# REW for measuring electrical response of Pre-amp and using external tones?



## jmcomp124 (Feb 27, 2007)

I would like to measure the electrical response of a pre-amp's subwoofer/.1 LFE output to examine the following..

1. When a room correction mechanism like Audyssey or Anthem ARC is used, is it consistent across sources, for example what happens when the player decodes vs when it bitstreams? This thread indicates a bug that was discovered in a Denon receiver but it only alludes to how it was done with REW with no specifics. 
2. I would like to use an external source like this DIY Audio test dvd or a movie clip with challenging LFE like WOTW pod emerges, and look at the electrical response in REW.
3. I would like THD and THD+N measurements for just the subwoofer output of the pre-amp with no actual subwoofer.

Basically, I want to measure the pre-amp LFE separately and then measure the actual room response with a subwoofer and Galaxy Mic. 

There are several conflicting reports (maybe I am misunderstanding) that kind of address if the above is possible or not with REW. For example
this says external source is not possible
this says you can use the internal tones to measure THD and THD+N which is also what I am interested in
While some have accomplised an external source like this post, is it accurate?

Please clarify.

I can think of a few ways of measuring the pre-amp LFE output electrical response. First if I want to use the internal tones in REW, it is pretty straightforward when the analog output of the sound card is used. Analog output of the card to analog input of the receiver, use a listening mode like stereo, set the Xover appropriately and then take the .1 LFE output from the pre-amp and connect it to the soundcard line-in, use REW internal tone, examine and have fun.

However, if I want to use a DD 5.1 source, bitstreaming from a player like the PS3 vs Decoding to PCM in the PS3 transfer via HDMI to the pre-amp and then measure the .1 LFE of the preamp, is that possible? I would to turn of Audyssey on/off, Dynamic EQ on/off, Dynamic Volume On/Off, and measure the electrical response and possibly examine the frequency response, THD, THD+N etc.

Is the above possible. I am pretty familiar with ETF so all that I need now is some direction. I plan on buying and external sound card like the turtle beach SRM. Maybe I need all it's capabilities like the optical SPDIF output in that to use DD 5.1 sources played on the PC. If one of you knowledgeable folks can give some direction, it would be much appreciated. I am asking for a lot, I promise in return, I will faithfully post all my findings so it woudl help a wider audience. 
Thanks in advance,
-Jai


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## glaufman (Nov 25, 2007)

Lots of questions here, I'll try and step through them 1 by 1...
First, yes, you can make a loopback connection from your soundcard out, through your pre-pro, to the soundcard in, and measure the response of your equipment. For this you don't need an external source.
Second, if you're just doing comparisons, there's nothing wrong with using a spdif output from the soundcard into the prepro, just understand that because the calibration will be off, you're not looking at an absolute FR, you can only compare FRs from mode to mode.
Third, yes you can use an external source with REW IN RTA MODE. For instance, there's nothing wrong with using an external source to generate pink noise with REW in RTA mode to get a picture of the response. You CANNOT, to the best of my knowledge, use an external source to run REW's sweep to get the response used to calculate the IRs and Waterfalls. REW gates the response it measures, and syncing the routine with the sweep is critical.
For THD or THD+N, feed REW a sine wave and use in RTA mode. The sine wave can be internal or externally generated.
The post you linked to regarding external sources, I'm not really sure what the poster was trying to do, how, or why. 
Feel free to read up on REW in the sticky at the top of this forum, and also the manual.


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## jmcomp124 (Feb 27, 2007)

glaufman said:


> Lots of questions here, I'll try and step through them 1 by 1...
> First, yes, you can make a loopback connection from your soundcard out, through your pre-pro, to the soundcard in, and measure the response of your equipment. For this you don't need an external source.
> Second, if you're just doing comparisons, there's nothing wrong with using a spdif output from the soundcard into the prepro, just understand that because the calibration will be off, you're not looking at an absolute FR, you can only compare FRs from mode to mode.
> Third, yes you can use an external source with REW IN RTA MODE. For instance, there's nothing wrong with using an external source to generate pink noise with REW in RTA mode to get a picture of the response. You CANNOT, to the best of my knowledge, use an external source to run REW's sweep to get the response used to calculate the IRs and Waterfalls. REW gates the response it measures, and syncing the routine with the sweep is critical.
> ...


