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Value of room correction?

Alexanderc

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I’ve been looking at the differences between the NAD C368 integrated amplifier w/BluOS for $1450 USD and the NAD C658 preamp paired with the C268 amplifier for a combined $2550. The only significant difference I see between them (besides one being separates) is the C658 has Dirac room correction. What is your opinion on spending $1100 for Dirac? Would it be better to spend that money on better speakers or more amplification (or something else)? Is good room correction always worth it?
 

stunta

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Room correction is definitely worth spending on. How much is up to you. Cheapest option is probably the DIY route with a UMIK mic and REW software. The advantage of room correction built into the pre-amp is that (hopefully, check with NAD on this) it will apply to all sources connected to it. With a standalone PC version, it can get really tricky applying it to all sources.

I didn't enjoy my experience with REW - I needed something easier to use; so I went with a minidsp 8 channel unit that has Dirac Live built in. I would not trade it for any electronics/speakers. Dirac took my room from unlistenable to irrelevant.
 

QMuse

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I can't speak to the NAD gear, but Dirac Live 2 software to run on a computer is $395 retail. I wrote a review of the software version. I would say good room correction software is always worth it :) but like I say, can't speak to how it is implemented in the NAD gear... Perhaps @Flak can comment?


Very interesting article, executed and written very professionallfy. There are however few things wrong in the REW Verification Measurements chapter:
"One should note that this is not fully representative as it is one measurement made at the middle listening position. Ideally, I would take 17 measurements at the same or similar locations as I did in Dirac and vector average them in REW. And even then it still will not be quite the same as Dirac is applying more than just a vector average. However, it does verify that Dirac is doing what it is supposed to do: "

Vector average in REW is designed to average measurements in time domain (phase) and will NOT give you correct result for averaging frequency response. If you want to average measurements for frequency response you should use Average the Responses under All SPL tab.

The other thing that is methodologically incorrect was using a single sweep to check the results of the spatial correction:

Capture.JPG


As we can see there is a 15dB peak to peak variation in the 100-300Hz region. This graph would probably look better if the measurement was taken as a spatial average taken over the same area that Dirac worked with instead with a single sweep.

Issues related to using single sweep measurement for the room EQ are well described in this AES paper written by Geddes and Blind. In the paper they also propose their own method to perform such measurements, but lucky for us moving microphone method gives retty good results as well.

So, to check how well of a job Dirac did it is not needed to replicate its 17 sweeps from the exact spots Dirac used. We are not here performing check on Dirac algorithm to see how well it processed its input measurement to design the filters. We can't do that even if we would like to, as we don't know how Dirac processing was designed - most probably it doesn't average all the sweeps, or at least not with the same weight. What we would like to check is the final result of the correction and using moving microphone method in RTA mode is the best and easiest way to do that. Take 50-60 samples over the same area Dirac measurements were taken and you will get a Correct spatial picture of the job Dirac did.
 

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mitchco

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Thanks @QMuse, as I already said, and you quoted, it is not representative... It would have been good of you to to include the top portion of my graph ;-)

Dirac also uses excess phase correction, so MMM technique or other just frequency response averages, weighted, whatever, would also not be representative. This is what separates commercial, purpose built room correction software from diy techniques and software that was not intended for room correction.

My simple point in the article is that Dirac is working appropriately in both the frequency and time domains as a "spot" check.

Wrt to a single measurement point is somehow flawed, other top DSP software disprove this as these software use a different analysis algorithm altogether that was not available to Earl, who I have discussed this point with. I go into gory detail in my book showing how a single transient response analysis can be used in a frequency and excess phase response correction to smooth the low frequency frequency (and timing) response over a 6ft x 2 ft area where I take 14 measurements in REW to verify this. It is predicable and repeatable.

The point I was making to the OP is that room correction is always worth it :)
 

Hipper

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On whatever system you have now, play some bassy music and walk round your room. You should notice that the bass sounds different in different locations. What this means is that if you move your speakers and/or listening chair you will hear a different sound.

This proves that the sound from any speakers will be interfered with by your room and its contents. The ideal solution is to get the best positioning of speakers and chair, add room treatment and finish off with DSP/EQ. For many that ideal is not practical, especially positioning and room treatment, so you are left with only DSP/EQ.

