Simplified REW Setup and Use (USB Mic & HDMI Connection) Including Measurement Techniques and How To Interpret Graphs - Page 454 - AVS | Home Theater Discussions And Reviews
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post #13591 of 14645 Old 02-25-2015, 06:15 PM
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Quote:
Originally Posted by jim19611961 View Post
I would be more concerned about what I am seeing at 200-300hz. +30-35ms at those frequencies looks like a problem to me. Why don't you post the whole FR range (and change the scaling)
I don't have a recent full FR. I'll try and do one in the morning.

Is this measurement just a single speaker, single speaker and subs or something else?
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post #13592 of 14645 Old 02-25-2015, 06:26 PM
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Quote:
Originally Posted by artur9 View Post
I don't have a recent full FR. I'll try and do one in the morning.

Is this measurement just a single speaker, single speaker and subs or something else?
Since I was focused on the bass region, the measurement I posted is all mains and all subs.

I also think 1/48th smoothing would be better for full range analysis.
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post #13593 of 14645 Old 02-25-2015, 09:16 PM
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Quote:
Originally Posted by Soulburner View Post
What does it mean if my speakers start to crackle after a couple of measurements? Crackles through the whole sweep. It did the same thing during SPL calibration. If I wait a couple minutes it's clean again. Don't tell me it's the AVR...
I had a similar sound while running one of the beta versions on my MacBook Pro. Updating to the most recent version made it go away.
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post #13594 of 14645 Old 02-25-2015, 09:20 PM
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Quote:
Originally Posted by Soulburner View Post
What does it mean if my speakers start to crackle after a couple of measurements? Crackles through the whole sweep. It did the same thing during SPL calibration. If I wait a couple minutes it's clean again. Don't tell me it's the AVR...
Just to clarify, is this the sound you are hearing?

REW Sweep Distorted Sound
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post #13595 of 14645 Old 02-25-2015, 09:35 PM
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Quote:
Originally Posted by AustinJerry View Post
Just to clarify, is this the sound you are hearing?

REW Sweep Distorted Sound
Similar, but a bit more crackly and it definitely screwed up the graph. I'll play with it again tomorrow and see if I can recreate it. I now have the X4000 hooked up so I'll know if it's the AVR.
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post #13596 of 14645 Old 02-25-2015, 09:50 PM
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Originally Posted by Soulburner View Post
Similar, but a bit more crackly and it definitely screwed up the graph. I'll play with it again tomorrow and see if I can recreate it. I now have the X4000 hooked up so I'll know if it's the AVR.
If it is the same sound, it is not the AVR, which should put you at ease. It is a bug in the ASIO driver. I have reported it on the Home Theater Shack REW forum, but have never received an acknowledgement or response from JohnM.

If the distorted tone occurs, you can clear it by opening the Preferences page and toggling the Output and Timing Reference fields with different selections several times. It is a very annoying problem, and it affects many of us.
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post #13597 of 14645 Old 02-25-2015, 11:07 PM
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I showed a graph on the last page of a peak around 10hz that is constant with my graphs. In my room of 12X18X8 that opens through a small hallway to a kitchen on the front left of the room, and a small walkway to bedrooms in the right corner of the room. I dont see this room causing a peak like this and it doesnt matter where I put the mic in the room. Its always there. I believe I may have a UMIK issue.

Well I turned off my subs and ran some sweeps with just my main L/R speakers. They were run configured Large. They are Salk Songtowers with a frequency response of 42hz-20000hz. As you can see in the graphs is that even with the main speakers I still get a peak down in the 10hz area. I dont feel these speakers should have anything going on down there room dependent or not. You guys are more knowledgeable with that than me so you guys decide.


The first graph is of me testing left and right speakers separately. The second graph is of both speakers running at the same time and a average of the left and right speakers.
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post #13598 of 14645 Old 02-26-2015, 12:29 AM
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Originally Posted by jim19611961 View Post
Depends how excessive the low frequency GD/Phase delay is. As previously stated, going from 60+ms GD to 30ms (40hz) was quite noticeable. But I agree that problems like this at higher frequencies definitely need attention.
did you change that without impacting frequency response?

Quote:
Originally Posted by jim19611961 View Post
BTW, whats that at 110hz on your graph?
2nd width mode (so varies by seating position significantly). You can see the effect more clearly on the phase graph, remember that GD is the slope of the phase response so you can see that the mode around 40Hz was causing a substantial phase rotation which is removed after correction & also that an attempt has been made to tackle the phase rotation around the 2nd width mode as it now just oscillates slightly rather than wrapping.



However note that SPL was not affected, the sharp null is still there.



I think this is a good example of where the GD chart alone is misleading and that you need to consider what is actually happening in the phase & frequency response to get a fuller picture. I think you also need to consider similarity of response across channels. There's a video of the trinnov people around (part 1 is
and there are parts 2 and 3 on youtube as well) where they talk about consistency of phase response across channels along with smooth changes as opposed to an absolute phase target. Intuitively this makes sense given that your ears are using interaural phase differences to image hence discontinuities between ears are presumably undesirable.

