DiyMobileAudio.com Car Stereo Forum banner

21 - 40 of 53 Posts

·
Registered
Joined
·
1,929 Posts
@jtrosky , I’ll take a stab at post #2 and post #8. Basically those small anomalies in the response you see, you probably can’t actually hear those. Many of the dips and comb filtering peaks and valleys you see on an RTA with a resolution of say more than 1/3 octave, and I would say definitely not more than 1/6 octave; you just cant hear those so don’t waste the DSP resolution and time to try and correct those.

This has been my experience anyway so often I tune using 1/3 octave and sometimes I flip back and forth from 1/6 to 1/3 for the lower octave FR response just to gain some additional insight at the larger dips and peaks. I think this is a trap that many people fall into when tuning and they fuss too much over that small stuff when it may not actually matter.
See, this is where I disagree. We go to great lengths to squeeze every last bit of performance out of our car audio systems. We spend insane amounts of money on speakers, amplifiers and DSPs. We spend insane amounts of money, time and effort to "sound deaden" our cars. We spend insane amounts of time tuning them to perfection.

So why shouldn't we spend 15-30 minutes to apply a few allpass filters to correct 5dB+ dips in the L+R combined response? It doesn't cost us anything. Those bands on the EQ would probably go unused otherwise (at least on a Helix since it has 30-bands per channel). I mean we're talking about (3) 5dB+ dips that each "span" ~50hz - and all three are in the 200hz - 600hz (approx) range. To me, those dips can certainly be audible - especially when all three are combined. As long as the allpass filters don't cause any "unwanted" side-effects, to me it's a no-brainer. Why purposely leave those dips there if you can easily fix them without any unwanted side-effects?

RTA's are specifically designed to make it easier to identify and correct things that we can't easily identify with our ears. I feel that it's useful to correct all of these small "anomalies", that when combined, actually do make an audible difference. If we just say "well, you probably can't hear that anyway" for everything we see on an RTA, but can't easily identify with our ears, then we might as well just not use the RTA at all, right? :) Again, I think it's the combined effect of correcting all of these minor issues that makes a difference.

That doesn't mean that I'm actually right though. :)
 

·
Registered
Joined
·
1,929 Posts
Great thanks - so you applied a all pass filter at your crossover points?

So for instance you crossed your mids at say 250hz do you apply the filter at 250hz?


Sent from my iPhone using Tapatalk
The original example really has nothing to do with crossover areas - it was just showing some phase-related dips that are caused when both my left and right midbass are played together (the green line). Those dips don't exist at all in the individual left and right measurements (blue and red lines) - only in the L+R combined response.

However, fixing these dips can actually impact the crossover area. Here is another example (I think these were completely different speakers) that includes both my midbass and my wideband measurements on the same graph - look how the midbass dips (before allpass filters) really screw up the crossover area:

Before allpass filters:


After allpass filters:


See how the dips in the midbass really change the crossover area? With the allpass filters in place, it really cleans up the crossover area as a result (at least based on L+R combined measurements only). If I was using a 400hz crossover, it would actually change the crossover point as well. I actually avoid using 400hz as a crossover point now because I always have a phase-related dip at 400hz, so I just avoid that area for crossovers now. However, you also need to take a measurement with left midbass + left wideband together, which may identify other issues with the actual crossover response itself.

When I take a "full" set of measurements, I'll take individual speaker, L+R speaker "pairs" (left midbass + right midbass, for example), left side (left midbass + left wideband), right side and "all together" measurements. This group of measurements really allows you to see any significant phase issues.


Good point - I would also like to know...

So you don’t apply the filter on the combined graph? Only one side?


Sent from my iPhone using Tapatalk
Personally, I usually start with the left side and try the allpass filter there first. If it fixes the issue, I leave it be. If not, I'll try it on the right side. You definitely only want it on one of the speakers (left or right - not both). In most cases, I find that the allpass filter works equally well on left or right side though. Can't explain why that is the case based on the explanation provided by @dumdum....
 

·
Registered
Joined
·
57 Posts
Ok, but they why wouldn't the phase issue that you created a couple of hundred Hz further up NOT show up as a phase issue on the RTA, if it really was a significant phase issue? I mean if two drivers are that far out of phase at that location, it should show as a dip on the combined L+R response, right?

To me, significant phase issues in the L+R combined response - those that actually dip lower than the individual measurements themselves - are obvious phase issues that can usually (in my experience) be corrected relatively easily with an allpass filter - without any ill side effects. I've always used high-q, 2nd-order allpass filters that only impact the phase over a very narrow range. I'm sure that there are situation where it's more complicated than that, but in my experience, the phase-induced dips have been very easy to resolve with any ill side effects. Maybe I've just been lucky - who knows. I can only go by my experiences.

Now of course you have to listen afterwards. I'm not saying to blindly throw allpass filters everywhere and leave them in place even if they do something horrible to the rest of the system - just to solve a narrow dip. :) But if I can correct a 5dB dip easily with an allpass filter, I don't see why I wouldn't.