Greg,
Thanks a lot for the answers. 
While using an external source with REW in RTA mode, I think you mean a source not in the REW software, but some other source material on the computer residing as a media file that can be loaded into REW, is that right?
Is it possible to use an external source that is a different program on the same computer, for example play a DVD through theatertek, use the SPDIF output into the preamp, loopback from the preamp to the sound card and sample the .1 LFE while your scene/track of interest is playing?
Secondly, is it possible to use an external hardware, like a PS3 to play a source and then use the .1 LFE from the preamp to the sound card while your scene/track of interest is playing, same as in the previous case?
From the above two, I would like FRs and as you indicated, since I would be comparing only mode vs mode (Example "preamp1 vs preamp2" or "mode 1 in preamp1 vs mode 2 in preamp1" etc) the relative difference is what I am interested in rather than absolute values. 
Thanks again :bigsmile:


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## glaufman (Nov 25, 2007)

jmcomp124 said:


> While using an external source with REW in RTA mode, I think you mean a source not in the REW software,


Yes.


> but some other source material on the computer residing as a media file that can be loaded into REW, is that right?


Not really. What I meant was a CD/DVD/BD/other connected to your system, playing test tones, and only using REW (in RTA mode of course) to record the sound produced in the room.


> Is it possible to use an external source that is a different program on the same computer, for example play a DVD through theatertek,


Hmmm... maybe, you'll have to try it, or someone more knowledgable than I will have to answer... it's possible it could create a conflict, but I'd go ahead and try it. The downside is that you'll never know for sure exactly what dB anything SHOULD be at, because it won't be calibrated, but for comparing one setting to another it should be fine if it works.


> use the SPDIF output into the preamp, loopback from the preamp to the sound card and sample the .1 LFE while your scene/track of interest is playing?


Should be ok as long as you accept that it won't be "calibrated". But for before/after comparisons, should be fine given the above caveat on conflicts.


> Secondly, is it possible to use an external hardware, like a PS3 to play a source and then use the .1 LFE from the preamp to the sound card while your scene/track of interest is playing, same as in the previous case?


That should work, but again you have to realize that what you read won't be calibrated. For the record, once it comes out of the preamp, the signal you mention shouldn't be LFE any more, it should be SUB, but that's mostly semantics. 
From the above two, I would like FRs and as you indicated, since I would be comparing only mode vs mode (Example "preamp1 vs preamp2" or "mode 1 in preamp1 vs mode 2 in preamp1" etc) the relative difference is what I am interested in rather than absolute values. 
Thanks again :bigsmile:[/QUOTE]

Hope it works, please post your results so we can all learn from them :nerd::T:wave::bigsmile:


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## jmcomp124 (Feb 27, 2007)

glaufman said:


> Yes.
> 
> Not really. What I meant was a CD/DVD/BD/other connected to your system, playing test tones, and only using REW (in RTA mode of course) to record the sound produced in the room.
> 
> ...


Hope it works, please post your results so we can all learn from them :nerd::T:wave::bigsmile:[/QUOTE]

Greg,
I sure will post as I learn more. For now, the sound card has been the showstopper since the RME did not work. I had another desktop that I moved to my HT room that has Sigmatel HD Audio with Line in/out and digital coax out on the motherboard. This should work right? I just have to deal with the inconvenience of the desktop for now. I should be soon up and running with REW if all goes well.

