MeasureTool » History » Version 13

Chris Cannam, 2013-03-05 11:48 AM

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h1. About the Measure tool and its limitations
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The Sonic Visualiser Help reference "describes the Measure tool":http://www.sonicvisualiser.org/doc/reference/2.0/en/#measurements like this:
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> The measure tool enables you to obtain measurements in scale units (such as time in the X coordinate, or whatever the Y coordinate of the current layer represents) corresponding to certain pixel positions. To measure a region, just click and drag a rectangle covering it, using the left mouse button with the measure tool selected ...
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> It's important to note that the measurements shown in this way are based entirely on the pixel coordinates of the measurement rectangle, not on properties of the data being displayed.
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The measure tool also has the ability to produce an automatic bounding box for a graphical feature, when double-clicked:
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> The area enclosed by the rectangle is based on the extent of similarly-coloured pixels surrounding the click position: it is entirely graphical, involving no audio analysis, and so depends on the gain and colour scheme in use in the spectrogram.
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Here's an example of what that means in terms of the practical limitations of this tool.
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!>measure.png!
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This is a recording of a singer, with vibrato. For this illustration I have switched off all of the spectrogram interpolation options in the preferences. The image shown here is a composite of three separately-highlighted measure boxes, because it isn't actually possible to highlight all three at once in SV.
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The *bottom* measurement box shows a frequency-scale range from 268.3 to 296.2Hz. These are the values that you would find if you took the green-line pixel positions and read them off against the scale on the left, interpolating appropriately (and taking into account that it's a log frequency scale). You can see that both of them look about right -- it's fairly easy to persuade yourself informally that these figures are OK, if wildly overspecified at 3 decimal places.