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Discussion Starter · #1 ·
Anyone know if any pre/pro makers will have room correction (besides TacT), ala Pioneer/Yamaha X400 series? I don't really want a reciever, as I'd rather have the money put into a higher quality front end.


My room is a rather hopeless size and configuration (is actually a grouping of rooms), so room correction seems to be a must have in my case, but I also am looking for the latest surround formats as well.


Thanks! Todd
 

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I was happy when I became a Lexicon addict. Mine does room shape, room size in CF (LxHxW), speaker tow, speaker angle, distance/phase plus the panorama, concert hall, music and stereo surround and even movie surround. eBay is a wonderful thing, you could pick up a DC-2V4 for cheap. They decode DTS, DD and load all the THX enhancements. Or you could get a MC-1, MC-8, or MC-12B.
 

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Meridian, if you have, oh, about $9000. 60 bands worth of room correction. And no one does the latest formats like Meridian. They already have PLIIx, I believe. Supposed to be the first, but then, Dolby and Meridian license technology to each other such as MLP.
 

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"Mine does room shape, room size in CF (LxHxW), speaker tow, speaker angle, distance/phase plus the panorama, concert hall, music and stereo surround and even movie surround."


Those are adjustments for the various surround modes though, that isn't room EQ.


Shawn
 

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No, surround modes are DTS ans Dolby Digital (and with Lexicon Logic 7). Electronic adjustments for room shape, room size, distance, phase/time adjustments all effect acoustics. Most prepros have a EQ adjustment as well. Todd wanted to know if a processor could correct for room acoustics. Of course adjustments in a prepro can only do so much. Materials need to be used to control sound.
 

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Listen to Shawn and learn, HT guy. ;)
 

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" Electronic adjustments for room shape, room size, distance, phase/time adjustments all effect acoustics. Most prepros have a EQ adjustment as well."


Those adjustments have nothing at all to do with *your* room. They have to do with the simulated room/hall in the reverb based modes on the Lexicons.


Load Concert Hall and play some music with it. Adjust the hall size or any of the RT parameters and listen to how the music changes. If you have a MC-1,DC-2 or DC-1 you can hear this *extremely easily by going into Effect Adjustment->Custom->Effect Only ON.


You can read more about how this works in your 'Theory And Design' booklet. If you don't have it you can download it here:

http://lexicon.com/products/download...ID=7&FileID=49


Shawn
 

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OK, member wannabe, I will just for you. So Shawn, you are saying you know without a shadow of a doubt that audio preamp processors do not have any adjustments for room correction? And Catapult, what are your feelings on this debate?
 

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Quote:
Originally posted by hometheaterguy
OK, member wannabe, I will just for you. So Shawn, you are saying you know without a shadow of a doubt that audio preamp processors do not have any adjustments for room correction? And Catapult, what are your feelings on this debate?
100%... Lexicon has been doing this for years and what you are referring to (as Shawn mentioned) is their reverb algorithms that have absolutely nothing to do with room correction... That doesn't mean that there aren't any processors out there that don't do room correction its just that the feature you are referring to has nothing to do with it. These are algorithms that Lexicon has been using for years to simulate the sound of concert halls, not improve the acoustics of your room.


That being said, the Tag 192 also does room correction.
 

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Quote:
Todd wanted to know if a processor could correct for room acoustics
Todd was wondering if there are any pre/pro's coming out in the near future that have Room Correction software built inside that, in addition to one or several microphones in your room, measure YOUR room's frequency response and adjust for any anomalies (peaks and dips in frequency response). Hence his reference to Pioneer's MCACC and Yamaha's YPAO.


Todd, I'm not sure whether or not you're aware of Krell's parametric room optimization software in their Showcase and HTS 7.1 processor's but they would go a long way in taming a room's negative effect on the sound of your system. The only problem with it, is that it's not automatic, and you need a way of measuring your room's response. You could use a microphone with a computer, or an RTA or even just an SPL meter, but it would take a rather long time to find the correct adjustments.


I am unaware of any other pre/pro's coming to market in the near future that have any automatic room calibration programs.
 

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Discussion Starter · #11 ·
Thanks for the leads and discussion!


I am indeed looking for the pre/pro to analyze a room via microphone and re-equalize the output to correct for poor room acoustics, although I have read many excellent things about Lexicon's ambiance modes (especially Logic 7) and overall quality and I appreciate hometheatherguy sharing his positive results. In any case, I should have been more specific in my initial post.


I have read many good things about all the product lines mentioned, and aside from the Tag & Meridian which I'm not (yet!) able to afford (although the new line looks a *little* bit more accessable), I was curious why this hasn't trickled down from ultra high end lines, yet we're seeing it in very reasonably priced recievers. Perhaps its simply not in the r&d budget for many companies... or maybe they are letting the receiver manufacturers test the waters.


It looks like many if not most of the recent pre/pros have the microphone input and capabilty to to automatic distance and level calabrations - it must be quite a leap to include the auto eq. The Pioneer people seem to swear by the MCACC, an the early YPAO discussions seem positive as well. I should just home demo a unit to see if it makes such a huge diff. Who knows, maybe I'll fall for one of these and have some extra cash to boot!


