I just used the o-scope function in REW to diagnose why the hell I could never get my subs and mids to blend properly. I have swapped the RCA's in this whole process and the output from the head unit (went from network mode to standard mode to eliminate HU xovers, two full range outputs now). I found that the subs are somehow delayed by 24 milliseconds. So, since I'm using a MiniDSP C-DSP 8x12, I only have the capability to add 25 milliseconds maximum delay, not enough to compensate for the additional delay plus time alignment. I looped one output back to an input (for the front stage) and added the additional delay to that loop, problem solved. Unexpectedly, there are no ill effects from the loop; and the subs and mids blend perfectly.
Experts please comment on this.>> I'm talking specifically to Justin Zazzi, ErinH, and any other electrical/acoustic engineering types.>> Before I move on, all xover slopes mentioned are acoustic slopes NOT electronic. I can only guess that this additional delay is from the LR24 slopes on either side of the 90hz xover point, one full wavelength delay from each slope adding up to 22.22ms delay, that seems about right. Should we calculate additional delay like this? >"Xover point wavelength in ms" X "number of cycles (360 deg) phase change in slopes (LP+HP)" = "additional delay to add to the high pass side". In my case, 90hz=11.11ms LR24=360deg xover -- (11.11ms)X(1+1)=22.22ms. This math may be different (it likely is different) for Butterworth xovers, but it works for Linkwitz-Riley at least.<< I feel like I rediscovered some math we've all been ignoring for the sake of simplicity. I'm sure this is covered somewhere, point me to it if you can. Or maybe I'm just measuring a single point of group delay, in which case this is fairly pointless.
Has anyone else used an oscillosope for time alignment?
The process is fairly simple:
1) Fire up your laptop
2) Plug in your trusty measurement mic
3) Fire up REW and your DSP software (I'm using REW V5.20 beta 48)
4) Click on the "Generator" button at the top of REW
5) On the "Tones" tab of the "Generator" select "Tone Burst" and set it to whatever frequency your sub-mid xover is
6) Make sure the "Cycles" button on the right is clicked, select 0.5 in that selection box (The half-wave is perfect to show phase and short enough to not clutter the o-scope with unnecessary info.)
7) Make sure "Repeat the burst" is selected
8) Set your level to whatever you normally use
9) Set output to L+R
10) Check your system volume
11) Click the play button
12) Select a sub and mid to align, mute all other speakers
13) Click the "Scope" button at the top of REW
14) Adjust the mV/div and ms/div to get the waves displayed properly (large enough and wide enough, also to show ALL of the waves from the separate speakers) You may also want to mute each speaker to get an idea of what each wave looks like alone, to better approximate the final product.
15) Now adjust T/A and/or phase to get those waves to merge into one coherent, clean wave. ( Compare the first or largest peaks, the horizontal time divisions will give you a very close approximation of how far off your T/A is, if at all) Take notes! Write down your initial delays (just in case this doesn't work for you). Subtract your initial delay from the final delay, now you have the additional delay. The difference in T/A (additional delay) may need to carry over to your tweets also to keep them aligned with your mids.
16) Repeat steps 12 and 15 for the other mid
This could also be used for mid to tweet alignment, but with an appropriate change to the frequency and an adjustment to the "Scope" settings.
I only used this method to determine the additional sub delay in my system, then added that delay (24ms in my case) to the front stage overall delay (in the loop mentioned above), the original tape measure determined delay now works as intended.
Or, you could just do the math.
I thought this might be helpful information for anyone else having T/A or phase problems that they can't diagnose.
Also, sorry about the formatting, don't laugh, I spent several hours on this, it's hard to be coherent when I keep coming back to edit.