problem with "Location, location, location" section of "What are the limits of applying EQ?" in manual - Home Theater Forum and Systems - HomeTheaterShack.com

 
Thread Tools
post #1 of 7 Old 01-21-15, 04:42 PM Thread Starter
Shackster
 
Join Date: Jan 2015
Posts: 12
problem with "Location, location, location" section of "What are the limits of applying EQ?" in manual

The RoomEQ software and its documentation is a wonderful resource that I am very thankful for.

I disagree with the specific wording of the subsection "Location, location, location" in the "What are the limits of applying EQ?" section of the manual. Overall, it's a great overview of the limits of applying EQ, per se. However, this section states:

Quote:
Before we start adjusting an EQ to alter the frequency response, we need to see a response to adjust, so we need to make a measurement. This brings up the first limitation. The measurement is made at a single position, and the frequency response of that measurement is only valid at that position, moving the mic elsewhere and making another measurement will produce a different frequency response. It may be a little different, or it may be (and usually is) a lot different. The changes made by an equaliser in the path to the speaker are the same no matter where we are in the room, so since the response is changing in different positions and the EQ isn't, it stands to reason that the EQ is only going to be good in places where the frequency response is the same as the one we used when setting the EQ.

Reading some of the advertising blurb for EQ products you could be forgiven for thinking that some clever guys somewhere have figured out a way around this. They haven't. The best you can do is to look at the frequency responses measured at many positions in the area where you need the correction to work, figure out which bits of them are sufficiently common, and come up with a compromise EQ setting that helps somewhat in most places and doesn't do too much harm elsewhere. It can help, but it is no magic bullet.

From http://www.roomeqwizard.com/wizardhe...theanswer.html
Overall, this section is true. However, some products (at least purport) to work on the time domain and not just EQ. Thus, to discuss some such tools, such as IK Multimedia's ARC 2 system, this section would provide more clarity by also addressing the limitations of EQ + time tools.

Next, I would like to reference my usage of the ARC 2 system and why it appears that some such tools, while definitely NOT a replacement for proper acoustic treatment, can get a user closer to accuracy than just an EQ.

I have performed some testing with ARC 2, which incorporates Audyssey MultEQ XT32, in a typical "home project studio candidate" sub-optimal room (rectangular, 12ft x 12ft 6" x 8ft 11") without any treatment. Test equipment is:
  • Dayton Audio EMM-6 (with on-axis calibration file)
  • REW software
  • Alesis Prolinear 820 speaker pair
  • Steinberg UR824 audio interface
  • IK Multimedia ARC 2 (version 2.2.1) VST plugin

I have performed many tests, and the below are representative of the typical/average results.


Graphs

The IK Multimedia ARC 2 correction display:


A stereo SPL graph with (blue) and without (magenta) ARC 2:


A left speaker SPL graph with (blue) and without (magenta) ARC 2:


A right speaker SPL graph with (red) and without (blue) ARC 2:


A left speaker waterfall graph without ARC 2:


A left speaker waterfall graph with ARC 2:


A right speaker waterfall graph without ARC 2:


A right speaker waterfall graph with ARC 2:



Observations
As someone with an interest in acoustics physics, I am dubious of claims by product manufacturers such as IK Multimedia's claims about this product: "Regardless of the acoustical issues in your studio, what you are recording, mixing, or mastering becomes immediately clear and reliable"; "ARC takes your room out of the mix", and "...you can trust the sound of your studio." The graphs above indicate that the EQ effect of ARC 2 is fairly minimal. However, when listening to familiar mixes with ARC 2 in this environment, it is apparent that something more than EQ is being applied to the signal. ARC 2 obviously improves the bass response in the room (per the target curve), but when going from bypassed to on, ARC 2 immediately clears up the stereo image quite a bit; the bass suddenly becomes more focused, and elements that sound diffused or hazy suddenly sparkle with clarity. Obviously, ARC 2 cannot overcome the fundamental physical problems of a room (modes, reflections, null-causing factors, etc.), but nonetheless it is working better than any corrective EQ alone could, in my opinion. I have to admit that the resultant sound is vastly improved over the unprocessed signal.


