# Number of Sweeps Anomaly



## AustinJerry (Apr 2, 2010)

I have encountered a peculiar anomaly with room response measurements when I vary the number of sweeps. Since the differences I observe are significant, I would like to share the results in this forum to get some feedback. Please take a look at the attached file and help me understand why I am seeing huge response dips when the sweeps are greater than 1. I hope there is a simple explanation!


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## AustinJerry (Apr 2, 2010)

Anybody have an idea?


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## jtalden (Mar 12, 2009)

This can be due to a very brief drop out of the audio stream during one of the sweeps. You may hear this, but it can be pretty subtle also. You may be able to see the interruption of the sweep by looking at the "scope" tab.

Some things that can contribute to this:
> Early beta versions of REW V5 - The release version is fine.
> ASIO drivers - particularly with small buffer size setting. There is no reason not to use 2k buffer size so far as I know.
> PC WiFi transmitter turned on - disable it while making measurements
> With ASIO drivers I sometimes even have this issue even with 2k buffers and WiFi disabled. I attribute this to heavy background PC activity and just repeat the measurement.


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## AustinJerry (Apr 2, 2010)

- I'm running the latest version of REW 5.0. "Check updates" says its current.
- I am using Java drivers, not ASIO. Should I change this?
- WiFi is definitely on, and the laptop I am using has WiFi running. I'll try turning it off.
- "Heavy background PC activity" doesn't seem to be applicable. This is a dedicated laptop, reasonably powerful dual-core system, and the CPU usage indicator shows less than 5-10% usage.


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## jtalden (Mar 12, 2009)

The java drivers are fine I would continue using them.


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## JohnM (Apr 11, 2006)

What soundcard are you using and at what sample rate? Do you see the same effect if you change the sample rate?

Multiple sweeps use synchronous averaging, if anything causes an interruption or gap in the generated or captured signals the averaging will fail. If your soundcard supports ASIO drivers it is worth trying them as the ASIO driver provides a completely separate route to the soundcard and higher priority access than the JavaSound drivers.


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## AustinJerry (Apr 2, 2010)

Hi John,

I am using the SoundBlaster X-Fi USB. The sample rate is 44.1 KHz.

I have not tried ASIO, nor do I know if the SoundBlaster supports it. It is my feeling that Java should be perfectly fine to use. There should be no resource contention on the laptop I am using.

This issue has caught me completely off guard. I have been expending a lot of time and energy (and money) trying to address what I thought was an issue with my room response. By simply selecting one sweep instead of multiple sweeps, my room response went from terrible to pretty good. (See attached--same subs, measured from same main listening point)

How could I have known that I had bad measurements? :scratch:


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## JohnM (Apr 11, 2006)

Try a measurement at 48k, it is possible that an OS-level resampling artefact is affecting the measurement (you would need to redo the soundcard loopback calibration at 48k or simply clear the calibration file for the purposes of making the check). The soundblaster installation usually includes ASIO support so that could be tried also, just see whether the X-Fi appears as an ASIO device when selecting REW's ASIO option. 

Such deep notches are unusual and would imply either a very strong reflection or some measurement issue. The impulse response would show multiple peaks.


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## AustinJerry (Apr 2, 2010)

I have discovered why my REW measurements were flawed. It turns out that I had a mismatch in the Sample Rate settings. The sound card calibration was taken with a sampling rate of 16-bit 41.1K, while REW Preferences screen was set to 48K and Windows was set to 41.1K. As soon as I made sure that the sampling rate in Windows matched the REW sampling reate, and re-calibrated the sound card, I am now getting consistently good measurements.

Lesson learned: double-check all settings, and then check tham again! Thanks to everyone for the suggestions.


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## groovbert (Aug 8, 2012)

Hi John, Austin,

I have the same symptoms, but have not found the cause yet. I spent a day to find out why on earth there would be a 40dB Notch around 10kHz with the monitors I am checking.
Since I did a calibration of the soundcard only first, which was fine, I did not suspect an issue with system settings.
After searching and not finding the "room-mode" I short-circuited the Monitor in and Mic-Cable and did the sweeps again - and there it was-nice and clear, without air: a notch around 10k in an otherwise linear frequency response. Calibrating again, no notch, so by chance reduced Sweeps from 2 to 1 and the notch was gone.
We are currently using the Focusrite Saffire 40 on the MacBookPro, since feeding back the signal to REW via FireWire did not work, we used the Analog Out instead and the Line In of the MBPro.

For the time being I'll be just using 1 sweep, but I am too curious to find out, what the issue actually is.
Anybody an idea - would be highly appreciated!

Regards,
Markus


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## AudiocRaver (Jun 6, 2012)

groovbert said:


> Hi John, Austin,
> 
> I have the same symptoms, but have not found the cause yet. I spent a day to find out why on earth there would be a 40dB Notch around 10kHz with the monitors I am checking.
> Since I did a calibration of the soundcard only first, which was fine, I did not suspect an issue with system settings.
> ...


Markus - did you ever figure out the cause of your problem? I am not a Mac expert, so have no suggestion, but was wondering if you had any further insight to share.