Please don't take this as an "argument" - it's an interesting discussion and I think it's worth having. One of these days, I'll invest in the gear to properly analyze phase, but between a good XLR MIC, a decent multi-channel USB sound card and the software to use it properly, it's an expense. Although, now that we have OpenSound Meter, that makes it a lot more feasible (vs needing Smaart).
As to why it might not be obvious further up, it may be that the phase rotates over a wider frequency range, so there would not be a single narrow dip. It is also harder to see phase dips higher in frequency, as summing reduces at higher frequencies due to reflections, combing, etc.

Bear in mind that we are talking about spacial issues fundamentally. And the psychoacoustics of how stereo fools the brain is not that simple. If you have a much wider dip, how are you going to work out where the All Pass needs to go? I'm sure you could put a 1st order All Pass at, say 700Hz and see no substantial RTA change. But it sure will sound different.

Which is the other point - narrow Q 2nd Order does not mean it affects a small frequency range. An all-pass affects basically the whole frequency range, by rotating the phase 180deg (2nd order) about the frequency you specify. High Q just makes the transition happen faster.
 

·
Registered
Joined
·
57 Posts
As a general point to this thread, nobody is saying don't use All Pass. Nobody is saying those dips are not candidates to fix and improve.

What is being said is a) don't worry about the audibility of the dip as such b) measure phase directly.

As has been said - we go to great lengths to extract performance from our system. It is not a huge leap to get the capability to measure phase directly, if you want to tackle that issue as well. It has also been said, we use RTA to measure things we can't hear easily. If you want to fix phase, use phase measurement, so you can correct it more accurately than you can by ear.
 

·
Registered
Joined
·
1,929 Posts
Understood - and I agree that actually measuring phase would be the best way - but at the same time, if you have a few dips like this and don't really have the ability (or desire, for that matter) to properly measure phase directly, in real-time, I feel that it's worth using a few allpass filters to address issues that are pretty obvious just using frequency response measurements. In my case, I don't yet have the ability to properly measure phase in real time. Until I get to that point, I think doing it this way is still beneficial - and may even be "good enough" for most people.

I actually learned about this from one of Skizers videos on how to verify phase through a crossover while remote tuning. He was responding to criticism that you can't possibly identify and address phase issues via remote tuning with a simple USB mic and frequency response measurements. His point was simply that you can actually identify and correct phase issues - even when tuning remotely, without actually measuring phase directly - based solely on the measured frequency response with multiple speakers playing. In the video (which has since been marked private), he used an allpass filter to address a big dip in the crossover region - that was identified with simple frequency response measurements. He simply applied a 2nd-order allpass filter and the dip was corrected. That was his "evidence" that you can use frequency response measurements to identify and fix some phase-related issues, quickly and easily. Is it the best way? No. Is it the most accurate way? No. However, it is still useful and good enough in a lot of cases.

It works for me until I have a better way to do it. :)

Hell, I used simple frequency response measurements to get both of my subs phase-aligned as well. I didn't use allpass filters, but just by altering the time delays, I could get huge differences in summation - and frequency response measurements allowed me to get the best possible summation. I initially tried doing it via phase measurements via sweeps - and that was U*G*L*Y. I feel that I got much better results, quicker - just by using frequency response measurements instead.
 

·
Registered
2017 Toyota Tacoma 4x4 double cab sport
Joined
·
1,351 Posts
Great post here. Learning a lot that I’ve not been able to figure out about tuning
 

·
Registered
Joined
·
185 Posts
How do you actually apply an all pass filter on a Helix Ultra? As in what steps in the software on which screens/settings/buttons.
 

·
Registered
Joined
·
1,929 Posts
Once you "Enable" allpass filters in the DCM->PC Tool Configuration screen, then there will be an "Allpass EQ" option in the filter type selection box for each EQ slider. So your choices will be "Fine EQ", "Parametric EQ" and "Allpass EQ" (plus Low or High Shelf EQ if you select the first or last slider).

Once you select "Allpass EQ", you can then specify the filter type (1st order or 2nd order) and then specify the Frequency and Q (if 2nd order) for the allpass filter.

To correct the type of dips in my earlier examples, I always use 2nd order - and start with using a frequency that is right around the middle of the dip, with a Q that you would normally use if your were EQ'ing the dip with a parametric filter. Then take a new measurement and see where you stand. You may have to play with the frequency and Q a little to get the desired results. Again, this method is not as accurate as actually measuring phase, but using this method, I usually get the dips corrected in the first few tries.

Just make sure to only apply the allpass filter on one channel. If you apply the same allpass filter to both the left and right channels in my midbass example, they completely negate each other and nothing changes...

Hope that helps.
 