Regarding THD/THD+N measurements, I want to be able to measure the pre-amp subwoofer output's distortion when approaching reference levels and maybe reference and beyond. I want to try this in the following configurations...
a) Audyssey on and off
b) Audyssey DEQ on/off
c) Audyssey DVOL on/off
d) Bitstreaming DD from player and let preamp decode and repeat a) b) c)
e) Decode DD to PCM from player and repeat a) b) c)

I have a suspicion that EQ is introducing mild distortion. Don't ask me why. Just by ear. I want to measure and find out if it is just imagination. Once I get the hang of REW I want to compare pre-amps like the Integra 40.1 to the 80.1 and maybe the Statement D2/v w/ARC

Is this possible only with the internally (from REW) generated sine wave or can I use an external BD/DVD player that plays a sine wave and then measure from within REW?
My desktop has an SPDIF output, so if I want to do uncalibrated mode to mode THD and THD+N comparisons, I either would like to use the SPDIF output on the desktop. This would not faciliate e) above. For e) Ideally I would like to play a sine wave in an external DVD/BD player and measure the THD/THD+N in REW while decoding in the player vs decoding in the preamp. Am I on the right track?.:scratch:

BTW, the reason I want to use the digital output from the sound card or the external player is to avoid the A->D conversion that would happen in the pre-amp if I were to use the analog line out of the sound card. Will such a test that I am contemplating be valid for THD/THD-N when uncalibrated but done mode vs mode?

Hope you understand what I am trying to quantify. I am tired of wondering, and asking and guessing. This time I want to measure and find out for myself. With help from folks like you here, I should be able to make progress.
Thanks a lot!
Here is some context about where I am coming from and why I have all these questions...
Anthem ARC and Audyssey MultEQ Pro
Why does Audyssey in some cases sound too bassy/not tight etc..I have a theory.. 
post# 21655 and 21659 showing Audyssey electrical response


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## Ricci (May 23, 2007)

Jai,

I've done some testing of random receiver and processor sub pre-outs that I own and they are surprisingly bad (Pioneer, Onkyo, NAD). Nothing fancy and just for my own information about my units. 

At a REF volume setting and the pre-out trim at -10db they are all distorting already with a cd level output. Movies have much hotter output levels with the LFE track so who knows how much is there. I simply watched it with Spectrum Labs waterfall display so i didn't calculate the THD % but you can clearly see it increase with the level. There are people who run their pre-out trim well above 0db or even cranked. They are probably clipping the snot out of them with any sort of movie playback volume. 

I'd be interested in seeing what you come up with.


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## jmcomp124 (Feb 27, 2007)

Ricci said:


> Jai,
> 
> I've done some testing of random receiver and processor sub pre-outs that I own and they are surprisingly bad (Pioneer, Onkyo, NAD). Nothing fancy and just for my own information about my units.
> 
> ...


As much as I appreciate your input, I am disappointed as this is the first time I get clear confirmation that my gut feelings were right. :unbelievable:. I was hoping to be wrong. 
The sad thing is most folks like those distortions as it gets pretty loud, walls shake, paint peels, their subwoofer cooks them dinner and all those things happen. For someone who is used to good quality sound this becomes a nightmare experience. I really would like to learn from your exeperience Ricci.Can you elaborate a bit more on how you saw distortion visually? Were you able to use digital inputs?
Thanks!


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## bjs (Jun 12, 2008)

Ricci said:


> I've done some testing of random receiver and processor sub pre-outs that I own and they are surprisingly bad (Pioneer, Onkyo, NAD). Nothing fancy and just for my own information about my units.
> 
> At a REF volume setting and the pre-out trim at -10db they are all distorting already with a cd level output. <etc>


Does your measurement INPUT chain have enough distortion headroom to measure this?

My soundcard input, for example, cannot handle the full output of my sub-out signal without distorting. This distortion occurs well before clipping and it doesn't help to reduce the input volume control as it is the input stage prior to the volume control that is overloading. 

One needs to verify this when making electronic loopback measurements (and also when using a mic as the source if measuring very high SPL levels).


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## JohnM (Apr 11, 2006)

You can use the REW Spectrum/RTA view to show and calculate THD for tones played on other sources. However, you need to take care that there is no clipping in the soundcard, a CD level output will easily clip a typical line level input. It would be unusual for just about any processor to show significant levels of clipping or distortion on its line outputs.