Inquiry to IK Multimedia
Before I launch into this extensive set of details, please note up-front that I am happy to use and recommend ARC 2 after receiving additional information from IK Multimedia. This post is not intended to raise concern or alarms over the product; rather, it is to help clarify the intended functionality and capabilities of the product for other users who think in similar ways to me. After testing the product and getting clarification from IK, I intend to use the product and recommend it to others who understand its positive effects within the context of its limitations. This product is effective because it works on both EQ and the time domain, using psychoacoustic techniques to improve the output for your targeted listening area. It cannot make a dorm bathroom sound like a world-class studio, but it can definitely get you much closer to a flat, correct response than (a) just EQ and treatment or (b) no EQ or treatment at all. Obviously, proper room treatment will enhance your experience with ARC 2 tremendously.

That said, the IK Multimedia marketing claims, combined with the very-flat target curves shown in the ARC interface, might lead a user (not doing his/her own measurements) to hear the dramatic improvement caused by the plugin and to conclude that "Wow, now that ARC is on, my frequency response is truly flat." But as the tests I performed indicate, this does not quite appear to be the case. There are some improvements in the low end, but higher than about 100K, the EQ changes ARC 2 makes are minimal. Thus the obvious followup question: Are there other tests I could have performed to clarify the effects ARC 2 makes?

The included manual states that ARC 2 also works on the time domain to address phasing and other issues, but the overall strategy/effect isn't clear from either (a) my own testing or (b) the ARC 2 interface, which doesn't represent anything other than frequency response. Thus, a user like me is left to guess what ARC may be trying to do. This was disconcerting, as I believe that overall results from ARC 2 should be quantifiable, but the actual results received (I tested many times to verify) are anecdotally (experientially) audibly dramatic but measurably minimal based upon the tests I performed, and definitely include a non-flat frequency response. In other words, if the product really aims to make a bad room sonically "reliable" in terms of frequency response and time domain issues, it seems that it should make the results measurably flat and time-corrected.

I opened a support ticket with IK Multimedia to follow up on these concerns, asking if (a) despite the marketing hype, the system is really just supposed to make its processed audio sound "a little bit better" in a conservative way, such that mixes get "a little bit closer" to the balanced sound someone would expect to achieve in a world-class facility; or (b) is it in fact doing something esoteric psychoacoustically, which can't be measured by traditional freq. response and timing processes, that really does somehow make the listener interpret the final sound as near-accurate. I also told them that I understand their proprietary licensing relationship with Audyssey and that I wasn't interested in sharing any algorithmic / technical specifics they shared with me; I am just interested in knowing, in a broader way, how much to really "trust" ARC in the overall scheme of things.

The concern I had is that, while the product sales literature, and the product box and manual, make sweeping claims about the efficacy of ARC 2, the actual tested results did not appear to fully live up to the claims. (I was also surprised to note that the capsule frequency response of the included mic is only 20Hz - 16KHz; IK Multimedia tech support explained that that is its response when used without ARC 2, but that its calibration with ARC 2 results in a 20Hz - 20KHz frequency response when used with ARC 2.)

I gave a rough analogy: Let's say that you are a car mechanic always looking for ways to improve your auto repair workflow. You come across an interesting product, the JL Multivehicle CAR2 System. Looking at the box, it says, "No matter what condition your vehicle is in, this product will get you going. Don't worry about traditional maintenance schedules or costly repairs; just pop the CAR2 Dynamic Fluid in the gas tank, attach the CAR2 Energy Interocitor to the chassis, and the car will run like it did straight out of the factory." As a mechanic, you are dubious about these claims, but then again, with the state of nanotechnology, energy field tech, etc. you are willing to stretch your imagination to consider the possibility that some very ingenious engineers at JL Multivehicle have actually worked out a solution that bypasses the need for car maintenance and repairs. Your experience and knowledge of how cars work tell you that this isn't likely, but you are willing to give the claims a fair review. So, you purchase the CAR2 kit and find a decently clunky vehicle with a rough idle and transmission problems - it even has a missing door - to test the kit on. You put the Dynamic Fluid in the gas tank and attach the Interocitor, turn the key, and the engine roars on. The first thing you notice is that, indeed, the engine is running more smoothly in idle; and when you take the car for a drive, the engine doesn't shake as much. However, you notice that the transmission problem still remains. Thinking maybe you didn't follow the directions precisely, you use the kit again, and get the same results. Looking under the hood, you can see that, indeed, the fuel system is cleaner (less deposits) and ignition is improved - but there are still deposits, and the ignition has room for more improvement. Plus, the serpentine belt is still worn out and the air filter is very dirty. Based upon the product box, you were expecting CAR2 to somehow really repair the car, or at least give the apparent measurable effect of fully repairing the car, which it hasn't done. So now you begin to ask some questions: OK, is this product really, truly designed to create a perfectly running, repaired car such that a mechanic could replace his tools with just the CAR2 kit? Or is it just designed to help the do-it-yourself home car repair customer to make his car repairs just a little bit better, but in the hands of a professional mechanic the limitations of the system will, by design, become obvious?

