The 2nd order all pass will eventually go to -360deg from 0 degrees. If you add a all pass at a point where the phase is moved by x in the highpass, whoever x is the allpass center is 90deg on both sides of the all pass. What happens when you put an all pass at the “knee” of a high-pass or a lowpass? Well they do the inverse of each other except on 12db LR or even order LR crossovers, than they behave the same. It has a built in calculator to take an “all pass filter” and set it down the highpass slope using the phase shift of the high-pass as a insertion point. The “phase” ona helix moves the entire magnitude of the speaker channelīy however many degrees it says it’s moving it. Here’s the real storey that you won’t find on audio frog tuning page.
I'll take a look at the videos I did get today, because it sounds like some of it might actually still be of use to some folks. Unfortunately, I found myself explaining all of this stuff using Smaart and SysTune, then when I would try to show how you'd do it in REW, I'd get so lost in the logistics of trying to pull it off with REW that I'd loose my train of thought. match the slope of the traces, use polarity to get them to overlay, if necessary.Ī lot of this basic concept I was able to capture on video before the lighting went to ****. The steeper the slope, the greater the delay. Then, of course, you use the overlay window to compare each set of measurements and decide which one needs more or less delay/phase shift.Īn all pass filter IS NOTHING MORE than frequency dependent delay.įinally, when comparing two traces to see which one needs adjusting, the slope of the phase trace shows you relative arrival time. It is imperative that you use the same reference delay number for each measurement that you're attempting to align. The logistical problem with performing phase alignments in REW is that after EACH AND EVERY sweep, you have to go in and manually set your reference delay (by clicking in the little gear in the upper corner of the IR window and inputting a previously arrived at time in ms) It does a Hilbert Transform of the magnitude to show the minimum phase response that such a system could, in theory, have). So, contrary to popular belief, REW DOES measure the actual phase, it CAN calculate the phase based on the magnitude (this is what happens when you click "show minimum phase. If you're adept at using measurement software, then the logistics of pulling off these basic concepts can be figured out. The basic concept can be explained with words. Ha ha little audiophile pun there for you. I could just label my woofer channels something else that would enable the phase shift on them.Īnyway if anyone knows any good discussions on this or wants to add some advice I'm all ears. To further complicate things, the Helix software does not allow phase adjustments on a channel labelled "low" ie they expect you to adjust the phase with the woofers being the zero point I guess. So does that mean delay the phase on the lower frequency driver by the number it calculates ? Is the phase control on the DSP "delaying" the phase ? (as opposed to "pushing" the phase forward in time. I think I'm supposed to enter the high pass xover value of the lower frequency driver and the time delay currently applied to the lower frequency driver (or would it be the delta time delay between the lower and higher frequency dirver ?) and then it spits out the phase of that signal. I've used the calculator on this page but I"m not sure how to utilize the numbers it gives. I've read that phase is especially important for the sub, so do I enter the proper time delay for the sub and then adjust the phase on the sub ? For example, am I trying to use phase to bring the LW and LM into coherence (primarily at the xover point) or am I using phase to focus the image between left and right ?
I'm mostly unclear if I'm supposed to use the phase adjustment to adjust the image or to improve coherence between the different drivers. I know phase and time delay are intrinsically linked which is what confuses me. Can anyone point to a good thread (or add info here) about the best way to use the phase adjustment in my Helix DSP ? (its the older unit but I have the latest tuning software version 4.x or something). I've found a few threads but they left me confused.
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