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## jmcomp124 (Feb 27, 2007)

bjs said:


> Does your measurement INPUT chain have enough distortion headroom to measure this?
> 
> My soundcard input, for example, cannot handle the full output of my sub-out signal without distorting. This distortion occurs well before clipping and it doesn't help to reduce the input volume control as it is the input stage prior to the volume control that is overloading.
> 
> One needs to verify this when making electronic loopback measurements (and also when using a mic as the source if measuring very high SPL levels).


bjs,
Ignoring SPL levels, how does one guarantee that distorton does not occur at the soundcard input?
Also, how did you know that such a distortion was happening at the line input in your sound card? Did it show up in REW?
Thanks,
-Jai


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## jmcomp124 (Feb 27, 2007)

JohnM said:


> You can use the REW Spectrum/RTA view to show and calculate THD for tones played on other sources. However, you need to take care that there is no clipping in the soundcard, a CD level output will easily clip a typical line level input. It would be unusual for just about any processor to show significant levels of clipping or distortion on its line outputs.


Again,
what are the best known ways to avoid this type of clipping in the sound card? Are there special high quality ones that are recommended?


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## JohnM (Apr 11, 2006)

It is not a sound card quality issue, just signal levels - driving the input with a larger signal than it is designed to accept. The audio spec for soundcard inputs is typically 1.0V rms full scale, whereas a CD player has 2.0V rms full scale output. AV processor subwoofer outputs can often generate even higher signal levels. You get a good indication by looking at the VU meters in REW to make sure the peaks on the input never reach 0dB FS. At levels close to full scale the inputs are more prone to distortion, but clipping in the input stage generates high levels of odd order distortion.


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## Ricci (May 23, 2007)

bjs said:


> Does your measurement INPUT chain have enough distortion headroom to measure this?
> 
> My soundcard input, for example, cannot handle the full output of my sub-out signal without distorting. This distortion occurs well before clipping and it doesn't help to reduce the input volume control as it is the input stage prior to the volume control that is overloading.
> 
> One needs to verify this when making electronic loopback measurements (and also when using a mic as the source if measuring very high SPL levels).


I'm fairly certain that it does, but I cannot absolutely rule it out. The levels are at well below the level of clipping the SC input when I see the distortion appear and start to increase. The SC I'm using is a Turtle Beach AA SRM. Nothing special to be sure, but I run the input sensitivity at minimum and use Spectrum Labs to look at the output of sine waves (you could use REW ). However the fact that the different units (even 2 different Onkyo's 806 and 886) all exhibit different spectra and at different levels does give me some confidence that I'm seeing THD from the receiver output. Could be a combo of both the SC input and the pre out signal....

Just for example in my 3 systems I run my amplifiers with input levels full and my SW pre-out is all of the way down at -10 or -15db from the processor or receiver. I use DCX's or a Rane PE-17 in between, both of which are pro units and allow for a wide range of signal boost or cut. I use those to get the level to the subs to where it needs to be for calibration. Interestingly in none of the cases is a signal boost needed of the receiver sub signal into the amps. It's in the pass through to -3db range. All of my amps have either adjustable input sensitivity or a modified one though.

When I was checking the SW pre-out I just used a sine wave out of REW like 30hz -3db into the cd input, while monitoring with Spectrum Labs (or REW) and run the pre-out into the sc with the input sensitivity at it's lowest setting and the sub pre-out at it's lowest level setting too. I set the main volume to whatever REF level would be and from there increase the sw pre out level until you start to see distortion show-up. Continue increasing the level and monitoring, paying attention to the input level. Try a few different frequency of sines like 10, 40, 63, etc. You may want to disconnect the mains BTW or turn them off.

Let me see if I saved any of the screenshots...


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## bjs (Jun 12, 2008)

Ricci said:


> ...The levels are at well below the level of clipping the SC input when I see the distortion appear and start to increase.


Yes, as I mentioned soundcard distortion can begin to rise well below clipping. That's why people need to check the actual distortion at the levels being used.



ricci said:


> I run the input sensitivity at minimum


As I mentioned previously reducing input sensitivity doesn't necessarily help as the problems occur prior to that stage. On my soundcards I leave the input volume on full for distortion testing since if the signal is too strong for full volume then I know that it will have excess distortion regardless of the input level setting. 

