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Old 19th Jan 2016, 7:56 pm   #1
wd40addict
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Default Audacity based wow & flutter calibrator

Recent acquisition of a faulty wow & flutter meter meant I needed a 'calibrated' source of known W&F. I had an idea that the modern way to do this would be to use Audacity together with a soundcard to feed the meter. I thought it might also be of interest to others.

The attached file needs to be unzipped and the resulting *.ny file needs to be copied to your Audacity Plug-Ins directory and, if you're using a recent version, enabled in the 'manage' function.

This done a new item 'Wow Flutter Cal..' will appear under the 'Generate' menu (see pictures). It defaults to 1% peak W&F at a frequency of 6Hz. The carrier defaults to 3kHz, but can be changed to 3150Hz. Clicking 'ok' will cause 10 seconds of the chosen signal to be generated. Strictly speaking a project rate of 44100 should be used for the 3kHz ref and 48000 for 3150Hz to avoid sub multiples.

The generated waveform can then be played back (using repeat if necessary) and the results observed on the connected meter. Different W&F standards use different detectors: peak or RMS, and different ref frequencies. 6Hz is the crossover region between wow and flutter so if your meter can select them individually the reading should be 0.707 of that displayed as combined W&F.

It all seems to work on my Leader, but I would be interested in others' results.

Regards,

Paul
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Old 20th Jan 2016, 12:38 pm   #2
dave cox
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Default Re: Audacity based wow & flutter calibrator

I guess your work could easily be adapted to calibration signals for THD ?

I've used 'sox' to generate various reference signals in the past but the command line interface is very fiddly !

dc
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Old 20th Jan 2016, 1:36 pm   #3
wd40addict
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Default Re: Audacity based wow & flutter calibrator

Hi Dave,

Yes a plug-in could be written, but it's also pretty quick to do by hand in Audacity using the Generate>Tone.. function :

Generate a tone at 997Hz, amplitude say 0.8. Now click on the grey area outside the track. Generate a second tone at 2991Hz amplitude 0.008, this will appear as a second track.

Now hit play, both tracks will play together producing a 1% THD cal signal.

Note this is not mathematically 100% correct, but is more than good enough for the typical notch based vintage THDN analysers such as the Heathkit IM-58 etc.

Regards,

Paul
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Old 20th Jan 2016, 1:42 pm   #4
Brian R Pateman
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Default Re: Audacity based wow & flutter calibrator

What an interesting idea.

I'll give it a try on my Philips W&F meter, when I get time. and post the results.
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Old 20th Jan 2016, 2:08 pm   #5
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Default Re: Audacity based wow & flutter calibrator

I should add that the THDN example above relies on a good quality soundcard with no spurious output of its own, otherwise this will also form part of the reading.
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