An additional thought to all, when making any kind of measurement, before chasing what appears to be a an odd problem, is to ask "Does this make sense? Might this not be real?" It can be very hard to tell sometimes, as the examples above show. The LF notches could very well have been real, although the regularity of the frequencies, multiples of 49, look a little suspicious. The lone notch at 10 KHz - probably not. Just something to remember.


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## groovbert (Aug 8, 2012)

Hi and no, we did not figure it out.
Apart from that, I am completely with you to use common sense and ask, if sg we see can be "real". We did a slow sweep to check if we could hear that notch and since we could not, started searching on the system side...
So, still, any solution suggestions welcome. Id like to increase the no of sweeps to bring down avg noise...
Regards,
M


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## AudiocRaver (Jun 6, 2012)

It sounds like you approached the problem wisely. I will keep my eyes open for possible explanations/solutions.


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## AustinJerry (Apr 2, 2010)

Ever since reporting this, I restrict my sweeps to one. I never did any more research.


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## weverb (Aug 15, 2008)

After reading the op's posts, I went back and remeasured my system. The blue trace is with sample set 4 versus the purple set to 1. Can someone explain to me what's going on. Needless to say I went back and redid everything with a sample rate of 1. I believe the purple trace is more accurate based on my listening. The system never sounded as bad as the blue makes it out to be. :dontknow:


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## AudiocRaver (Jun 6, 2012)

I will try a multi scan when I get home, see what I get.

edit: I meant "multi-sweep"


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## AudiocRaver (Jun 6, 2012)

On my main desktop system, I see no problem. Not saying it does not exist, only that I can not reproduce on that one system: Windows 7 Pro 64bit, Avid/M-Audio Fast Track C600 interface. Tried it with 256K x1, x2, x4, x8, 128K x1, x2, x4, x8, 1M x1, x2, x4. All overlaid perfectly.

If someone is still having problems, they should re-read *posts 6, 8, & 9* of this thread, they contain the explanation for what would have to be _most_ problem occurrences (groovbert, I know you have already done this).

Obviously, we all have lots to do and prefer to spend our time on fun projects rather than something like this. If anyone does find an anomaly, or has seen one recently that is not explainable by John's responses above, please note as much detail as possible with your post.

My own experiences with audio application, driver, operating system interaction over time is that setting changes that affect driver &/or operating system or buffers or the like often require an application close and restart before they will run clean, sometimes more than once for the affects to ripple through all the interacting levels. This is a frustrating aspect of computer audio that does not promise to go away in this dimension of the omniverse. If anything about the audio - the sweep, in this case - sounds odd or glitchy, do not trust it.

As John suggested, double-checks can be helpful, like checking the impulse for multiple peeks, etc., and asking self "does this make sense??" As AustinJerry reminds us, "double-check all settings, and then check them again!."

If still having problems after checking all the above, I suggest noting the following detail with your posts:

have you followed the suggestions in posts 6, 8, & 9
operating system, version, 64 vs 32 bit
mic (if USB), audio interface, driver version
driver type - java or asio
amount of memory
ASIO4ALL version (if used)
REW version: everyone should be using the latest beta version unless there is some reason they can not - beta17
any conditions that might be out of the ordinary or might be helpful


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## akajester (Mar 4, 2009)

Quick question. does the sample rate setting in windows matter if you're using ASIO for your input and output? It seems like a good reason to use ASIO if that solves many PC settings issues. Thanks.


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## AudiocRaver (Jun 6, 2012)

akajester said:


> Quick question. does the sample rate setting in windows matter if you're using ASIO for your input and output? It seems like a good reason to use ASIO if that solves many PC settings issues. Thanks.


From what I have seen, ASIO4ALL _usually_ is able to force windows drivers to the right sample rate, but there could be exceptions. Generally speaking, what you suggest is true. It can be handy to keep your audio interface driver control window open so you can see what streaming rate it reports, at least until you fully trust your system's behavior.


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## akajester (Mar 4, 2009)

I remember in the advanced settings of Asio4all a checkbox (checked by default) that resamples all to 48hz. Perhaps that's what fixes it.

Thanks for the response.


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## andy_c (Aug 8, 2006)

JohnM said:


> Multiple sweeps use synchronous averaging, if anything causes an interruption or gap in the generated or captured signals the averaging will fail.


Does the synchronous averaging require a loopback timing reference to work properly?


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## AudiocRaver (Jun 6, 2012)

andy_c said:


> Does the synchronous averaging require a loopback timing reference to work properly?


Not that I am aware of.

==========================

I saw the multi-sweep problem last night on my low-powered laptop. It is a Windows 7 64-bit Acer laptop, 4 GB memory, with M-Audio MobilePre USB interface (latest drivers), latest Beta17 of REW. Immediately after starting up REW I ran one single 256K sweep, then switched to 128K x4 and then x8 sweeps and got insanely crazy high-frequency results relative to the single sweep. All the craziness was above 1 KHz. After switching between x4 and x8 and x1 sweeps a few times, it went away and would not repeat.


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