Anyway, the test you describe in your last paragraph isn't valid for these reasons. You really do need to independantly verify the soundcards distortion at the levels you are using. Feel free to prove me wrong, but I'm fairly certain you are mostly measuring soundcard distortion.


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## bjs (Jun 12, 2008)

JohnM said:


> ...You get a good indication by looking at the VU meters in REW to make sure the peaks on the input never reach 0dB FS...


Unfortunately it is not sufficient to look at the VU meters *if* you've had to reduce the input sensitivity to achieve a decent reading.

This is a critical point for anyone trying to measure high (ie more than 1v RMS) voltages with their soundcards.


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## jmcomp124 (Feb 27, 2007)

bjs said:


> Unfortunately it is not sufficient to look at the VU meters *if* you've had to reduce the input sensitivity to achieve a decent reading.
> 
> This is a critical point for anyone trying to measure high (ie more than 1v RMS) voltages with their soundcards.


I did a quick measurement with a voltmeter on the subwoofer pre-out. A 30Hz sine wave at reference level (played from an external player, PS3 in this case) was well below 1v. It was around .5 in the integra 40.1 with subwoofer trim level at 0. However playing pod emerges WOTW DTS, I saw spikes of about 3v. Even my Rane PE-17 had it's overload LED light up bright. With a 30Hz sine wave at least if I am measuring correctly, chances of overload at the sound card line in should be very low even at reference levels.
Since Ricci used sine waves in the 30Hz range, I don't think he must have encountered clipping and distortion at the line it at around .5 v unless his receivers are very different from the Integra.


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## jmcomp124 (Feb 27, 2007)

Ricci said:


> I'm fairly certain that it does, but I cannot absolutely rule it out. The levels are at well below the level of clipping the SC input when I see the distortion appear and start to increase. The SC I'm using is a Turtle Beach AA SRM. Nothing special to be sure, but I run the input sensitivity at minimum and use Spectrum Labs to look at the output of sine waves (you could use REW ). However the fact that the different units (even 2 different Onkyo's 806 and 886) all exhibit different spectra and at different levels does give me some confidence that I'm seeing THD from the receiver output. Could be a combo of both the SC input and the pre out signal....
> 
> Just for example in my 3 systems I run my amplifiers with input levels full and my SW pre-out is all of the way down at -10 or -15db from the processor or receiver. I use DCX's or a Rane PE-17 in between, both of which are pro units and allow for a wide range of signal boost or cut. I use those to get the level to the subs to where it needs to be for calibration. Interestingly in none of the cases is a signal boost needed of the receiver sub signal into the amps. It's in the pass through to -3db range. All of my amps have either adjustable input sensitivity or a modified one though.
> 
> ...


I have the Rane PE-17 too which is a very nice unit.
Ricci, where were the sliders set at? (input gain and output gain)?
Are both all the way up? That is what is recommended to reduce noise. To avoid overload,
I have them set at unity gain, two dashes below the top. My crown CE4000s are set at input sensitivity 
at 1.4 (33.8dB gain) and the gain knob is at 3/4th in bridged mode. The subwoofer output is at -12 trim level. Even with the most intense movie bass (WOTW example) the overload on the rane never lights up.
It looks like at approx 2v the PE-17 begins to overload. 


Excerpt from RANE PE-17 manual...

For optimum noise performance, you should take as much
gain at the Input as you can, with a corresponding reduction at
the Output. This is done by moving both Gain sliders upward.
Note that the In control is calibrated top-to-bottom +12 dB to
-12 dB, while the Out control is calibrated just opposite. Thus,
by moving both sliders together, you always maintain unity gain
through the Equalizer, but you vary how much gain/attenuation
occurs at the Input or Output of the unit. Therefore, by operating
the unit with both sliders as high as possible, you take a lot
of gain up-front, while simultaneously reducing the signal (and
noise) at the Output. Use the Overload light as a guide: occasionally
lighting is set just right. Continuously lighting is too
much gain, while never lighting is too little gain.