In the meantime, I will look into the Tag, Meridian, and Krell info in more detail. Thanks again for the leads!


Todd
 

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Should the room correction be recalibrated when large curtains are closed over a large window? If so, does room correction software typically store multiple calibrations, that you can select?


Best,


Will
 

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The Krell room correction is not parametric AFAIK, so it's more akin to the Pioneer and Yamaha solutions.

The Tag AV192R and the dual processor AV32R allow up to 8 filters per channel for L/C/R/Sub and 5 filters per channel for the others. The filter centre points can be configured in 1Hz increments and the gain and Q in 0.1 increments.

The Meridian EQ is similar, but I understand it has 60 filters that can be assigned to whichever channel you need.

The TacT TCS probably has the most mature room EQ since they've been doing it in their 2 channel pre-amps for years.
 

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Quote:
I am indeed looking for the pre/pro to analyze a room via microphone and re-equalize the output to correct for poor room acoustics.
Just a note, Todd. An equalizer can't correct for the most common and severe room acoustical problems, which are in the time domain. These include high or uneven reverberation, low dialog intelligibility, and large early reflections.


I always recommend an equalizer (3rd octave and/or parametric), but for "fine tuning" the frequency response, after the real acoustical problems have been corrected. Putting money directly into room acoustics has much greater bang for the buck than spending more on a fancy equalizer.


Regards,

Terry
 

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"you are saying you know without a shadow of a doubt that audio preamp processors do not have any adjustments for room correction?"


I am saying without a shadow of a doubt your Lexicon doesn't have room correction/eq in it.


It is hoped that Lexicon will be adding room correction to the MC-12 in a future upgrade using the four mics that can be used with MC-12 v3 software to automatically calibrate your speakers.


Shawn
 

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Thanks for the knowledge Shawn
 

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Discussion Starter · #17 ·
Terry - I would love to be able to treat the room properly, but as my installation will be the main living space in a very modern house, my options are next to nil. There are many debates as to the usefulness of room re-eq, but I figure that while I won't have "perfect" sound, I'll have at least better sound than I had without.


I could be mistaken, but from what I have read the pioneer models at least do preform some type of time domain correction, but of course contacting pioneer to confirm this is an exercise in futility. From your signature you appear to be in the acoustical business, so I'm sure you've run across many different setups so you might be in a much better position to confirm/deny this from your readings.


In either case, the reason I haven't just jumped on one of the new receivers with this ability is that I figure that at least a good chunk of the budget has gone towards the amplifier section, which I wont be using, and would rather be spent on the front end. Also I presume (never a good thing!) that the amp section produces no small amount of noise even when not in use, but I could be totally off base with this.


The reason I haven't jumped on the pre/pros that do have this is mainly cost, and in the case of the TacT, previous-generation surround processing (I will be listening 70% music / 30% TV/DVD) and enjoy the envelopment that the 6.1 processing from a 2 channel source *can* give in the poorly shaped (acoustically) room. It can also sound like dog meat if not done articulately with music!


But to cap a long reply - I agree that room treatement should be first, but is in some cases just not possible.


Todd
 

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tjsdesign


Go over to the Meridian website and read their

"white paper" on room correction. Also Tag has

some interesting reading. Try echo busters also.

I have the same problem as you but there are workable

solutions to your problems and looks good too.


Jim


:)
 

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Quote:
The Krell room correction is not parametric AFAIK, so it's more akin to the Pioneer and Yamaha solutions.
"...the Krell Showcase processor offers a parametric equalizer for each channel, giving an unheard of amount of flexibility to the end user. Filters for the equalizer include Notch, Peaking, High Pass, Low Pass, High Shelf, and Low Shelf. These filters allow you to dial in the sound to your room. For those not familiar with parametric equalizers, they allow you to flatten the response of the room by dealing with peaks and valleys in the overall room response. While this function cannot completely eliminate these problems, it can go a long way in dealing with certain issues. If you want to go the extra mile and use acoustic treatments as well, you can be sure that almost any problem with your room’s response can be handled."


That's an excerpt from the Hometheaterhifi.com review of the Showcase. Sure sounds parametric to me unless the reviewer severely misunderstood the details given by Krell. The Home Theater Mag review also makes reference to Krell's "...interesting room-equalization feature that lets you use three discrete parametric bands to adjust frequency response."
 

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Low bass can be room-corrected with a normal EQ, because they tend to be minimum phase phenomena (explanation of this to follow below). However, not all EQs are created equal for room correction, and here are some questions to ask any manufacturer who claims to have room correction:


1. What frequencies do they try to correct? Bass frequencies can be corrected well, but higher frequencies are problematic, especially over a wide listening area. To properly correct higher frequencies for one spot, you necessarily create pre- and post-echoes for other listening locations, so most room correction systems either correct only bass, or allow you to shelve down higher-frequency correction.