This is a very important point, because as a professional mechanic, you need to make informed decisions about how to precisely use the tool. As CAR2 is intended, will you still need to make manual repairs? How does CAR2 actually affect a treated vehicle, in what systems, in what ways? You need to know to be able to compensate for any potential limitations.

So you contact JL Multivehicle and ask them questions about the remaining fuel system deposits and the unfixed transmission problem. The product said it would literally fix the car so that no additional repairs are needed, but instead, the product appears to be performing incremental "it's better than it was" improvements, which is a discrepancy. Despite being slightly irritated with the hyped marketing that helped to sell you the product, you are willing to accept that the product has limitations (which you fully and skeptically expected anyway), but you need to know what the product's true effects and limitations are, precisely. Your eyes, ears, and measurements tell you that the product does have a positive effect, but it does not really completely fix a car. You begin to suspect that this may be just another typical fuel additive (which also has positive effects but doesn't repair the whole car) combined with a proprietary whizbang doohickey distraction device (the Interocitor) in order to create a sense of secret mystery as a means of selling the kit, as opposed to providing a genuinely real full car repair tool.

We all have experiences where someone has told us, "Just use this dietary supplement / multilevel marketing system / mastering plugin / thing and everything will be totally better." When you ask specifics about the thing, you are told, "It's a secret blend of Icelandic yak spit and other proprietary DSP processing that can't really be explained but it does work. You don't need a scientific explanation about yak spit, it just makes your brickwall limiter glue the mix better." And just about 100% of the time, when a person/company resists the explanation, it's because the product doesn't really do everything it claims to.

Like the CAR2 system, the ARC 2 literature very precisely and technically avoids making outright statements like, "You will get a perfectly flat frequency response and perfected timing," but the marketing verbiage is strong enough to lead a customer to believe that the product is supposed to bypass the need for acoustic treatment, etc. by creating a flat response in just about any room. If the product just said, "This will sort of help you get incrementally better response in most rooms," then we would all relax and say, "OK, that makes sense." But the ARC 2 interface shows a ruler-flat target curve, and the product literature could lead a non-expert to believe that, once they switch ARC 2 on, they are literally hearing perfectly flat response for their mixes. As aforementioned, when someone bothers to measure the results, they discover that a flat response is not what actually happens (like the remaining fuel deposits and transmission problem) and then they want to understand, "OK, so what did actually get fixed, and what do I still need to look out for?" Does ARC 2 only affect EQ? EQ plus phase/polarity issues? Is there some psychoacoustic process which isn't readily measurable in freq. response and time domain terms but which somehow alters the perception of the sound? Without details, a professional cannot know, and if the effect cannot be measured, then the pro is left to guess.

Pro audio guys don't like to guess. Gut instincts are great when working on a mix, but when selecting gear, we want to know the specifications of the gear we're buying and implementing.

As another analogy: If you were flying an airplane and someone gave you an "absolute altimeter" that is supposed to tell you exactly high off the ground you are, but you notice that it reads "100ft" when you are at 20ft, would you use that altimeter ever again? Probably not. Should you just believe the altimeter is correct, despite your measured result, because its manufacturer said so? Absolutely not. So if you asked the altimeter manufacturer about it and they said, "Well, it's not very accurate, but it's better than nothing," then you would say, OK, well I can use it, but I will trust it just a little. But if the manufacturer asserts, "It is a proprietary altimeter acting in ways that we cannot explain via quantum tunnelling, but it is perfect," yet you know from experience that it is not perfect, then you have to assume that (a) your particular altimeter is defective, or (b) the altimeter company is distorting the truth. In either case, a professional pilot isn't going to use the altimeter because the pro pilot is looking for precision instrumentation, as his life depends on it.