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## Ricci (May 23, 2007)

bjs said:


> Yes, as I mentioned soundcard distortion can begin to rise well below clipping. That's why people need to check the actual distortion at the levels being used.
> 
> 
> 
> ...


You are probably right. I was not trying to measure voltage actually at the time. I was measuring the pre-out FR and trying to set the gain structure up to avoid clipping the inputs and just happened to try what I described above as a side test. If I was to set my sc's input sensitivity at maximum it would be clipping all over the place with even a modest level. I do distinctly recall that I got different results with different units though.:scratch:

How can you possibly get a valid distortion test with a mic if your SC is limiting you to a very low level input before it adds distortion of its own? (seems like from what you've described this is the case with yours as well?) I guess this limits to keeping the level low with very low output from the mic mixer? This before even getting to possible microphone distortion issues. 

How would you recommend measuring SC input/ output distortion?


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## Ricci (May 23, 2007)

jmcomp124 said:


> I have the Rane PE-17 too which is a very nice unit.
> Ricci, where were the sliders set at? (input gain and output gain)?
> Are both all the way up? That is what is recommended to reduce noise. To avoid overload,
> I have them set at unity gain, two dashes below the top. My crown CE4000s are set at input sensitivity
> ...


I use my Crown CE4000's and Crest 8002's at the highest sensitivity input settings. The Rane is in use with one of the CE4000's and is set to attenuate about -3db total if I remember right. It's been a while since I set that up. The input is about at 2 marks down and the output attenuation is up a little higher.


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## jmcomp124 (Feb 27, 2007)

:scratch:


Ricci said:


> You are probably right. I was not trying to measure voltage actually at the time. I was measuring the pre-out FR and trying to set the gain structure up to avoid clipping the inputs and just happened to try what I described above as a side test. If I was to set my sc's input sensitivity at maximum it would be clipping all over the place with even a modest level. I do distinctly recall that I got different results with different units though.:scratch:
> 
> How can you possibly get a valid distortion test with a mic if your SC is limiting you to a very low level input before it adds distortion of its own? (seems like from what you've described this is the case with yours as well?) I guess this limits to keeping the level low with very low output from the mic mixer? This before even getting to possible microphone distortion issues.
> 
> How would you recommend measuring SC input/ output distortion?


Ricci,
I was wondering about the same thing...:scratch: but then came up with the following...
Couldn't you use the Rane PE-17 in the path to the sound card line-in to attenuate the signal to be less than 1v (use a voltmeter) ? This would avoid clipping in the line-in.


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## jmcomp124 (Feb 27, 2007)

Ricci said:


> I use my Crown CE4000's and Crest 8002's at the highest sensitivity input settings. The Rane is in use with one of the CE4000's and is set to attenuate about -3db total if I remember right. It's been a while since I set that up. The input is about at 2 marks down and the output attenuation is up a little higher.


Rane PE-17, CE-4000, almost same calibration, almost same worries... looks like we are operating in about the same frequency :bigsmile:
Do you have the same insane 18" LMS-5400 based woofers by any chance?
:heehee:


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## jmcomp124 (Feb 27, 2007)

Ricci said:


> You are probably right. I was not trying to measure voltage actually at the time. I was measuring the pre-out FR and trying to set the gain structure up to avoid clipping the inputs and just happened to try what I described above as a side test. If I was to set my sc's input sensitivity at maximum it would be clipping all over the place with even a modest level. I do distinctly recall that I got different results with different units though.:scratch:
> 
> How can you possibly get a valid distortion test with a mic if your SC is limiting you to a very low level input before it adds distortion of its own? (seems like from what you've described this is the case with yours as well?) I guess this limits to keeping the level low with very low output from the mic mixer? This before even getting to possible microphone distortion issues.
> 
> How would you recommend measuring SC input/ output distortion?


