Eliminate Noise in FRF

[malkoba]

Hi

I am testing the transfer function of two audio signals genarated by a white noise source through two mics. I am using a Dual Channel Spectrum analyser programme attached below. I seem to pick up noie spikes on the curve I get. Usually commercial dual channel spectrum analysers have ensemble averaging of the separate estimates of
the auto and cross spectra of the two input time signals (x,y). How can I obtain a stable transfer function in my programme.
Thanks

Mal

[johnsold]

You do not specifiy the hardware you are using. If the DAQ device does not perform simultaneous sampling on the two channels, the response is measured at at a slightly different time than the stimulus. This could be part of the problem as the response may not be well correlated with the (random) stimulus. For testing purposes try sweeping the frequency using a continuous sine wave for the stimulus. Make the sweep slow enough that the system response stabilizes at each frequency before movign to the next one. I think the FRF VI will be happier with the sinusoidal stimulation.

On another topic, your diagram is hard to follow. Look at the Style Guide for suggestions. In particular it is preferred that signals flow from left to right rather than zig-zagging all over the diagram. What is accomplished by multiplying by 1?

With a 1 Hz cutoff on the filter and 0.1 second duration samples the filter may not fully settle to a steady state value. If the filter is to eliminate a DC offset, try subtracting the mean.

Lynn

[malkoba]

Hi

Thanks for the advice. I have include a clearer programme. Can you please run through it. I seems to not get the averaging done. (ie when I keep the restart average false,set the number of averages say 100, and run the programme,I do not get the number of averages and the averages completed icon lit.

Thanks

Mal

[johnsold]

The help file for the FRF.vi says that when the Exponential Averaging mode is selected, averaging is continuous and the Number of Averages is ignored. I have not tested, but I would expect that the Averages Done indicator wil never light in this mode because the averaging never stops. Try the Linear averaging mode or set the loop to stop after the desired number of averages.

Lynn

[malkoba]

Hi

Many thanks. How can I replace the HPF filter with a 1 Hz cut off so as to eliminate the DC offset by subtracting the mean.

Thanks

Mal

[johnsold]

See attached. I removed your filters and inserted the mean subtraction. I do not have any DAQ devices so I have not tested this VI.

Lynn

[malkoba]

Hi

Thanks. It works fine.

Mal