Analyze Menu

Analyze contains tools for finding out about the characteristics of your audio, or labeling key features. Plug-ins that accept input but produce no output will also be placed in the Analyze menu.

[ToDo]Sort the table out

Is this menu image usable? Will the items not in default release builds confuse? If these are VAMP plug-ins, maybe OK if fully explained?

If we don't want table and cell borders with Wikitable class (and if we still use tables for layout, we probably don't) it looks if borders have to be turned off in Common.css if this behaves like other Wikis, but http://www.audacityteam.org/manual/index.php?title=MediaWiki:Common.css seems to have no content? I'll ask Buanzo - Gale

Menu	Description
	Plot Spectrum text Find Clipping text Vamp Plug-ins text

Plot Spectrum

Frequency Analysis window of a music track

This takes the selected audio (which is a set of sound pressure values at points in time) and converts it to a graph of frequencies against amplitudes. This is done using a complicated piece of maths known as a Fast Fourier Transform or FFT. This gives a value for each narrow band of frequencies that represents how much of those frequencies is present. All the values are then interpolated to create the graph. Note that currently only the first 23.8 seconds of selected audio can be analyzed.

The following drop-down boxes let you configure the way the plot is calculated:

Algorithm

Determines what type of processing is done on the audio data.

• Spectrum: (default) - Plots the FFT of the data as described above.
• Autocorrelation: These options measure to what extent the sound repeats itself. This is done by taking two copies of the audio, and moving one forward by one sample. The two copies are then multiplied together, and all the values added up. This is repeated for two samples difference and so on, up to the number of samples in the size option. This gives a small result if the waveform is random (for example, noise) and a large result if it is repetitive (like a musical note). By looking at the peaks in the plot, the key frequencies present can be determined even if there is a lot of noise.
• Cepstrum:

Size

This controls how many frequency divisions are used for the spectrum, or how many samples are used for the autocorrelation. In the Spectrum, a larger number gives more accurate frequencies (narrow bands) but averages the result over a longer period of time (because more samples are needed for the calculation). In the Autocorrelation, a large size looks for repeating patterns over a larger range of time offsets, and so will detect lower frequency patterns.

Function

Selects which mathematical function is used to pre-process the data. The basic forms of the FFT and autocorrelation algorithm require infinitely long sections of audio to work on, and so take infinitely long to complete. Hence the available audio must be pre-processed in such a way that the finite length of the audio has minimum effect. The available functions are known as "windows":

• Rectangular: is the simplest - it just cuts off the given block of samples with a sharp cut, and so leaves a sharp click at each end of the data. This means the results are often poor with a lot of random frequencies in them.
• Hamming, Hanning, Bartlett: do a smooth fade in and out of the audio data, and so give cleaner, more accurate results. Can someone explain what the differences are? I've only done Hamming!
• Blackman, Blackman-Harris:
• Welch:
• Gaussian:

Axis

When using the Spectrum, the frequencies can be displayed on a linear scale (default, which gives equal width to each increment on the scale) or on a logarithmic scale.

Beatfinder

no documentation available yet