But like I indicated in one of my previous posts, you may not have reached clipping levels at all if you were simply using sine waves that always stay under 1v (at least in my integra). The only variable is that you were using CD analog inputs and I measured the voltage with a digital signal fed to the pre. Can't see why a A-D conversion like in your use-case would cause the voltage to be higher. If you have a voltmeter in hand and can take a quick measurement if you still have those avrs handy, that would lay to rest some lingering questions. Thanks for your participation and feedback Ricci. Much appreciated :clap:


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## Ricci (May 23, 2007)

Jai,

I'm using the CE4000 and Rane PE-17 on the ported XXX 18. The LMS 18 gets a bridged Crest 8002/DCX2496 combo. :sn:

What SC do you use? That could explain some differences. Also I'm using an Onkyo unit and I never drove the output to clipping, just to the point where I started to clearly see distortion in SL.


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## jmcomp124 (Feb 27, 2007)

Ricci said:


> Jai,
> 
> I'm using the CE4000 and Rane PE-17 on the ported XXX 18. The LMS 18 gets a bridged Crest 8002/DCX2496 combo. :sn:
> 
> What SC do you use? That could explain some differences. Also I'm using an Onkyo unit and I never drove the output to clipping, just to the point where I started to clearly see distortion in SL.


Ricci,
I have been using the RME Digi 96 PAD with ETF. Just getting started with REW. I haven't even done one measurement yet with REW. I am in the data collection and setup phase now. Want to start things right. I do have a CM140 and I have setup a desktop that has line in (Sigmatel) so I should be up and running soon. I ran into issues with REW and RME that stalled things. BTW, the voltage tests I mentioned here in this thread does not have any Sound Card involved. Tones were played with a PS3 and outputs at the preamp subwoofer output was measured.


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## bjs (Jun 12, 2008)

I had some time before breakfast so did a quick test of my Pioneer Receivers Sub-out (VSX-816). I raised the sub-out Xover to maximum (200Hz) and then generated a 20Hz sinewave in REW and cranked the volume on the Receiver up until I was getting 4.2v RMS out (that's right it was swinging almost 12Volts peak to peak out of the Sub-out on the Receiver!!!).

Since my power amp (Behringer ep4000) clips at full power around 1.7v RMS (as I recall) this is way in excess of anything reasonable. Attached is the REW distortion plot. As you can see THD distortion of the entire chain remains below 0.017% even at these crazy levels (and that inclues the pesky little 60Hz hum skewing the results...!).









For the test I looped the Sub-out into the Soundcard input via a -20dB resistor pad to keep the Soundcard operating within it's linear region (which on this particular card is anything below 1.4v RMS...they all seem to be a little different).

Also 4.2v RMS was not the maximum clean output from the sub output. I just picked that level for a healthy example and didn't bother to find the maximum clean level.

Hopefully this helps rather than confuses the discussion...


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## bjs (Jun 12, 2008)

Ricci said:


> ...How can you possibly get a valid distortion test with a mic if your SC is limiting you to a very low level input before it adds distortion of its own? (seems like from what you've described this is the case with yours as well?) I guess this limits to keeping the level low with very low output from the mic mixer? This before even getting to possible microphone distortion issues.


Yes, when measuring at very high SPLs one has to pay attention to details. Don't assume you mixer can handle the peak output from your microphone either. Of course it depends on your microphone but even a simple $2 Panasonic mic capsule by itself (ie no other electronics) can clip the LINE level input on a soundcard (and some mixers) at 135dB (for example). Input distortion will have begun to rise below that level. This is partly why cheap measurement mics fall apart at high SPLs...their electronics can't handle the capsule output at those levels.

If your mixer can't handle the mic output then you'll have to pad the mic down with a resistor pad. Signal to noise will suffer a little but usually not a problem for speaker measurements.




> How would you recommend measuring SC input/ output distortion?


I wouldn't worry about the soundcard output except to loop it back at a safe level and verify it is extremely low.

If you suspect the input is overloading then add a resistor pad to the input and re-measure (without changing anything else). If the distortion drops then your input is indeed overloading. Using a variable pad from a fixed (higher than clipping) drive level will allow you to find the point where distortion begins to rise (typically somewhat below clipping).


