together and there's a big null at 800Hz.
So I go into the Helix software, and start trying various flavors of AP filters at 800Hz, 1st order (+ inverted), 2nd order (+ inverted) and I watch and see what sums the null the best without causing weirdness elsewhere. Later during listening tests I click the "bypass" button back and forth on each AP to decide if it sounds better or worse.

I didn't realize the AP affected the phase over the whole frequency range but I always wondered why some implementations of an AP would completely whack the whole response (while other implementations solve the null perfectly with no obvious downside, and almost always sound better as well).
 

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I've continued to follow Nick's advice and ignore the Helix phase slider.

However I have been playing with AP filters more, and his other statement is starting to make more sense to me "That phase adjustment is a second order all pass filter that uses degrees relative to the crossover point" (which I understand to mean the xover pint being the starting frequency of the AP filter). I understand "degrees" is supposed to refer to the degrees of phase change, but GotFrog's last statement says "whether the change says its in degrees or frequency you are simply changing the frequency of the filter".

So is this a correct understanding
"if your HP xover is 80Hz, adjusting the phase slider is moving a 2nd order all pass filter from 80Hz up to ?? (some frequency at which the effect of the filter has come around 360 degrees from where you started at 80Hz and will behave the same as if you still had the filter at 80Hz)"

Is the following a decent practical approach to using an AP filter ?

I play each mid-range, they look good and delays are properly set. I play them together and there's a big null at 800Hz.
So I go into the Helix software, and start trying various flavors of AP filters at 800Hz, 1st order (+ inverted), 2nd order (+ inverted) and I watch and see what sums the null the best without causing weirdness elsewhere. Later during listening tests I click the "bypass" button back and forth on each AP to decide if it sounds better or worse.

I didn't realize the AP affected the phase over the whole frequency range but I always wondered why some implementations of an AP would completely whack the whole response (while other implementations solve the null perfectly with no obvious downside, and almost always sound better as well).
Multi Sub Optimizer can be a learning aid for this, it doesn't have the same buggy display problems for predicted phase as REW.
 

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Phase is adjusted in degrees and delay is adjusted in mS. The number of degrees per delay adjustment varies by frequency because the number of cycles per second varies.

At 80Hz, there are 80 sine waves per second. So, one 80Hz sine wave takes 1second / 80 cycles = 12.5mS.
At 20Hz, it's 1/20 or 50mS
At 1kHz, it's 1mS

Sound travels at about 1132 ft/sec. So...

At 80Hz, the wavelength is 1132/80 or 14.5 feet.
At 20 Hz, it's 1132/20 or 56.6 feet
At 1kHz it's 1.132 feet

So, at 80Hz, one degree of phase is equal to 14.5 ft /360 degrees or 0.48"
At 20Hz, it's 56.6 / 360 or 1.86"
At 1kHz, it's 1.132 feet / 360 or 0.0377"

So, delaying the signal in mS adds more phase shift as frequency increases. A first order all pass filter rotates phase by 180 degrees below the filter frequency. At the filter frequency, the phase is 90degree.

A second order all pass filter rotates the phase a total of 360 degrees and the phase is 180 degrees at the frequency you choose. The graph looks like this one below. When you adjust the all pass filter, no matter whether the adjustment says it's in degrees or frequency, you're just shifting the frequency of the filter.
View attachment 318032

Sometimes there's also a Q adjustment and that changes the slope of the line, which you can think of as the rate of phase change or the number of adjacent frequencies over which the change occurs. The filter above has a Q of 0.707

This one is a Q of 2.

View attachment 318033
Thanks for taking the time to write this up.
 

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So is this a correct understanding
"if your HP xover is 80Hz, adjusting the phase slider is moving a 2nd order all pass filter from 80Hz up to ?? (some frequency at which the effect of the filter has come around 360 degrees from where you started at 80Hz and will behave the same as if you still had the filter at 80Hz)"
The easiest thing to do is to switch all pass on in helix (this will let you view the textbook version of what crossovers, all pass and the phase slider does to phase), then get two channels and set an 80hz low pass and name them both subwoofer channels

now highlight the p from both channels in the main view

you can now see a text book phase graph for the channels

now apply to one the phase slider to 180 degrees, it will now show an 80hz all pass applied, adjust it to 90 degrees and it will raise the freq of the all pass so it then crosses 80hz at 90 degrees so the all pass will be higher than 80hz in frequency, and if you set it for 270 degrees the all pass will be lower than 80hz so the phase crosses 80hz at 270 degrees
 

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Discussion Starter · #26 ·
That sounds illuminating I will try it.
 
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