With this line of thinking I thus contacted IK Multimedia.


Response from IK Multimedia
Jason Williams and Paul Hudson of IK Multimedia were gracious enough to provide me with excellent, thoughtful replies. They were truly helpful, and I encourage anyone who has questions about IK products to contact the support team directly.

Here is a substantial portion of Mr. Hudson's reply, which I am including for public consumption because it doesn't reveal anything beyond information already publicly available as published elsewhere by IK Multimedia and Audyssey:

Quote:
"I can certainly understand your concerns and your pursuit of precise detailed information here.... There are limitations on the proprietary technology [due to licensing restrictions with Audyssey] and what can be provided to you....

From your provided images, I see quite a bit of improvement from the captured measurement file. The low end is corrected quite a bit from 20-300Hz which has some peaks and dips, with smoothing out the higher end as best as possible....

The ARC System 2 is a unique product and it is using a psychoacoustic trick for what you hear. However, it is not a miracle maker and achieving a perfectly flat or near-flat response, while possible, greatly depends on a large number of factors including room shape, room dimensions, acoustic treatment in place, speaker type and positioning and measurement accuracy.

What the end result is depends on how the initial condition of the corrected environment is to start with. You cannot expect to turn a small bathroom into an A-list mixing room but it will certainly help to improve a room within it's own set of limitations. The purpose here is to flat out peaks and dips in the frequency response and do time domain corrections so that most anomalies are removed. If the anomalies are out of the plugin's range, then the compensation will only be as much. ARC is not meant to replace a properly designed room with treatment but that it will actually make an average room better and a fine-tuned room smoothed out with its own minor imperfections.

ARC does aim to correct both EQ and time domain issues that can often be caused by large corrections from the EQ issues it found. In addition, it is used to reduce ear fatigue, to achieve a quicker mix that translates better across multiple systems, and is good for engineers traveling to a new studio where they can ARC the room and get a better flat feel than having to 'learn' what the new room sounds like to achieve a better mix.

Finally, please do visit the Audyssey website here for some more information:
MultEQ FAQ | Audyssey

Reaction to IK Multimedia response
I am satisfied with IK Multimedia's response. Knowing that they are not intending for a user to believe that a truly ruler-flat frequency response is intended (or possible) in a room lets me place the product along an efficacy spectrum in my mind as a useful tool for getting closer to an approximate ideal result - just not an actually ideal/perfect result.

Further, the additional referenced materials from Audyssey (some links below) state that the overall effect of ARC 2 is not particularly easy to measure with the usual techniques because of the proprietary psychoacoustic techniques being employed. While that may sound like a cop-out to some (and indeed, I am very skeptical of "secret sauce" answers in general), I do understand that a company like Audyssey would need to protect the fundamental techniques and technologies that differentiate them from competitors; after all, giving everyone full and precise knowledge of how to measure their products' effects could lead to reverse engineering efforts. Thus, while I am disconcerted and made somewhat skeptical by not being able to see major changes in frequency response, waterfall, impulse, group delay, and other charts, I can accept a certain amount of "fuzziness" in understanding the product's results due to commercial concerns. In use, the product really does work nicely, so I tend to accept the IK Multimedia / ARC 2 assertions (with a healthy knowledge that the resulting sound as processed by ARC 2 is not exactly correct).

In essence, ARC 2 is a very useful tool that can help bring out a closer-to-ideal sound in a far-from-ideal environment.

My view is that the ARC 2 sales literature would be more useful, believable, and effective if it were edited a bit to skew toward this conservative understanding. As it is marketed now, the product sounds "too-good-to-be-true" and causes skeptics to see red flags; however, it really is a useful, effective product, and if the marketing was adjusted to sound more realistic and pro-friendly (less "we'll fix everything" and more "here's what it really does") then I think more audio pros would be apt to give it a shot.

Additional information

Quote:
I've seen some people attempt to verify what Audyssey is doing by using more conventional measurement methods like RTAs or FFT swept sine measurements. Is there correlation between these methods and Audyssey? Can Audyssey's EQ be verified by any other measurements?

