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Gear / Technical Help => Post-Processing, Computer / Streaming / Internet Devices & Related Activity => Topic started by: greenone on March 02, 2010, 10:06:50 AM
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Evening all...transferring an old recording for a friend that has a constant, high-pitched squeal through the whole show. I'm pretty sure a notch filter would take care of it but I haven't the foggiest as to how to a) figure out the frequency of the squeal and b) construct/apply such a filter with the tools I have available to me - Sound Studio 2.2.4 and Audacity. Anyone willing to teach a man to fish? ;D Sound Studio is a Mac-only program but hopefully this can be done in Audacity or some other cross-platform tool.
Actually, it occurs to me I could use Windows via VMWare but obviously I'm not interested in buying a program for one function.
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Here's how you can do it in Audacity.
Figure out the frequency of the squeal: Select a section where the squeal is particularly prominent. Under the Analyze menu select "Plot Spectrum". Audacity will display a frequency spectrum of the selected waveform. If you scroll over the plot there is a readout of the frequency and level at the cursor position.
You can create a notch filter using the "Equalization" item under the Effect menu.
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Do you have the notch filter nyquist plug-in for Audacity?
You can find it here ----- http://audacity.sourceforge.net/nyquist/notch.zip (http://audacity.sourceforge.net/nyquist/notch.zip)
Here's a wiki ----- http://wiki.audacityteam.org/index.php?title=Noise_Removal (http://wiki.audacityteam.org/index.php?title=Noise_Removal)
Halfway down the page it deals with finding the offending frequency, and designing a notch filter. You can do what it recommends there (the hard way).........you will have to enter values at the Nyquist prompt
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(the easy way) If you download and install the Nyquist Notch Filter tool, it will show up in your "Effects" list.
Here's the way that works for me best:
(1) Analyze > Plot Spectrum > set graph to Spectrum, Hanning window, 8192, Log frequency
(2) Slide cursor over graph and find the peak resonant frequency that you want to remove
(3) Effect > Notch Filter
(4) enter Notch Frequency you want to remove, enter "4.0" for the Notch "q" (gives a steep-sided notch, won't affect other frequencies around it)
(5) Analyze > Plot Spectrum > see if what you did was what you expected.
(6) Repeat for any other resonances...
Viva Audacity!
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Yes, I've used that notch filter nyquist plugin and it worked pretty well.
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Also, the Audacity site has a package of LADSPA plugins (http://audacity.sourceforge.net/download/plugins) - I've made use of the "Mag's Notch Filter" plugin quite a bit for dealing with things like squealing hearing aids. One thing you need to be careful of, though - that can sometimes put a click in the output. I used to use trial and error to figure out the best place to begin and end the notch to avoid clicks.
However, I've found that it's best to copy the offending segment plus a second of padding on either end to a separate track, split-delete the bad section (without the padding) from the main track, apply the notch filter to the copied section and cross-fade the notched result with the main track. A lot of work, but it can make a huge difference in listenability.
For the crossfades, the LADSPA package I mentioned has crossfade plugins as well.
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Thanks for the help, all - it worked outstandingly well!
My only concern was being able to isolate the exact frequency - I could see the peak in the graph but found it hard to interpret exactly what I was looking at and if I'd chosen the right one, because I couldn't get the graph big enough to make sure I had the exact right range. Fortunately I nailed it on my first try, and equally fortunately, the squeal was consistent through the whole recording so I could take it out at one go. I'll relisten to make sure it's completely gone but initial result seem good. Thumbs up!
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Thanks for the help, all - it worked outstandingly well!
My only concern was being able to isolate the exact frequency - I could see the peak in the graph but found it hard to interpret exactly what I was looking at and if I'd chosen the right one, because I couldn't get the graph big enough to make sure I had the exact right range. Fortunately I nailed it on my first try, and equally fortunately, the squeal was consistent through the whole recording so I could take it out at one go. I'll relisten to make sure it's completely gone but initial result seem good. Thumbs up!
In Analyze > Plot Spectrum, try fiddling around with the "numbers" field.....the box that goes from 128 to 16384. You will see the ranges change. Use "log frequency" too.