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## jmcomp124 (Feb 27, 2007)

bjs said:


> I had some time before breakfast so did a quick test of my Pioneer Receivers Sub-out (VSX-816). I raised the sub-out Xover to maximum (200Hz) and then generated a 20Hz sinewave in REW and cranked the volume on the Receiver up until I was getting 4.2v RMS out (that's right it was swinging almost 12Volts peak to peak out of the Sub-out on the Receiver!!!).
> 
> Since my power amp (Behringer ep4000) clips at full power around 1.7v RMS (as I recall) this is way in excess of anything reasonable. Attached is the REW distortion plot. As you can see THD distortion of the entire chain remains below 0.017% even at these crazy levels (and that inclues the pesky little 60Hz hum skewing the results...!).
> 
> ...


bjs,
Excellent :T
Doesn't confuse but really helps the discussion. Thanks!
It is encouraging to know that the receiver is not distorting at the subwoofer pre-out. A few clarifications, what was the master volume at and the trim level of the subwoofer?
I would like to do a similar test and exactly what parts should I buy to get the -20dB attenuation in the signal path? I don't want to introduce any noise when I do the attenuation to bring the levels down to < 1v.
One reason I went with the Rane rather than the Behringer (which I used to use for EQ a while ago) is exactly because of the overload and clipping issue. The Rane had a lot more tolerance to input levels. I put up with it though it is not as precise as the digital behringer. I use ETF to set the subsonic filter. 

Now with my subwoofer trim at -12 and making sure the Rane is not overloading, the bass sounds nice and clean. So I don't think it was the sub pre-out with the distortion but may have been the Rane and other combination of things. I would still like to check this out. With a room correction like Audyssey turned on, these levels at pre-outs can get higher.

The lesson is to be careful with subwoofer pre-out levels. I wonder how subs with built in amps and crossovers handle this issue. 

SOme of us who have pro amps for subs have the luxury to play with input sensitivity levels. Question is, are subwoofer line-ins with filters in the path, are they really designed to handle 4v rms? onder:


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## bjs (Jun 12, 2008)

I can't remember the Receiver volume setting (11?, 5? 3? somewhere in there). The subwoofer trim was at 0dB. But neither mean much as the actual output volume is also determined by the REW signal generator level used and the actual output (ie gain) of the soundcard.

As for the probes, see Figure 1.3 in this link (http://www.fesb.hr/~mateljan/arta/download/STEPS-user-manual.pdf). I didn't bother with the protective zener diodes. Noise won't be an issue with these high level signals.

You mentioned the Rane has a lot more tolerance to high input levels versus the DCX2496. But I note they are spec'ed similarly (22dBu versus 23dBu). That's around 10v RMS for maximum input for either one. :yikes:


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## Ricci (May 23, 2007)

Thanks Bjs,

I'll try to get some time to investigate later this week.


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## jmcomp124 (Feb 27, 2007)

bjs said:


> . But I note they are spec'ed similarly (22dBu versus 23dBu). That's around 10v RMS for maximum input for either one. :yikes:


I am a bit surprised by this. You are right that it is speced at 23dBU. The data sheet for the rane says 
Overload LED Threshold +20 dBu Output or any internal level which is about 7.5v RMS. However,
the Overload LED lights up during very intense bass scenes (Master volume 0, subwoofer trim -6) depending on where the input and output gain sliders are set. If both are pushed all the way up, then the LED light up earlier, but if they are around 2 dashes below the top (still unity gain) which is about 
+6dB at input and -6dB at output. Internally this would translate to about 2vrms. But the led lights up
even at an output voltage from pre amp subwoofer output around 2 volts. In other words if more than 2 volts is sent to the PE17 input, it begins to overload. Another thing to factor is that I help Audyssey
with a boost of about +6 in the 40hz range and that maybe eating my headroom. 
This makes me wonder, about built-in eqs like Audyssey and ARC should use their filters very carefully to
avoid overload. With a unit like rane we know when it overloads, but our pres don't come with an overload light telling us when it is on the threshold or started clipping. I now have my LFE trim at -13.
The Rane still has some headroom left even with War of the worlds played at reference.


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## Ricci (May 23, 2007)

I think that you would need to have the Rane's input sensitivity bottomed out to be able to use the full 10v headroom. Things could get noisey though.


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