It's usually tough to match results with different methods because they don't have the IP used by Audyssey. However, reasonable results for verification can be achieved if:
  1. You have a calibrated microphone (i.e. a file used by the measurement program)
  2. You take multiple measurements (in the same locations used when calibrating with Audyssey)
  3. You use an FFT method to find the impulse response of each measured position (RTA only looks at magnitude and ignores time so it doesn't match at all)
  4. You average the multiple measurements to come up with a representation of the listening area response.

From https://audyssey.zendesk.com/entries...ation-methods-
Quote:
How does Audyssey MultEQ measure and correct problems with room acoustics?

The technology is a result of a multi-year university research effort to understand the key factors that influence sound reproduction. From this research came two key findings:
  1. Acoustical problems in the room are more accurately measured in the time domain. This type of analysis provides information about the direct sound and the effects of reflections from room surfaces. This was a departure from traditional EQ methods that only looked at data in the frequency domain.
  2. Measuring in a single location does not capture sufficient information and often results in equalization artifacts. Multiple measurements are required to capture the spatial distribution of acoustical problems, particularly in the low frequencies where the problems are more serious.

MultEQ captures multiple measurements in the time domain and then groups them in clusters based on similarities in the data. Using Fuzzy Logic mathematics, the clusters are allowed to overlap so that each measured response belongs to each cluster with a certain probability. In each cluster a representative response is then created that is weighted by the acoustical problems in that cluster, but also by those in the other clusters. MultEQ then re-combines these representative responses to create a final room representation and then inverts that to create the correction filter for each loudspeaker. The type of filter used by MultEQ simultaneously corrects the time and frequency domain problems to produce a smooth response.

MultEQ also measures the time it takes for the signal to arrive from each speaker to the first microphone position. Delays are then applied to the speakers that are closer to match the timing of the signals coming from the speakers that are farther away. Finally, the sound pressure level produced by each speaker is adjusted with the trim controls so that they match each other....

Audyssey uses time domain measurement and correction and the resulting filters are not based on bands. Instead they operate on the amplitude and phase of the signal simultaneously to avoid any such anomalies.

There is other research (in addition to our own) from the BBC that shows the importance of equalizing speakers all the way up to beyond 10 kHz. In fact, the BBC work showed that a ±1 dB match is required between speakers otherwise stereo imaging suffers. Even the best speakers have difficulty matching each other to within such tight requirements when placed in different locations in the room. Correction all the way up to the highest frequency range can fix that if properly applied.

From https://audyssey.zendesk.com/entries...om-correction-
Quote:
What is room distortion and why do I need MultEQ?

Room distortion is caused by reflections from walls and furniture in a room (or car). Sound from the loudspeakers reaches your ears directly but also bounces off surfaces and is reflected. You hear these unwanted reflections a bit later than the direct sound causing distortion. MultEQ analyzes these reflections and corrects the problems they create. Movies, music and games sound the way they were created. Audyssey founders spent 6 years and $6 million of university research to create MultEQ and solve the room distortion problem.

From MultEQ FAQ | Audyssey
Quote:
What makes MultEQ work when so many others have tried to solve this problem for decades?

MultEQ is different from the many systems in the past because of two differentiators:
  1. The way it measures your room
  2. By correcting both time and frequency problems

In terms of calibration measurement methods, there are two types of systems out there today:
  1. Those that attempt to correct for only one seat in the room. This typically makes other seats in the room sound worse because a single measurement does not provide an accurate representation of the problems of the entire room.
  2. Some EQ systems do try to correct for multiple positions. They simply average the measurement results that they gather. Averaging tends to smooth-out common acoustical problems without really fixing them. For example if there's a peak at 200 Hz in one seat and a dip at 200 Hz in another seat (typical room), then the two measurements average each other out and there's no correction made.

In either case these systems only attempt to correct frequency response problems and not time domain problems and this leaves a big part of the problem unsolved.

From MultEQ FAQ | Audyssey
Quote:
How does MultEQ measure your room differently?

MultEQ looks at patterns in the time domain responses and classifies them into clusters based on the similarities in those patterns, typically in 3-5 groups. A representative response is created from each cluster, and a final response is then created from grouping the representatives. That response is then used to create the EQ filter. It is a complicated process based on the complex mathematics of pattern recognition and fuzzy logic. But there's nothing fuzzy about the results. If you want to know more, we offer copies of selected publications we've written for technical conferences.