For these reasons DSP is important. How you implement it then becomes the question. As mentioned there are software solutions that require a computer, and hardware ones, such as MiniDSP. All require measuring in some way using a microphone. They are not easy to use but have been made as easy as possible. DSP needs to apply to everything that will come out through your speakers, but not for your headphones if you use them.

My attitude would be that DSP is essential whatever equipment you buy.
 

QMuse

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Thanks @QMuse, as I already said, and you quoted, it is not representative... It would have been good of you to to include the top portion of my graph ;-)

Dirac also uses excess phase correction, so MMM technique or other just frequency response averages, weighted, whatever, would also not be representative. This is what separates commercial, purpose built room correction software from diy techniques and software that was not intended for room correction.

My simple point in the article is that Dirac is working appropriately in both the frequency and time domains as a "spot" check.

Wrt to a single measurement point is somehow flawed, other top DSP software disprove this as these software use a different analysis algorithm altogether that was not available to Earl, who I have discussed this point with. I go into gory detail in my book showing how a single transient response analysis can be used in a frequency and excess phase response correction to smooth the low frequency frequency (and timing) response over a 6ft x 2 ft area where I take 14 measurements in REW to verify this. It is predicable and repeatable.

The point I was making to the OP is that room correction is always worth it :)

I included only the corrected part of your graph as that is the one addressed with my comment - single point sweep measurement of uncorrected response is equally not representative. :)

Spatialy averaged measurement of both, frequency and phase, is needed for a precise room EQ and also for the precise control of the room EQ results. For spatially averaged frequency response you have 2 options with REW - either take a number of sweeps from different points around the listening area you are optmising and than use Average tghe Responses under All SPL to average them or use MMM RTA. If executed properly both will give practically identical results but with MMM you can process more measurements in a less time.
For spatial averaged phase response you have only one optionwith REW - take a number of sweeps from different points around the LP and use vector averaging. Frankly, I have rarely used that as phase response doesn't change so "wildly" from point to point as frequency response might, so a single sweep is usually enough.

And I don't agree that anything has changed from the time Geddes and Blind wrote that article - it is a scientific fact that single sweep contains no spatial information but only informatioin about the response in the particular point in space where mic was located while measuring the sweep. For that reason no magical algorithm could be applied to extract spatial information from a single sweep because spatial information is simply not there. Situation where FR taken from single sweep matches the averaged response from spatial measurement is a pure coincidence related to the particluar room you're doing meaurements in so it's certainly not something that should be repeatable in any given enclosed space.
Unfortunately, the only way to collect spatial data is to take measuremets at multiple points in space.

But unlike your single sweep, most high-end room EQ systems (including Dirac) actually make spatial measurements. Now, these room EQ systems may, and best of them certainly do, have clever algorithms to get the same result from 9 spatial sweeps vs the 50 samples MMM RTA. As we don't have details of their algorithms we cannot use their 9 points method to control the results as they have done more "magic" with them than a simple average. But luckilly for us we don't need to do it their way - we can simply make MMM RTA and get very acurate picture of FR. We can also take a number of sweeps at different points, apply vector average and some FDW and we will get very acurate phase response of the correction.

Regarding time corection - it is nice you showed step response as a proof of time-alignement of the drivers and also that no pre-ringing was introduced during phase correction, but the truth is those things simply don't matter much in audible terms, no matter how nice they look on the various time-domain graphs. There is one thing that might, and very often it does, cause an audible issue but you didn't show it, nor todays room EQ systems are addressing it with their time domain correction, and that is frequency response cancellation in LF caused by phase mismatch between speakers. Unfortunately I have yet to see modern room EQ systems that tries to correlate the phase response of left and right speaker to avoid cancellation.
 
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QMuse

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The point I was making to the OP is that room correction is always worth it :)

I forgot to comment this so I'll do it here:

One cannot emphasize enough the importance of room EQ as it solves THE most important characteristic of the speaker and that is flat frequency response. Sure, speaker's position matters very much and room treatment may help if done properly but, as @Hipper noted in his post, so many times we simpy cannot move the speakers and apply room treatment so we are left with room EQ as the onyl way to make things better. Your past and present work in that context is very much appreciated. :)
 
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QMuse

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I’ve been looking at the differences between the NAD C368 integrated amplifier w/BluOS for $1450 USD and the NAD C658 preamp paired with the C268 amplifier for a combined $2550. The only significant difference I see between them (besides one being separates) is the C658 has Dirac room correction. What is your opinion on spending $1100 for Dirac? Would it be better to spend that money on better speakers or more amplification (or something else)? Is good room correction always worth it?