·
Registered
2017 Toyota Tacoma 4x4 double cab sport
Joined
·
1,351 Posts
Ok so I have one dip in my tune at around -4db that I cannot get level by boosting. The dip is in the mid or in tweeters crossover slope which I’ve tried equing both. Seems it moves a little more when I play with the tweeters slope in eq. The center of this dip would be at 2.16khz roughly. Which all pass filter order would you recommend and which driver should I apply it to? I have one more dip about the same amount more in the mid range and the center is around 1.38khz and I can’t get it to go away with eqing the crossover slope of tweeter or mid range eq. Any guidance on which all pass to use would be greatly appreciated. @jtrosky Thanks
 

·
Registered
Joined
·
1,929 Posts
Ok so I have one dip in my tune at around -4db that I cannot get level by boosting. The dip is in the mid or in tweeters crossover slope which I’ve tried equing both. Seems it moves a little more when I play with the tweeters slope in eq. The center of this dip would be at 2.16khz roughly. Which all pass filter order would you recommend and which driver should I apply it to? I have one more dip about the same amount more in the mid range and the center is around 1.38khz and I can’t get it to go away with eqing the crossover slope of tweeter or mid range eq. Any guidance on which all pass to use would be greatly appreciated. @jtrosky Thanks
Can you post graphs showing what you are referring to? Keep in mind that allpass filters are not for correcting dips when a single speaker is playing - only when multiple speakers are playing the same frequencies. I think some graphs would clear up what you are referring to.
 

·
Registered
Joined
·
3,042 Posts
Did you apply the APF on the left Mid-base or Right Mid-base to make the above corrections? Dumdum mentioned that there is a 'bad' side and a 'good' side, so how does one determine which side to apply the APF?
If you put the all-pass on one side and it makes things worse, that was the good side. Trial and error.
Although, if you can measure and view phase, it is quite apparent which is bad and which is good. In REW a good phase response (viewed unwrapped) will slope down gently from left to right, a bad phase response will deviate pretty significantly from that. All-pass filters will steepen the phase slope, so if the bad response slopes upward or less downward (than the good response), it gets the all-pass, if the bad response slopes more downward then the good response gets the all-pass to match it.
 
  • Like
Reactions: tjk_bail

·
Registered
Joined
·
19,216 Posts
So why shouldn't we spend 15-30 minutes to apply a few allpass filters to correct 5dB+ dips in the L+R combined response?
Because what you see on the RTA isn't exactly what you are hearing.
 
  • Like
Reactions: dumdum

·
Registered
2017 Toyota Tacoma 4x4 double cab sport
Joined
·
1,351 Posts
Well the one at 2200 is now gone and I have one around 650 and it’s more like a 2.5dB null
549F52D0-BEB0-44D1-953F-174A084BFCE4.jpeg
 

·
Registered
Joined
·
19,216 Posts
Well the one at 2200 is now gone and I have one around 650 and it’s more like a 2.5dB null View attachment 302922
tbh, I wouldn't be using bar-style rta with that low resolution to figure out phase issues. way too low hard to get a proper reading of whats going on.
 

·
Registered
2017 Toyota Tacoma 4x4 double cab sport
Joined
·
1,351 Posts
How would you recommend me do it? Any tips and help would be greatly appreciated
 

·
Registered
Joined
·
1,929 Posts
How would you recommend me do it? Any tips and help would be greatly appreciated
You want to go into the RTA screen settings (the gear icon) and set the "Mode" to "RTA 1/48 octave" (you can always adjust the response "smoothing" level later while analyzing the responses, but this will capture much more detail in case it's needed).

You also want to disable the "bars" setting. If you are using a newer version of REW, click on the "Appearance" button in that settings window and uncheck the "Use bars on RTA" box.

IMO, you really don't want to use the "bars" anywhere, whether it be the RTA window, the EQ window or any other REW window. :)
 

·
Registered
2017 Toyota Tacoma 4x4 double cab sport
Joined
·
1,351 Posts
E321A06D-5770-432B-80FC-D46CCD347D5C.jpeg
E321A06D-5770-432B-80FC-D46CCD347D5C.jpeg
E321A06D-5770-432B-80FC-D46CCD347D5C.jpeg
Here is the screen measurement with bars off and 1/48 res
 

·
Registered
2017 Toyota Tacoma 4x4 double cab sport
Joined
·
1,351 Posts
How is this? Also here is how my eq is set up right side is first 3 left side is 2nd 3 pics. Sub isn’t pictured
Right tweeter
7968DCAC-6167-4483-8F39-A4BDA4BAA85A.jpeg
right mid range
78D6801D-BDC0-4F3D-ABD5-01EE6794F1D6.jpeg
right mid bass
D643DB76-CA5F-47C5-9798-F8D76E70B6B0.jpeg
 

·
Registered
Joined
·
1,929 Posts
Actually, images that are smoothed down to 1/12th or even 1/6th should be good for this task. Just open the "Overlays" window and in there, you can select which measurements to display and the smoothing level of them. You basically just capture the data in 1/48th mode - but then you smooth it down to various different levels as needed, depending on what you are looking for. But at least you have the highest level of detail if you need it. Taking measurements at 1/6th, for example, may not always have enough detail, depending on what you are looking to analyze.

The Overlays window in REW is where it's at, IMO - it lets you see multiple measurements at once and the "Predicted" tab will show what the measurements will look like after your proposed EQ is applied.

You may know this already, but just in case you don't. Also, REW has a little camera icon in the upper-left-hand corner to take screenshots of the measurements - much nicer than phone pictures of the screen.
 
21 - 40 of 53 Posts
Top