Time and Frequency Correction:
  1. The time domain is where many of the problems are. Parametric and graphic equalizers can only correct for the frequency response and do so in a very coarse manner because they have limited resolution (bands).
  2. Further, whether they have fixed or adjustable bands doesn't matter because bands cause phase problems that most people hear as "ringing" or "smearing." That's why, after thirty-plus years of trying this method, most people don't like the results. And they turn it off.

From MultEQ FAQ | Audyssey
Quote:
How does MultEQ address time and frequency problems?

MultEQ filters start in the time domain. They aren't just a few parametric bands. Instead they use several hundred points to represent the room response in both the frequency and time domains.

The trick is to use enough filter points to get the needed resolution but not so many that it overwhelms the processor inside the audio component. So we came up with a way to reduce the number of points without sacrificing accuracy while providing more filter power at lower frequencies where it's needed the most. MultEQ can correct 8 channels by using only a fraction of a single DSP chip. This gives you the best of both worlds: time and frequency correction. Result: room correction that works for the first time ever.

From MultEQ FAQ | Audyssey
Quote:
How is MultEQ different from other equalization methods?

There are two fundamental differences from every other method available in AV receivers today. The first is that MultEQ is not based on parametric equalization. Parametric equalization relies on a few bands that are centered at certain frequencies. These bands do not provide sufficient resolution to address many room acoustical problems. Also, parametric bands tend to interact so that changes at one frequency have undesirable results at nearby frequencies. Moreover, parametric equalization methods use a particular type of digital filter called Infinite Impulse Response (IIR) that only attempts to correct the magnitude response in the frequency domain. These filters can cause unwanted effects, such as ringing or smearing, in the time domain particularly as the bands get narrower. MultEQ uses Finite Impulse Response (FIR) filters for equalization that use several hundred coefficients to achieve much higher resolution in the frequency domain than parametric bands. Furthermore, by their nature, FIR filters simultaneously provide correction in the frequency and time domains. FIR filters had been considered to require too many computational resources. But Audyssey solved this problem by using a special frequency scale that allocates more power to the lower frequencies where it is needed the most.

The second major difference is that MultEQ combines multiple measurements to create equalization filters that better represent the acoustical problems in the room. Most other methods only perform a single point measurement and this can result in making other locations in the room sound worse than before equalization. There are some methods that use spatial averaging to combine multiple room measurements. Although this is a step above single-point correction, it does not provide optimum correction when discussing spatial averaging. For example, it is common to find a peak at a certain frequency in one location and a dip at the same frequency at another nearby location. The averaging methods will add the peak and the dip and this will result in an apparent flat response at that frequency, thus causing the equalization filter to take no action. MultEQ uses a clustering method to combine measurements so that acoustical problems are better represented, thus allowing the equalization filter to perform the appropriate correction at each location.

From https://audyssey.zendesk.com/entries...ation-methods-
Quote:
....What are the differences between the different levels of MultEQ at mid- and upper-frequencies?

MultEQ filters operate in the time domain so octave band notation doesn't apply. Impulse responses are measured at multiple locations and then they combined by weighting the most significant problems first. These generally happen in the lower frequency range and so higher resolution is applied there. We don't publish the filter resolution, but the relative resolutions are shown here for the various flavors of MultEQ.

Because the filters operate in the time domain, any phase issues that arise from misalignment of drivers will be addressed. Obviously the results will be even better if we are allowed to control the delay of each individual driver, but this doesn't happen in home speakers. Our automotive customers do take advantage of that portion of the algorithm.

