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Digital Audio/Video - Audacity

We use Audacity. An Open Source and comprehensive audio editor. If you are not you should be - and to be ever helpful you can donload and install it now.. Most documentation is found at the Wiki site and most of what is there is pretty good. But Audacity is a deceptively simple program that does a lot of stuff. These notes are designed either to augment exiting documentation or pull it out of some hard to find places.

While Audacity has a strong GUI interface many of the functions are implemented in the nyquist audio manipulation language. For full details about nyquist commands use the on-line manual.


  1. Frequency Handling

Frequency Handling

Notes in no particular order:

  1. Frequency Removal with Nyquist Prompt: When working with a mono channel the format is trivial:

    notch2 s frequency parameter
    # examples
    notch2 s 1400 20
    # - parameter needs experimentation
    #    the lower the value the deeper (higher db reduction) and narrower the frequency cut
    #    the higher the value the fatter (wider frequency) and shallower (lower db)
    #    so in the above example a narrow (5 Khz) range centered on 1400 is removed
    #    to a depth of approximately 10 db

    When working with stereo (or multi) channels the format is more complex:

    (vector (notch2 (aref s track) frequency parameter) 
       (notch2 (aref s track) frequency parameter))
    (vector (notch2 (aref s track) frequency parameter) 
       [(notch2 (aref s track) frequency parameter)] [...] )
    # - any number of tracks may be referenced so if tehr are 
    #   five tracks there will be 5 notch2 expressions
    # - the brackets are vital!!
    # - examples - stereo equivalent of mono example
    (vector (notch2 (aref s 0) 1400 20) (notch2 (aref s 1) 1400 20))
    # - parameter - see notes above

Audacity supports the nyquist programming language for audio manipulation. For full details about nyquist commands use the on-line manual.

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