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Author Topic: Dual FFT Excitation  (Read 8781 times)

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CR500

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Dual FFT Excitation
« on: April 25, 2011, 11:09:25 PM »
Ok, what am I doing wrong.  I can't get the output channel enabled when using the excitation tab of the dual FFT.

Output would be from the MacBook Pro headphone jack.  Input from either the built-in mic or built-in input.

Thanks

FaberAST

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Re: Dual FFT Excitation
« Reply #1 on: April 26, 2011, 04:54:34 PM »
Within the Dual FFT Analyzer tool, you'll want to select a device that includes both input and output channels. Recent Macs present their built-in inputs and outputs as separate devices. In this case, you can use the Audio MIDI Setup application (in the /Applications/Utilities folder) to create an aggregate device that includes the input and output paths that you want. (You'll want to make sure that both physical devices have the same sample rate selected.) Then, you can go back to the Toolbox and select the aggregate device. I suspect that that will solve the issue you are experiencing with the excitation signal.

Ben

CR500

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Re: Dual FFT Excitation
« Reply #2 on: April 26, 2011, 06:16:53 PM »
Thanks, I'll give it a shot!

Is there a way (maybe using the MIDI setup) to port the audio out to the audio in as well, or do you always need to put a cable between the two as a reference?

FaberAST

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Re: Dual FFT Excitation
« Reply #3 on: April 27, 2011, 07:14:51 AM »
If your goal is to employ an internal reference signal within the Dual FFT Analyzer tool, then you can already accomplish that. Just select the same output channel for your reference signal and for the excitation signal.

Otherwise, take a look at Soundflower for internal signal routing between apps.

Ben

CR500

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Re: Dual FFT Excitation
« Reply #4 on: April 30, 2011, 07:53:35 PM »
That worked!

Is there a way to limit the frequency response of the when using the excitation tab?

If I use the signal generator, should the swept sine duration be equal to the FFT block size?  Without a triggering option in the Dual FFT, how is the signal positioned in the acquisition window?

Off topic, how do I delete data captured in a project?

Thanks

FaberAST

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Re: Dual FFT Excitation
« Reply #5 on: May 02, 2011, 11:23:39 AM »
Quote
Is there a way to limit the frequency response of the when using the excitation tab?
Perhaps, limiting the frequency span of the FFT is what you want?

Quote
If I use the signal generator, should the swept sine duration be equal to the FFT block size?
Not necessarily, but I would recommend it, since doing so may reduce or eliminate the need for averaging.

Quote
Without a triggering option in the Dual FFT, how is the signal positioned in the acquisition window?
FFT-based measurements assume periodicity, so it shouldn't matter for frequency domain measurements. If you would like to shift the position of one of the two signals relative to the other, you can add delay to your reference signal.

Quote
Off topic, how do I delete data captured in a project?
Each captured measurement has an associated delete button in the project outline of the main project window.

Ben

CR500

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Re: Dual FFT Excitation
« Reply #6 on: May 02, 2011, 06:38:57 PM »

Quote
Perhaps, limiting the frequency span of the FFT is what you want?

Not really, I'm measuring the low frequency response of a speaker system (15Hz to 100Hz).  But I don't want to drive it below 15Hz.

Quote
FFT-based measurements assume periodicity, so it shouldn't matter for frequency domain measurements. If you would like to shift the position of one of the two signals relative to the other, you can add delay to your reference signal.

With my HP3562, I'll typically set it to trigger on the signal so its always centered in the window.

I was wondering about windowing a signal that is delayed (microphone a fair distance away from the driver / reverence signal).  I'll try delaying the reference.

Quote
Each captured measurement has an associated delete button in the project outline of the main project window.

Thats where it is!!  Thanks.  Is there a manual somewhere with all this info?

Here's another one off topic (should I start a new thread for these)?
When looking under the Display tab, I can only resize the columns for Name and Auto but none of the others.  What do the Fre... check boxes do?   I also noticed that I can reposition the columns under the Display tab of the FFT Analyzer but not the Dual FFT.

Thanks