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Analysis of the NAD T752 bass management


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#1 of 14 OFFLINE   Jonathan M

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Posted April 06 2003 - 12:17 PM

Hi All, As promised, I analysed the bass management using a method I developed to measure it's frequency response over the weekend. It was quite successful, and I was able to easily verify that the NAD acts like one would presume it should under most conditions. For a start, the manual is incorrect. It states in the manual that with the sub on, and mains set to large, then any bass from other channels set to small is sent to both the sub and the mains. This is incorrect (Luckily!) and the bass gets sent ONLY to the sub which is good news. (No bass doubling.) I have measured the frequency response of a number of different scenarios and have concluded the following: High pass xovers on channels set to SMALL are at the specified xover frequency (If using a sub) OR at 100Hz (If no sub is present). They are 2nd order Butterworth filters. Low pass xover for any filtered bass is a 4th order Linkwitz Riley aligned xover at the xover frequency mentioned above. This is per the THX spec, I believe. The above applies to ALL digital sources. The LFE channel (If present) is boosted by 10dB relative to the other bass, before being mixed and sent to the sub, as it should be. I'm pretty sure that the LFE channel is not filtered in anyway before being sent to the sub (As it is already filtered on mastering) but I will have to confirm this. Analog sources do not have any bass management on them, unless they are being processed by a DSP. ie. they are analog direct - just passing through the volumea and tone control (If in place) circuitry. If the sub is set to ON, then a summed signal that is low passed at around 500Hz (I will confirm this later) at around 1st or second order is sent out the subwoofer pre-out. If the sub is off, then nothing is sent down the sub channel. The mains receive a full bandwidth signal - regardless of whether they are set to LARGE or SMALL. The best plan for people with this receiver is to use digital only inputs as much as possible so all bass management is effected. If you must also use analog inputs, then you may either apply a DSP to them, or do the bass management for the mains externally via the pre-out/main-in loop. I am just checking over my measurements over the next week or so, and will hopefully be posting a website with info including graphs of the frequency response etc. including how anyone can perform these same tests. If anyone has any questions - feel free to ask.
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#2 of 14 OFFLINE   Rich Wenzel

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Posted April 06 2003 - 06:33 PM

excellent useful post. thank you Rich
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#3 of 14 OFFLINE   Chris PC

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Posted April 07 2003 - 04:09 PM

Very useful. Confirms what others have said, unless you are the same one posting this in AVS. If so, well, thats cool too. So what DSP's can you apply to the stereo analog input? Are there any totally benign DSP's? I'd love to upgrade to the T752 or T762 from my Marantz SR 6200.
Going from projector to flatscreen for a while.... :P

#4 of 14 OFFLINE   Jonathan M

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Posted April 07 2003 - 04:21 PM

Indeed I have reported these results on the AVSforum. The DSPs are: Pro Logic, Pro Logic 2, DTS Neo:6, EARS, Matrix 7.1, and Enhanced Stereo 1 and 2. I'll try all of these, but the best setting will likely be using Enhanced stereo 1 with all speakers set to off except the mains (Set small) and sub. Assuming this doesn't alter the signal to the mains (It shouldn't) other than doing proper bass management, then that is the solution. You just have to have a speaker settings preset defined with any other speakers turned off. With the HTR-2 remote, it should be easy to set up a macro so that when you switch to an analog stereo input, it switches the speaker settings as well. I'll test this out tonight with any luck.
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#5 of 14 OFFLINE   Chris PC

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Posted April 08 2003 - 01:27 AM

Cool. Hopefully NAD hasn't done anything "goofy" in the "enhanced stereo" mode. Unfortunately, "enhanced" is not quite ideal. Worth a try though. Perhaps its normal sounding. I guess you can test and see. Should request that NAD provide a totally blank stereo DSP mode designed simply to provide bass management. Oh well, ony T762 has the RS232 port.
Going from projector to flatscreen for a while.... :P

#6 of 14 OFFLINE   James Edward

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Posted April 08 2003 - 03:03 AM