From https://audyssey.zendesk.com/entries...er-frequencies
Attached Thumbnails
problem with "Location, location, location" section of "What are the limits of applying EQ?" in manual-arc2-measurement2.png  

problem with "Location, location, location" section of "What are the limits of applying EQ?" in manual-l-narc2-waterfall.jpg  

problem with "Location, location, location" section of "What are the limits of applying EQ?" in manual-l-spl.jpg  

problem with "Location, location, location" section of "What are the limits of applying EQ?" in manual-l-yarc2-waterfall.jpg  

problem with "Location, location, location" section of "What are the limits of applying EQ?" in manual-r-narc2-waterfall.jpg  

problem with "Location, location, location" section of "What are the limits of applying EQ?" in manual-r-spl.jpg  

problem with "Location, location, location" section of "What are the limits of applying EQ?" in manual-r-yarc2-waterfall.jpg  

problem with "Location, location, location" section of "What are the limits of applying EQ?" in manual-stereo-spl.jpg  


Last edited by ShackLLC; 01-22-15 at 04:53 PM.
ShackLLC is offline  
Sponsored Links
Advertisement
 
post #2 of 7 Old 01-22-15, 04:56 AM
Elite Shackster
Wayne Myers
 
Join Date: Jun 2012
Location: Lincoln, NE
Posts: 4,838
Re: problem with "Location, location, location" section of "What are the limits of applying EQ?" in manual

Thanks for the treatise

Kidding, of coarse, that is a lot of good detail about Audyssey MultEQ and ARC2, thank you for the contribution.

Manufacturer advertising is all best taken with a grain of salt. That is why we will always have a need for independent expert evaluations and reviews, and for the sharing of user experiences and findings.

One statement about Audyssey MultEQ caught my eye. There are two fundamental differences from every other method available in AV receivers today. This statement was made 4 and 1/2 years ago and is no longer true. There are many excellent room corrections available, even a few receiver-based, that create complex FIR filters from multiple room measurements and compete strongly with MultEQ. Of course all toot their own horns pretty loudly in their advertising. And while none is perfect, most of them do pretty well, each usually addressing the issues of room correction with its own special emphasis.

Edit: While the detailed "under the hood" workings of MultEQ or ARC2 are proprietary for good reason, I doubt that there is any special magic to any "psycho-acoustical trick" being employed. The principles of room correction are fairly will understood, while the implementation details are certainly well guarded.
AudiocRaver is offline  
post #3 of 7 Old 01-22-15, 05:54 AM
Shackster
 
Join Date: Feb 2011
Location: Rochester, MN
Posts: 84
Re: problem with "Location, location, location" section of "What are the limits of applying EQ?" in manual

Holy long posts Batman! I think time domain correction is limited just like frequency correction via IIR filters. I believe John covered that pretty well in the "Minimum Phase" section IIRC.
natehansen66 is offline  
 
post #4 of 7 Old 01-22-15, 12:30 PM
Elite Shackster
Wayne Myers
 
Join Date: Jun 2012
Location: Lincoln, NE
Posts: 4,838
Re: problem with "Location, location, location" section of "What are the limits of applying EQ?" in manual

Quote:
natehansen66 wrote: View Post
Holy long posts Batman! I think time domain correction is limited just like frequency correction via IIR filters. I believe John covered that pretty well in the "Minimum Phase" section IIRC.
I believe John covered IIR and FIR filters and their limitations quite well. As a rule, when John speaks, I listen and take notes.
AudiocRaver is offline  
post #5 of 7 Old 01-22-15, 03:44 PM
REW Author
 
JohnM's Avatar
John
 
Join Date: Apr 2006
Location: UK
Posts: 6,707
Re: problem with "Location, location, location" section of "What are the limits of applying EQ?" in manual

Quote:
Overall, this section is true. However, some products (at least purport) to work on the time domain and not just EQ. Thus, to discuss some such tools, such as IK Multimedia's ARC 2 system, this section would provide more clarity by also addressing the limitations of EQ + time tools.
The info in that section is valid regardless of what filtering method is used.

As a simple example, let's suppose there is a reflection from one sidewall that generates a spike in the impulse response 8 ms after the main peak, due to the reflection path being about 8 feet longer than the direct path. It would be possible to completely remove that reflection with an FIR filter, so that at the original measurement position the response is magically corrected. However, moving the measurement position means the reflection spike is no longer at the same place in the impulse response, it will be earlier or later depending on whether we move towards or away from the reflecting surface. The naive correction filter then produces a worse result than if we hadn't bothered with correction at all, since we still have a reflection spike and we also have a filter that generates a signal that tries to correct a spike at 8 ms that is no longer present. All attempts at room correction have to strike a compromise, reducing the amount of correction and the frequency range it spans to try and deliver a result that balances improvement and degradation across a large enough area to be useful. There's more info in this wikipedia article and particularly in its references and external links: http://en.wikipedia.org/wiki/Digital_room_correction
JohnM is online now  
post #6 of 7 Old 01-22-15, 04:46 PM Thread Starter
Shackster
 
Join Date: Jan 2015
Posts: 12
Re: problem with "Location, location, location" section of "What are the limits of applying EQ?" in manual

Quote:
JohnM wrote: View Post
The info in that section is valid regardless of what filtering method is used.

