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Arb Data (Interpolate / Round / Step)

Controls: Gen Dlg >> Stream >> Wave >> Arb >> Interp
Macro: ArbMode

This control is at the bottom of the Arb dialog. Normally, you will want to leave it in Interpolate mode. This means that Daqarta will interpolate between Arb file values to find the proper output value at any given instant of time, just as the normal internal Wave types do. That's especially important if you only have 1024 points in the Arb file.

But sometimes you don't really want that, and instead want to have the output go only to the nearest value from the file. That's what Round does. This might be needed, for instance, if the Arb output is used as a Stream Modulator to provide different predetermined modulation levels in a fixed sequence (usually with Freq set to a low value).

If the application requires only specific output values, you can just build a list of them with a text editor such as Windows Notepad. (But an editor that can handle columns can make things simpler.) Values must be in the range of +/-32767, and decimal fractions will be rounded to the nearest integer. This text file can be loaded directly, or you can first convert it to a .DAT file with the Convert Text File to .DAT option in the File menu.

The Step mode is an even more powerful extension of this concept, since instead of the Freq control it uses the Timing dialog. This allows you to step very slowly through the Arb file, holding each value for a step interval that may be set to any arbitrary value, up to more than a day. You also have the option of smoothing the steps, for example to prevent abrupt jolts to mechanical equipment being controlled.

Suppose, for example, you are conducting vibration tests on a product, using an electro-mechanical shaker system. Each test cycle consists of several stages at different frequencies and amplitudes, then repeats.

You can set this up using a Step Arb output as the FM modulation source for the main output stream that will drive the shaker. At each step the Arb output produces a different frequency, the range of which can be adjusted via the FM deviation and the main Freq setting on the output.

If you want some stages to be longer than others, just repeat that value in the .DAT file as many times as needed. Assign each stage a duration in samples, such that the total comes to 1024, 2048, 4096, 8192, or 16384. (But see Random Selection of Arb Values for a way to avoid this.)

In a similar way, a second Step Arb can be used to step through various amplitudes by using it as the AM modulation source on that same output.

In Step mode, Gen Sync defaults to the Timing Step size, even if Spec or Slow are active. If you set Timing Step to the Arb size (shown on the message line when you select an Arb Wave, or when you right-click on an Arb Wave button), then the sync point will be the start of the Arb cycle.

Macro Notes:

Use L.1.ArbMode=Interp, L.1.ArbMode=Round, or L.1.ArbMode=Step to set the Left Stream 1 Arb Data mode. Alternatively, you can use numeric values, variables, or expressions in place of names like 'Interp':

    0 = Interp
    1 = Round
    2 = Step

Note that setting the Arb Data mode directly via macro does not automatically set the Wave to Arb. Use Wave=Arb to do that.

See also Wave Dialog, Arb Wave.


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