Chris- I think what you mean about 'enhanced' does not apply to the 752. Why they use that word, I don't know. It is not processed in any way- the enhanced 1 mode is the same as '5 channel stereo' with no processing. Enhanced 2 mode turns on only the back(surround) speakers, with no other processing. This could be useful at night.
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#7 of 14 OFFLINE   BruceD

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Posted April 08 2003 - 03:29 AM

So, do these enhanced stereo modes, or ALL DSP modes, offered by NAD essentially run the analog input signal through an ADC-DAC cycle? There are only two ways to get bass management (sub output)from an analog input source in a digital prepro/receiver; 1) Run the analog input signals through an ADC-xover-DAC cycle sending output to mains, sub, etc. or 2) Take a copy of the analog input signal (run it through an ADC-xover-DAC cycle) sending the bypassed (untouched) full range signal to the mains and a copy of the low-pass bass to the sub. This is typically how Rotel and Outlaw do it and it also causes the "double bass" issues.

#8 of 14 OFFLINE   Jonathan M

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Posted April 08 2003 - 08:47 AM

Ok, I tried the various DSP's out last night on an analog input in order to see if there is a benign DSP that allows bass management to be involved. First off, the 5 (or 7) channel stereo option (Extended Stereo 1) and the rear-only stereo option (Extended Stereo 2) are not enabled if one turns the rears etc. off, so this is not an option. EARS is also not available when the rears are off. It is also unclear whether any A/D D/A conversion is taking place in these modes. I tried the other modes out, and the "best" sounding ones (With all speakers off except the mains) were Prologic 2 Music and DTS:Neo6 Music. It was unclear in the few minutes I tested whether these were removing anything from the left and right channels. I know with PL2 that one can have a fully phantom centre which is what I enabled. The differences I heard between these modes and stereo may be due to a change in levels, so it is not clear to me whether this would be an option or not. I'll email NAD regarding this. I'll also email NAD regarding which DSP modes run the analog signals through the ADC/DAC and which don't. Whether or not they would be able to add a "blank" DSP mode which does the ADC/Bass management/DAC procedure to the T762 I really don't know. Probably worth asking, however. This "flaw" with the T752/762 is NOT a huge issue IMO. Most analog stereo sources are in the form of TV broadcasts etc. a lot of which are either in Dolby Surround already or which are going digital anyway. I much prefer that there is an analog direct mode, rather than there being only a ADC/DAC mode. Ofcourse, both would be the best solution. I'll report back if/when I get a reply from NAD.
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#9 of 14 OFFLINE   BruceD

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Posted April 08 2003 - 09:32 AM

Jonathan, By the way, I'll be most interested in your procedures and look forward to your website link. For 2-channel sources, I use a turntable and I prefer the DACs in my CD Player to those I have heard using the CD player's digital connection to a number of prepros. This means I don't want to run my analog sources through an unnecessary ADC/DAC cycle. Unfortunately many prepro/receiver manufacturers don't understand this very well and simply don't provide well designed analog passthru. Good discussion!

#10 of 14 OFFLINE   Dan Lindley

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Posted April 08 2003 - 03:51 PM

Jonathan, Please understand that I may be clueless, but why the concern with 'bass doubling'? If the mains are set to large, isn't that supposed to mean that bass is sent to the mains (and to the .1 channel?)? Or did you mean by 'other channels' that the 752 does not allow bass from the rears to go to the mains? Thanks for helping. Had NAD in past and thinking about them in the future... Dan
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#11 of 14 OFFLINE   Jonathan M

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Posted April 08 2003 - 05:16 PM