As a simple example, let's suppose there is a reflection from one sidewall that generates a spike in the impulse response 8 ms after the main peak, due to the reflection path being about 8 feet longer than the direct path. It would be possible to completely remove that reflection with an FIR filter, so that at the original measurement position the response is magically corrected. However, moving the measurement position means the reflection spike is no longer at the same place in the impulse response, it will be earlier or later depending on whether we move towards or away from the reflecting surface. The naive correction filter then produces a worse result than if we hadn't bothered with correction at all, since we still have a reflection spike and we also have a filter that generates a signal that tries to correct a spike at 8 ms that is no longer present. All attempts at room correction have to strike a compromise, reducing the amount of correction and the frequency range it spans to try and deliver a result that balances improvement and degradation across a large enough area to be useful. There's more info in this wikipedia article and particularly in its references and external links: http://en.wikipedia.org/wiki/Digital_room_correction
I entirely agree with you. My point is that the way the user manual section in question is written, it doesn't specifically discuss anything beyond EQ. A reader might be inclined to believe that, "Well, he didn't mention products that work on both frequency and time, just EQ-focused products; so maybe products made after this manual was written somehow supersede the physics this section is describing." By incorporating what you just wrote into the manual more fully, it will help people to understand that ANY such signal affectation product, working on time domain or not, will not be able to overcome the limitations of a room without treatment.

In other words, the main point of my post is not to attempt to contradict what you rightly state, but rather to suggest a more full-featured rewrite of a manual section that currently addresses EQ products but ignores products that work both on frequency and time domain issues. This way, people can be more fully educated; quite a few people are first introduced to the world of acoustics via your program, and your manual may be the first (and most authoritative) thing they initially read.

The rest of my post is just an illustration of how I perceive one particular EQ+time product, the ARC 2 system.
ShackLLC is offline  
post #7 of 7 Old 01-23-15, 02:09 PM
REW Author
 
JohnM's Avatar
John
 
Join Date: Apr 2006
Location: UK
Posts: 6,707
Re: problem with "Location, location, location" section of "What are the limits of applying EQ?" in manual

Ah, understood. I have a revamp of the help on my list of things to do anyway, I'll revisit the section then.
JohnM is online now  
Reply

Bookmarks

Tags
manual , problem , section

Quick Reply
Message:
Options

Register Now




PLEASE COMPLETE ALL REQUIRED FIELDS BELOW... THANKS!

REQUIRED FIELDS ON THIS PAGE
YOU MUST COMPLETE ALL OF THESE

Username
Password
Confirm Password
Email Address
Confirm Email Address
Random Question
Random Question #2




User Name:
Password
Please enter a password for your user account. Note that passwords are case-sensitive.

Password:


Confirm Password:
Email Address
PLEASE READ BELOW PRIOR TO ENTERING AN EMAIL ADDRESS!

ATTENTION!

YOU MUST ACTIVATE YOUR ACCOUNT!

Activation requires you reply to an email we will send you after you register... if you do not reply to this email, you will not be able to view certain areas of the forum or certain images... nor will you be able download software.

AN INVALID EMAIL ADDRESS WILL CAUSE YOUR ACCOUNT TO BE DELETED!

See our banned email list here: Banned Email List

We DO NOT respond to spamcop, boxtrapper and spamblocker emails... please add @hometheatershack DOT com to your whitelist prior to registering or you will get nowhere on your registration.


Email Address:
OR

Log-in










Thread Tools
Show Printable Version Show Printable Version
Email this Page Email this Page



Posting Rules  
You may post new threads
You may post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML is not allowed!
Trackbacks are Off
Pingbacks are Off
Refbacks are Off

 


For the best viewing experience please update your browser to Google Chrome