I'll see if I have a graph for that later.
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post #13599 of 14645 Old 02-26-2015, 03:58 AM
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Quote:
Originally Posted by artur9 View Post
Is this just the GD tab in REW? If so, here's mine. I have a room mode at 31Hz but I do not understand why the group delay would be negative.
AIUI group delay is not an actual absolute time delay unless the delay is constant, it is instead a measure of the relative delay of different frequencies within the envelope. I think it also means it can only be calculated for a steady state signal and I'm not sure whether it directly applies to transient content.

Technically it's the negative derivative of the phase response hence it is a measure of the rate of change in the phase response & hence why looking at the phase response can be informative to see what underlies that GD graph. This means a negative group delay is possible when the phase response does weird things and that is exactly what happens when your measurement captures the direct sound and reflected content. It also means the group delay view of an in room measurement will be seriously affected by how you window that response hence then the Q is which view is the "right" view (as usual but worth pointing out).

There's an interesting thread on the subject here -> http://www.audiocircle.com/index.php?topic=24932.0 (though pictures no longer there)

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post #13600 of 14645 Old 02-26-2015, 04:43 AM
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Quote:
Originally Posted by jim19611961 View Post
I would be more concerned about what I am seeing at 200-300hz. +30-35ms at those frequencies looks like a problem to me. Why don't you post the whole FR range (and change the scaling) like this:
Don't forget that this data probably contains all room interaction so the GD plot is misleading. Our hearing gets better at separating direct from reflected sound as frequency is rising. One would need to use a sliding window on the IR in order to get more meaningful GD readings.
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post #13601 of 14645 Old 02-26-2015, 05:23 AM
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Originally Posted by jim19611961 View Post
Since I was focused on the bass region, the measurement I posted is all mains and all subs.

I also think 1/48th smoothing would be better for full range analysis.
Here it is. Looks terrible.

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post #13602 of 14645 Old 02-26-2015, 06:31 AM
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Originally Posted by 3ll3d00d View Post
simplest way to do that in REW is generally to import an appropriate FRD file

mine is attached, blue is pre EQ, purple is post.

Attachment 568177
Here is a GD graph from my system, with Dirac Live RC. What does it tell us?

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post #13603 of 14645 Old 02-26-2015, 07:04 AM
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Quote:
Originally Posted by AustinJerry View Post
Here is a GD graph from my system, with Dirac Live RC. What does it tell us?

Jerry,

OK I'll take a shot (with the qualifier - that I need to learn about the GD feature).
However, my guess:

There are issues at 35, 40, and 78 Hz.....?

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post #13604 of 14645 Old 02-26-2015, 07:22 AM
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Originally Posted by cuzed2 View Post
Jerry,

OK I'll take a shot (with the qualifier - that I need to learn about the GD feature).
However, my guess:

There are issues at 35, 40, and 78 Hz.....?
Jerry,

Disregard my comments above.
With just a bit more reading >> What your graph tells me is I have a LOT to learn about GD

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post #13605 of 14645 Old 02-26-2015, 07:23 AM
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Originally Posted by cuzed2 View Post
Jerry,

OK I'll take a shot (with the qualifier - that I need to learn about the GD feature).
However, my guess:

There are issues at 35, 40, and 78 Hz.....?
Other than you picked three spots where the graph shows dips, "issues" doesn't say much.

Edit: Saw your update, so never mind.
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post #13606 of 14645 Old 02-26-2015, 07:31 AM
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Originally Posted by AustinJerry View Post
Here is a GD graph from my system, with Dirac Live RC. What does it tell us?
I'm curious, if you compare with no Dirac, if it'll show that Dirac is adding 10ms of GD.
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post #13607 of 14645 Old 02-26-2015, 07:33 AM
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Originally Posted by artur9 View Post
Here it is. Looks terrible.
I wonder if it'll look better if I only do L or R or C?

With all subs and L+R maybe the messiness is from that and all the under-treated reflections.
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post #13608 of 14645 Old 02-26-2015, 07:36 AM
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Quote:
Originally Posted by 3ll3d00d View Post
did you change that without impacting frequency response?

I think this is a good example of where the GD chart alone is misleading and that you need to consider what is actually happening in the phase & frequency response to get a fuller picture.
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post #13609 of 14645 Old 02-26-2015, 07:44 AM
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Originally Posted by artur9 View Post
I wonder if it'll look better if I only do L or R or C?

With all subs and L+R maybe the messiness is from that and all the under-treated reflections.
Like when doing FR, running all mains and subs will look skewed unless the mic is EXACTLY the same distance from the higher frequency sources. If not, I would definitely look at L+subs, R+subs separately.