Room correction is definitely always worth it but it can be done for much less than $1100.
 
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Alexanderc

Alexanderc

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I like the idea of having the correction built in and, aside from AVRs, this NAD is the least expensive option I have seen. Compared to the Anthem STR and Lyngdorf equipment it seems like a bargain. In the end I may not go the “built in” route, but I appreciate everyone’s suggestions and will make sure room correction is part of my plan moving forward.
 
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Alexanderc

Alexanderc

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I'm currently writing a review of the NAD implementation of Dirac with measurements which I hope will shed some light here.

Spoiler: HOLY SHIT!

Ha! Can’t wait.
 

Soniclife

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I'm currently writing a review of the NAD implementation of Dirac with measurements which I hope will shed some light here.

Spoiler: HOLY SHIT!
What RC have you played with previously?
 

SIY

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What RC have you played with previously?

Sonarworks, which is quite excellent.

edit: Also ARC 2.5, which likewise was quite good.
 
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mitchco

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I included only the corrected part of your graph as that is the one addressed with my comment - single point sweep measurement of uncorrected response is equally not representative. :)

Spatialy averaged measurement of both, frequency and phase, is needed for a precise room EQ and also for the precise control of the room EQ results. For spatially averaged frequency response you have 2 options with REW - either take a number of sweeps from different points around the listening area you are optmising and than use Average tghe Responses under All SPL to average them or use MMM RTA. If executed properly both will give practically identical results but with MMM you can process more measurements in a less time.
For spatial averaged phase response you have only one optionwith REW - take a number of sweeps from different points around the LP and use vector averaging. Frankly, I have rarely used that as phase response doesn't change so "wildly" from point to point as frequency response might, so a single sweep is usually enough.

And I don't agree that anything has changed from the time Geddes and Blind wrote that article - it is a scientific fact that single sweep contains no spatial information but only informatioin about the response in the particular point in space where mic was located while measuring the sweep. For that reason no magical algorithm could be applied to extract spatial information from a single sweep because spatial information is simply not there. Situation where FR taken from single sweep matches the averaged response from spatial measurement is a pure coincidence related to the particluar room you're doing meaurements in so it's certainly not something that should be repeatable in any given enclosed space.
Unfortunately, the only way to collect spatial data is to take measuremets at multiple points in space.

But unlike your single sweep, most high-end room EQ systems (including Dirac) actually make spatial measurements. Now, these room EQ systems may, and best of them certainly do, have clever algorithms to get the same result from 9 spatial sweeps vs the 50 samples MMM RTA. As we don't have details of their algorithms we cannot use their 9 points method to control the results as they have done more "magic" with them than a simple average. But luckilly for us we don't need to do it their way - we can simply make MMM RTA and get very acurate picture of FR. We can also take a number of sweeps at different points, apply vector average and some FDW and we will get very acurate phase response of the correction.

Regarding time corection - it is nice you showed step response as a proof of time-alignement of the drivers and also that no pre-ringing was introduced during phase correction, but the truth is those things simply don't matter much in audible terms, no matter how nice they look on the various time-domain graphs. There is one thing that might, and very often it does, cause an audible issue but you didn't show it, nor todays room EQ systems are addressing it with their time domain correction, and that is frequency response cancellation in LF caused by phase mismatch between speakers. Unfortunately I have yet to see modern room EQ systems that tries to correlate the phase response of left and right speaker to avoid cancellation.

I won't bog down the thread with a lengthy response. Wrt your last point, actually Dirac (and the other software I am referring to), does indeed correlate the phase response of the left and right speaker: "the latest Dirac Live 2.x algorithm might even slightly compromise on FR response of one of the speakers in favor of the best possible phase coherence between them that is important for imaging."

What hasn't been addressed and my point with a few commercial software being used is "excess phase" correction at low frequencies. I have tried MMM technique several times and it does not deal with excess phase at low frequencies. I have also tried MMM with rePhase for excess phase correction and is not the same, but closer, to the commercial software. What I am discussing isn't "magic." In this article, one can see in the time domain a maximum phase peak caused by low frequency room reflections, that are corrected in the time domain. It certainly is audible to my ears.
 
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