Weird Looking Calibration

**thechairman** · **07-24-09, 02:23 AM**

Hi

I've just calibrated my soundcard (via loopback), and while the routine seemed to run fine, I was surprised by how rough the frequency curve looked. The device is an Apogee Duet, so it's got extremely good converters - I would have thought that this would have meant I'd get a pretty flat calibration graph.

Here's what it looks like:
Name: Duet Freq Response.png
Views: 219
Size: 104.4 KB

Name: Duet Freq Response.png
Views: 219
Size: 104.4 KB

Obviously I could be overestimating the unit, and this might be completely correct, however, any pointers would be great...

**waldo563** · **07-24-09, 03:09 AM**

I'm no expert but it looks like you're getting feedback. Is there a setting for "Record Monitoring" that might be enabled?

**JC74** · **07-24-09, 06:13 AM**

Hi, take a look-see at this post

It might help a bit??

J

**thechairman** · **07-24-09, 07:39 AM**

OK, so I did a bit more investigation, and decided to record what REW was actually sending as the output signal. The left channel sounds fine, just a regular 1kHz tone, but the right is just a wierd buzzing sound. I use the Duet all the time for recording and playback, and nothing else makes that noise, so I think that isolates it to REW. Furthermore, it don't think it could be a feedback issue, as a) the left channel with the same setup has no problems, and b) it generates the noise irrespective of how loud I set the right channel's input volume.

Click here for a stereo capture of what's happening. I think it's safe to ignore the glitching noises occurring on the left channel, as they're not there when I'm listening - I figure they're just artifacts created by the recording software....

Thanks for all the assistance so far, I'm fairly new to all this so the help's really appreciated

**brucek** · **07-24-09, 08:40 AM**

You're not the first one to notice noise in an audio channel with a Mac. Seems a bit strange that it's only on one channel though. Be sure to have the Default Device selected.

brucek

**Beranga** · **08-21-09, 01:01 AM**

I'm also having a problem with the right channel signal. Left work flawlessly, but in the right channel there's always a buzzing sound. I'm going digital out from the Mac, and if I try to get a measurement for both channels, the right will give me trouble. It was not a problem when I used a Windows machine. ¿Does anyone know what going on?

**brucek** · **08-21-09, 07:44 AM**

Quote:

I'm going digital out from the Mac

How do you accomplish a soundcard calibration by using digital out?

brucek

**Beranga** · **08-21-09, 11:41 AM**

Quote:

brucek wrote:

How do you accomplish a soundcard calibration by using digital out?

brucek

Using the DAC's analog out into the sound card's analog in.

Regards,

**brucek** · **08-21-09, 01:56 PM**

Quote:

Using the DAC's analog out into the sound card's analog in.

But then the cal file includes the response anomalies of the DAC's analog out, yet you aren't using that stage when you do your measurement?

brucek

**Beranga** · **08-21-09, 02:17 PM**

Quote:

brucek wrote:

But then the cal file includes the response anomalies of the DAC's analog out, yet you aren't using that stage when you do your measurement?

brucek

I understand my calibration file includes the DAC's behavior, and I think it's all right since I do my measurements using the DAC's output. That's the way my stereo system is connected, so I measure using the the same setup I would use when listening to music. Please let me know if you think there's something wrong with my approach.

Regards,

**brucek** · **08-21-09, 02:35 PM**

Quote:

Please let me know if you think there's something wrong with my approach.

It's not the DAC's behavior that is a problem, its the fact that you are using the analog line-out to create a calibration file, but you're not using it in the measurement since you've bypassed it to use the digital out to your receiver. So the calibration file isn't calibrating at all.

brucek

**Beranga** · **08-21-09, 03:12 PM**

Quote:

brucek wrote:

It's not the DAC's behavior that is a problem, its the fact that you are using the analog line-out to create a calibration file, but you're not using it in the measurement since you've bypassed it to use the digital out to your receiver. So the calibration file isn't calibrating at all.

brucek

My system consist of a Mac Mini -(USB)-> M-Audio Fast Trac Pro -(Digital Coax)-> Behringer DEQ2496 -(Digital XLR)-> DAC -(Analog RCA)-> PreAmp -(Analog RCA)-> 2Ch Amplifier -(Speaker cables)-> Stereo Speakers and one Active Subwoofer

I take the DAC's analog output and loop it back to the M-Audio analog input for calibration. For measurements I route the analog out of the DAC back to the PreAmp and plug the microphone to the analog in of the M-Audio.

I hope this makes sense.

Regards,

**brucek** · **08-21-09, 03:34 PM**

Quote:

I hope this makes sense.

It does, if you intend to use the [PC + M-Audio Soundcard + DEQ2496 + DAC] as your calibrated test equipment. The 2496 can't be adjusted without it changing the calibration.

What are your intentions using it in this format?

brucek

**Beranga** · **08-21-09, 04:18 PM**

Quote:

brucek wrote:

It does, if you intend to use the [PC + M-Audio Soundcard + DEQ2496 + DAC] as your calibrated test equipment. The 2496 can't be adjusted without it changing the calibration.

What are your intentions using it in this format?

brucek

Brucek, thanks for putting up with me.

By going digital from the M-Audio when playing music, I'm trying to avoid to have the soundcard do a D/A conversion, only to have the 2496 do A/D so it can apply the filters, and then do another D/A, either by the 2496's own or a separate DAC, going out to the preamp.

I tried to calibrate the equipment with no filters on the 2496, hoping to get a cal file that compensates for any misbehavior. I still don't understand why new calibration would be needed when filters are applied.

I thought it best to take measurements with the same setup I use for listening, but if it causes a problem I'd readily use the soundcard's analog out for testing, I don't think the triple conversion would make much of a difference when playing a sweep tone. However I'd greatly appreciate if you could clear things out for me.

Regards,

**brucek** · **08-21-09, 04:39 PM**

Quote:

I still don't understand why new calibration would be needed when filters are applied.

Because you are adding filters to your calibrated test equipment.

Whatever you use for the loopback to create a calibration file, then that is your test equipment and must not be altered. It is the standard. You use it to measure your 'system'. Then any results are from the 'system' and not the 'test equipment'.

brucek

**Beranga** · **08-21-09, 05:02 PM**

Quote:

brucek wrote:

Because you are adding filters to your calibrated test equipment.

Whatever you use for the loopback to create a calibration file, then that is your test equipment and must not be altered. It is the standard. You use it to measure your 'system'. Then any results are from the 'system' and not the 'test equipment'.

brucek

Brucek, thanks for pointing this out, it's clear now. I'll do my measurements again keeping the 2496 literally 'out of the loop'.

Regards,

**assemblyworker** · **10-21-09, 07:28 PM**

Hi there, just found this search on a thread and thought it was fairly close to my problem. I'm also having issues calibrating an M-Audio Fast Track Pro.

I've read through the instructions, have everything set up, however I have no options to disable the feedback loop. I'm getting a very erratic reading.

Can anyone help? I've been through a few threads but other than it being a mac issue I can't find much else.

I'm using an M-Audio Fast Track Pro and a PPC Mac on 10.4.11

I've also tried the test with a Digidesign 192 and got a similar reading. I also tried the M-Audio with my Macbook which is an Intel model but I'm still getting the same crazy read outs.

**assemblyworker** · **10-22-09, 08:05 AM**

Problem solved.... 2nd computer, 3rd soundcard! Good to go....

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