The problem with bass doubling is when bass that is supposed to go to the mains ends up being sent both to the mains and the sub. Thus the bass is emanating from 2 sources simultaneously. This will cause either constructive (Bass doubling) or destructive interference throughout the room (ie in some places it'll be real loud, in others there'll be virtually nothing). The NAD operates as it should on digital sources. With mains set to large, and the rest set to small and the sub off, all bass in the rest of the channels below 100Hz or so goes to the mains as does the LFE .1 channel. With the mains set to large, the rest small, and the sub on, the bass info for the mains is sent to the mains, and all other bass is sent to the sub only. ie. all redirected bass only ever ends up in 1 place - it is never replicated in more than channel pair. In ANALOG STEREO mode, the NAD does have flaws. If the sub is turned on, then all bass info from the source up to around 500Hz or so is sent to the sub (Which will no doubt filter a lot of 150Hz + stuff out) and the mains get a full signal also. Thus the bass is replicated. This can be worked around by turning the sub off. Hope this helps.
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#12 of 14 OFFLINE   Mike Up

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Posted April 08 2003 - 06:17 PM

Jonathan,

Are you sure about 500Hz? My Denon AVR-3803 will also do bass doubling in the direct modes if I don't turn off the subwoofer output for those individual sound modes. The L/R main speakers get analog bypass while the subwoofer output is derived from digital bass management and uses it's lowpass crossover. I have mine set to 80Hz. I usually set my subwoofer off for Direct and Pure Direct modes but usually listen to stereo mode that includes highpass and lowpass filters through(which I believe) digital bass management using Denon's parallel bass management circuit.

It sounds as the NAD is the same but without the option of using full bass management as the Denon does in 'STEREO' mode. Since the NAD's analog stereo mode looks to be the same as Denon's 'DIRECT' mode, I would think that the subwoofer would still use the digital bass management's lowpass crossover setting. If it doesn't, that's a double negative as now frequency doubling or cancellation can occur all the way up to 500Hz while using a LFE input/crossover bypass of most subwoofers. Even if the sub can't go up to 500Hz, a good portion of high bass, lower midrange will be affected.Posted Image

I have my subwoofer positioned so that it doesn't cause bass cancellation when using it along with the Direct modes. It just causes bass doubling. With my current Infinity 2000.4 tower speakers, Direct modes sound better with bass doubling as my speakers don't go much under 45Hz. The bass doubling isn't as bad as the lack of bass. I find stereo mode to sound best even though I don't think it's an analog bypass. I leave the sub off in Direct modes just so it's immediately noticeable if it gets switched from STEREO by accident. While there's bass doubling, it's not severe and quite mild, yet STEREO still sounds better without any doubling.

Have a good one.Posted Image

#13 of 14 OFFLINE   Jonathan M

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Posted April 09 2003 - 09:30 AM

Mike, Yes, I'm sure about the output filter on the subwoofer pre-out. It is purely an analog filter likely positioned at the output of the final buffer. The reason it is at 500Hz is so that it does not interfere with any other filter implemented for digital bass management (Which can go from 40-200Hz on the NAD). In analog stereo mode, the Mains get a full signal (pure analog) and that signal is summed (pure analog) and sent to the sub-woofer output (Then filtered at 500Hz by the output filter) if the subwoofer is set to ON. If the sub is off (Either in the speaker settings, or physically off on the sub itself) one gets no problems with bass doubling etc, as nothing is output from the sub. The ONLY time you get bass "doubling/cancellation" is when you have the sub ON and you are playing an analog signal. Ofcourse, most subs have a lowpass filter on them anyway, so for most people, there is always a solution to be found. The NAD has the distinct advantage of the pre-out/main-in loop, so if anyone really wants to take care of all scenarios, a simple xover between this loop can be added in. I have talked to NAD technical people who have confirmed my findings. They have also offered to send me upgraded software for my unit. I don't know what this does yet, exactly, but I'll be sure to post how it goes and what (if anything) it changes. If you're interested in testing your 3803, I'll be posting my testing technique probably in a week or so. It's easy and quick to do - you just need a computer with soundcard and CD burner.
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#14 of 14 OFFLINE   Chris PC

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Posted April 10 2003 - 12:32 PM

Take the offer. Try the software and let us know what it does. Thanx for the insight. The xover in the main loop is the simplest answer and provides the best results for both music and home theatre I imagine.
Going from projector to flatscreen for a while.... :P




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