# Improving imaging with REW and DRC Fir?



## goodkeys (Oct 18, 2014)

Hello everybody

Recently I tried the demo of Dirac live and got really good results. The Dirac filter didn't affect the overall sound of the speakers, but the imaging improved quite a bit. The stage was more clearly defined, and instruments better separated.
So I thought I'd try and see whether I can achieve a similar effect with REW and DRC Fir. Using the REW for measurement, and DRC to calculate the filters. However I must be doing something wrong, as the filters I created this way affect the sound signature of the speakers, but not the imaging. Too bad my Dirac trial has expired. I would have liked to compare the measurements with the ones made with REW.
Would anybody be so kind to help me interpet my REW measurements? I have only a perfunctory understanding of the graphs so any help is welcome. By the way, I made some eq filters with REW and they are quite good (not dull like the DRC filter I compiled), but they don't affect the imaging (which is only natural as far as I understood, as it is eq only and doesn't correct the phase, right?).
So here are my measurements. I'd appreciate if anybody could throw a glance at them. Thanks in advance.


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## jtalden (Mar 12, 2009)

Welcome to HTS!

The data you posted is not usable.

You selected 1M Sweep length and 2 sweeps. 

It is okay to use 1M length, but unnecessary for normal SPL/EQ work. 256k is very adequate for most all work except maybe distortion measurements. The larger length will not hurt anything. It just makes larger files. 

The problem appears to be the selection of 2 sweeps. REW normally works fine with multiple sweeps, but in this case it looks like the timing between the 2 sweeps was lost. Possibly this is because you have a device in the signal chain that disrupts the timing between the 2 sweeps. If the timing is lost, the results will be corrupted. 

Just use 1 sweep and that should fix the measurement problem. There is no disadvantage to 1 sweep for most work. Again, multiple sweeps can be an advantage for distortion measurements, but don't bring any benefit to SPL/EQ work.


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## goodkeys (Oct 18, 2014)

Thank you jtalden, that's a helpful information. I'll measure a single sweep tonight and repost the result.


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## goodkeys (Oct 18, 2014)

Here's the new measurement. Would you mind taking another look? The graphs seem to be right now to my unschooled eye. Would be great to have some help with the interpretation.


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## jtalden (Mar 12, 2009)

The measurements now seem to be okay. Your room acoustics, speakers and measurement methods all affect the results so it is impossible to say much that is helpful. 

Your SPL response is pretty typical looking and that is the major contributor to sound quality. There is still a lot of ringing in the impulse response. If Dirac was activate and not setup properly that may have contributed to the ringing? I would expect the overall measurement to be better with a proper setup. I don't use Dirac though so I am only stating my expectation as to what would result.. 


*Here* is a link to helpful information. REW help and sticky threads also have lots of good info.

Imaging and sound stage are complicated matters. *Here* is a recent thread.

Someone else will need to help with any Dirac related questions.


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## goodkeys (Oct 18, 2014)

Thanks jtalden for your continuos help. These are interesting links that I'll investigate.

My measurements are without the Dirac filter. My setup consists of two Geithain RL 906 in a nearfield setup (I'm listening from little more than 1m distance) in an untreated room (it's not a studio but a living room). It's roughly 35 square metre, and it has a good shape as it has no parallel walls (it's an attic room). I'll try and see if I can come up with a floorplan.
I made the measurements with a calibrated mic. The SPL response looks pretty okay to me. I guess I should try to extend the Dirac trial period and measure again with REW with the Dirac filter on. Maybe that would show where the perceived improvement in imaging came from.


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## jtalden (Mar 12, 2009)

The measurement looks a little rough for a measurement with a mic a ~1m distant.

What soundcard / audio interface are you using? 

If there is monitoring activated that would impact measurements and explain some of the remaining irregularity. We should not hear any output from the speakers when we speak or tap the mic.

If you haven't already done it, it may be good idea to take a loopback measurement of the soundcard to assure all is working well with the REW measurement settings. Just remove the mic from the left channel input and run a cable from the left channel output to left channel input of the soundcard. You also need to reduced the level of the REW sweep so that the input does not overload. A measurement that way will help confirm that all is well. If the impulse response is not very clean then there is still something wrong.