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post #13610 of 14645 Old 02-26-2015, 07:47 AM
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Originally Posted by jim19611961 View Post
Like when doing FR, running all mains and subs will look skewed unless the mic is EXACTLY the same distance from the higher frequency sources.
If I read it correctly the phase blog I mentioned yesterday says there's 0.1mm or such of wriggle room :-)
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post #13611 of 14645 Old 02-26-2015, 07:48 AM
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Originally Posted by artur9 View Post
I'm curious, if you compare with no Dirac, if it'll show that Dirac is adding 10ms of GD.
AFAIK the Dirac filter is not long enough to be able to perform non minimum phase correction at those frequencies hence any impact on GD will be via the effect of taking out minimum phase room issues. Why do you think it will add a specific amount of GD? (given that that would imply a step change in the phase response at some specific frequency)
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post #13612 of 14645 Old 02-26-2015, 07:53 AM
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Originally Posted by jim19611961 View Post
the thing that stands out from the FR is that it rolls off a little earlier and is that smoother through the lower (for music) bass. I would think that that would be the audible aspect over and above the reduced pace of phase rotation through the 40-60Hz range. What did you actually do to implement that change?
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post #13613 of 14645 Old 02-26-2015, 07:55 AM
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Quote:
Originally Posted by AustinJerry View Post
Here is a GD graph from my system, with Dirac Live RC. What does it tell us?

I would argue not much more than you probably have a sealed sub and that you have a fairly normal, but recently controlled, room.
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post #13614 of 14645 Old 02-26-2015, 07:56 AM
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Quote:
Originally Posted by 3ll3d00d View Post
the thing that stands out from the FR is that it rolls off a little earlier and is that smoother through the lower (for music) bass. I would think that that would be the audible aspect over and above the reduced pace of phase rotation through the 40-60Hz range. What did you actually do to implement that change?
My rear (main) sub had been phase reversed, and now it is not.

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post #13615 of 14645 Old 02-26-2015, 07:59 AM
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Originally Posted by cuzed2 View Post
Jerry,

OK I'll take a shot (with the qualifier - that I need to learn about the GD feature).
However, my guess:

There are issues at 35, 40, and 78 Hz.....?
I think the lower bass, or the region shown here looks fine. Its above this frequency range where I am not sure whats going on with your setup.

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post #13616 of 14645 Old 02-26-2015, 08:30 AM
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Originally Posted by 3ll3d00d View Post
AFAIK the Dirac filter is not long enough to be able to perform non minimum phase correction at those frequencies hence any impact on GD will be via the effect of taking out minimum phase room issues. Why do you think it will add a specific amount of GD? (given that that would imply a step change in the phase response at some specific frequency)
Because the graph rarely dips below 10ms. Seems to me that something is adding 10ms across the board.

In comparison to Jim's GD graphs which have more variance and does approach 0ms. IIRC, Jim doesn't use any EQ.
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post #13617 of 14645 Old 02-26-2015, 08:37 AM
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Originally Posted by artur9 View Post
Because the graph rarely dips below 10ms. Seems to me that something is adding 10ms across the board.

In comparison to Jim's GD graphs which have more variance and does approach 0ms. IIRC, Jim doesn't use any EQ.
you're misunderstanding what GD is then, it is a measure of the relative delay of different frequencies within the envelope, i.e. within the impulse itself, i.e. it is a measure of how long those frequencies take to pass through the filter relative to other frequencies. It is not, as I said in the earlier post, absolute time unless group delay is constant.

An example to illustrate, the absolute time delay incurred by my filter is ~650ms but you don't see my GD as showing up at 00s of ms
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post #13618 of 14645 Old 02-26-2015, 08:39 AM
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Quote:
Originally Posted by AustinJerry View Post
Here is a GD graph from my system, with Dirac Live RC. What does it tell us?
It tells you there's probably an overall delay that you should remove from the IR.

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post #13619 of 14645 Old 02-26-2015, 08:45 AM
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Originally Posted by 3ll3d00d View Post
An example to illustrate, the absolute time delay incurred by my filter is ~650ms but you don't see my GD as showing up at 00s of ms
If I'm following along then your GD shouldn't show 00s of ms because all the frequencies are equally delayed by your filtering.

I am assuming that 0ms of GD means that nothing is delaying that frequency relative to other frequencies.

What does it then mean if all the frequencies show a 10ms GD?

Simpler question, I think, what would cause 1 (or a handful) of frequencies to show significant GD?
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post #13620 of 14645 Old 02-26-2015, 08:46 AM
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Quote:
Originally Posted by 3ll3d00d View Post
you're misunderstanding what GD is then, it is a measure of the relative delay of different frequencies within the envelope, i.e. within the impulse itself, i.e. it is a measure of how long those frequencies take to pass through the filter relative to other frequencies. It is not, as I said in the earlier post, absolute time unless group delay is constant.

An example to illustrate, the absolute time delay incurred by my filter is ~650ms but you don't see my GD as showing up at 00s of ms
In an absolute or most accurate sense, yes. But I think what most of us are after, with any measurement really, is what we measure vs what that same measurement should look like. In the case of GD, in a perfect world, it would say 0ms from top to bottom. In the real world, there will some delay in the lower end. How much and at what frequency range is debatable. But from what I have studied, it should be possible to keep most of the GD pretty flat until you get to the low end. If you have 10 or 20ms bumps in the mids or upper bass, something is amuck.

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