2. For the bass region, do they only attenuate peaks, or do they try to fill up notches and nulls? They should only try to attenuate peaks, because they are more audible, and nulls require unrealistic amounts of power and excursion to do any correction. Some nulls may require infinite amounts of power.


3. For the peaks that they correct, how precisely do they locate their filters? Craven, Bean, and Gerzon (CB&G from here on) have found that 1 Hz resolution is necessary to properly correct typical listening room bass problems. Put another way, their filter length is 1 second long, which corresponds to many room bass resonance lengths.


This can be difficult to do, because you will need a digital filter with 1 second of length. Given a 96 kHz sampling rate, a 1-second FIR requires 96000 taps, and so has to perform 96000*96000 accumulates (additions) per second, or over 9 billion additions per second. This is clearly beyond any reasonably affordable computer, so, if they claim 1 Hz resolution, ask them how they perform such computations. CB&G decimate low bass frequencies to 1 kHz (which requires only 1 million accumulates per second), filter, and then upsample the result back to the output sampling rate (96 kHz in this case, but 48 kHz when their papers were written). You can then ask how the upsampled bass frequencies were merged back into the main signal.


4. For higher frequencies, it can be useful to correct only for the speakers, because unlike higher frequency room acoustics, their output is relatively constant over a wide listening area (like a sofa). Ask them if they do this, and how they do this. Common solutions involve making the speakers flat and linear phase.


5. Ask them how much delay or latency the room correction adds. We talked about a 1-second filter length, so the delay has to be at least that long, and ask them how they compensate the video signal to line up with this.


6. Ask them how many measurement points they take. CG&B found that a set of 6 measurements over the listening area worked well, and tended to smooth out the bumps and notches (they aren't sure why yet).


7. Ask them how those measurements are averaged.


8. Ask them how they fit or derive their correction filters. There's some kind of ideal curve that they're going to be correcting to, and there's the actual curve that's measured. You can't usually come up with a precise filter to map the actual curve to the ideal curve, especially with multiple measurements, so you've got to get a close approximation. Some people use standard statistical methods like least-mean squares, which minimizes the energy of the difference. What does this mean? Nothing if you hear like a human being, because LMS really isn't perceptually relevant. So ask how their fitting algorithm works, and whether and how it accounts for the human perceptual system.


That's all the questions I can think of for now, and here's an explanation of minimum phase I promised above. This is a simplified explanation, and there's lots of hairy math behind it, but I think it's fundamentally correct and intuitive.


Every filter takes some time to do what it does --- there's some finite amount of time between the moment a signal enters a filter, and when it exits, analog or digital. For a filter to do something to the signal (like high-pass or low-pass or notch filter) requires it to do some amount of "processing" to the signal, or for the signal or parts of the signal to pass through more components so that the right kind of filtering is done. As a result, there will be some phase shift to the signal.


Phase shift is just a frequency-dependent time delay of a signal. It means some frequencies of a signal are delayed more than others. In a filter, some frequencies are delayed more than others because they're being processed more than others, hence the phase shift you see when applying filters. Phase shift in a filter is neither good nor bad --- it is just part of the filter, and it's the designer's job to make sure everything, including phase shift, is properly accounted for in the final design.


Knowing all this, we can get to what minimum phase means. A minimum phase system is a causal system whose phase shift is the minimum possible phase shift necessary to get the amplitude frequency response it exhibits. Causal means the system cannot predict the future: it only outputs something after the input has entered the system. Examples of the amplitude frequency response is what you see for most speaker measurements in magazines: the magnitude of the signal across frequency. An implication of minimum phase is that the phase response of a system is exactly determined by the amplitude frequency response of the system. You only have to measure one, and you get the other automatically.


Why do we care about minimum phase? Every 1/3 octave EQ and parametric EQ, and analog speaker crossover (passive or active), and RIAA vinyl correction curve is minimum phase. The great thing about minimum phase phenomena is that they can be perfectly corrected to flat amplitude and phase with another minimum phase device. This is why the RIAA vinyl curve works so well, and why low bass in a room can be, in theory, perfectly corrected, because they're minimum phase. The trick is to find the right correction filter to exactly complement what we're trying to correct, and this is the essence of the later questions in the list above.


There are also such things as acausal systems, but they aren't really predicting the future, because they just wait until the whole signal is in, and then output what they would have output if they could predict the future. This is usually done digitally because digital delay is straightforward. One result, for the linear phase digital filters usually used for over-/up-sampling, is that there is a pre-response that comes out before the main response. In some cases, despite the popularity of having linear phase, this isn't desirable, because you can create pre-echoes that are audible and unnatural (unnatural because, in the real world, there are no acausal systems and hence no pre-echoes), so sometimes having minimum phase instead of linear phase is desirable. Speaker correction comes to mind immediately.


Finally, to read a good and only slightly technical article on room correction, check out Michael Gerzon's article on it:

Digital Room Equalisation


Audiosignal is building an archive of Gerzon's harder-to-find papers, too, which is great because Gerzon was one of the deepest and broadest thinkers audio's ever had.


The paper's also far more useful and correct than some of the other stuff out there, like the POS paper Meridian has unfortunately put out.


--Andre
 
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