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## goodkeys (Oct 18, 2014)

The soundcard is an ESI Julia, which has no recording channel. My speakers are connected to the ESI, the microphone to the onboard Realtek soundcard. I checked and didn't hear any microphone feedback on the speakers. I don't know if a loopback can be configured when the soundcard doesn't have input channels. But I could take another measurement with a reduced sweep level if that would tell us more.


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## jtalden (Mar 12, 2009)

Using 2 different soundcards for input and output is likely the reason that multiple sweeps did not work properly.

What mic are you using. I assume it's one that plugs into the realtek mic input and requires 5V phantom power?

With this type of setup I suggest the same sample rate be used for both soundcards and the REW signal. Converting sample rates 2 or 3 times may cause a problem?

REW is now set to 44.1k per the posted file. That is a good safe choice. Match that setting on the Realtek mic input in the Windows device settings. Do the same with the ESI Julia output in its control panel. 

Having all 3 match is best for measurements. For music you can switch back to your preferred settings.

If the Realtek input is mic/line then I would expect a loopback cable connection from the ESI Julia should work just fine. You could select the RCA output on the ESI Julia for cable convenience. If a measurement with the loopback shows a clean SPL and Phase measurement then that confirms that your setup is okay for measuring no matter the settings.


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## goodkeys (Oct 18, 2014)

The microphone is an individually calibrated Technika from Tesco. Indeed it is plugged into the Realtek mic input. I checked that samplerates match between ESI, Realtek, and REW.
Thanks to your help I managed to create a loopback and measure the soundcard. However, I had to make some adjustments to do so. Maybe that's the source of the problem? To measure with the microphone, in the realtek control I have to set recording level to 100, and microphone boost to +30 (the maximum). Otherwise the input level in REW is too low. For the loopback measurement these settings gave all sorts of clipping! I had to set mic boost to 0, and recording level to 40. Even then I had to measure with a rather low sweep (-24db) level. Does this mean the the microphone sensitivity is reeeaally low? Could this be the problem?


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## jtalden (Mar 12, 2009)

The soundcard measurement is okay. The SPL variation is about 0.1dB so it will not affect measurements in a significant way. You could use the REW soundcard calibration process to create a soundcard calibration file that will cleanup the measurements even more, but it is not really necessary. 

The decrease in input/output levels you made for the loopback measurement is normal. 

I wasn't able to find anything but a simple desktop mic from Audio-Technika that plugs directly into a PC mic input. I am not sure how that could be calibrated. If it was though, it could be satisfactory for use. It could be omnidirectional mic as is recommended, but it may not be. If you use a mic like this I would point it toward the speaker rather than at the ceiling or at an angle in case it is pretty directional.

The max mic boost needed does sound to be higher than I would expect, but there is no real evidence that there is a problem with that. Did you follow the REW procedure to set the REW SPL Meter calibration procedure. If so, and that full boost and full level resulted in the appropriate headroom for measurements, then all may be okay for casual usage.

For extensive usage or highly accurate work it would be better to purchase a true measurement mic. The MiniDSP UMIK-1 is a popular and cost effective USB connected mic.


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## goodkeys (Oct 18, 2014)

Here's the mic I'm using:

http://lasip.hifi-selbstbau.de/inde...n=com_virtuemart&Itemid=64&vmcchk=1&Itemid=64

It's omnidirectional, and individually calibrated. I guess that should be good enough for casual measuring?
But if the necessary mic boost doesn't cause the ringing, I'm at a loss where else the problem could lie. It seems like REW is set up properly. Is there anything that I could be doing wrong with the speaker placement? They are standing on the bridge of my desk (the bridge is designed specifically to hold speakers), and are toed in only slightly. Do you have any other ideas or might it just be the room?


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## jtalden (Mar 12, 2009)

The mic looks fine for this use.

The "ringing" not measurement system ringing after all, but is very likely just the impact of the reflection off the desk. My experience is lacking with very close significant reflections, however the measurements are consistent with what I might expect.

So I think you are good to go for EQ. I will let others help you with EQ questions/direction as again, I have no experience with the particular challenges and tradeoffs of this type of application.


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## goodkeys (Oct 18, 2014)

Thank you, you have been a great help! If anybody has tips concerning eq that would be great. In any case I'll play around with REW EQ and DRC Fir and see how far it can take me. Good to know that my measuring setup is now working as it should.


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