Data AcQuisition And Real-Time Analysis
Scope - Spectrum - Spectrogram - Signal Generator
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Spectrum Analyzer

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Decimate Factor (X)

Controls: X-Axis Dialog >> Slow >> X
Macro: DecX

When Decimate is active, this specifies how much the true sample rate will appear to be reduced to get the effective sample rate used for the display. For example, if the true sample rate is 48000 Hz and you set 480 X here, the effective rate will be 48000 / 480 = 100 Hz. (The Rate control below this will show either 100 Hz or 0.01 sec, depending on the state of the units button next to it.)

In waveform display mode, the Factor multiplies the X-axis time. The waveform shows up to 1024 samples on the screen at one time (or less, if Xpand is active), so with a true sample rate of 48000 Hz (Decimate off) the display shows up to 21.3 msec of the waveform. If Decimate is on and Factor is set to 10, it will show up to 213 msec.

In Spectrum mode, the Factor divides the maximum frequency range of the display. In the above example with Factor = 10, with Decimate off the maximum frequency would be 24000 Hz, whereas with it on it would be 1/Factor or 2400 Hz.

The maximum Factor is 2147483648 (which is 2^31). At a true sample rate of 48000 Hz this would result in one effective sample every 44739.24 seconds, or 12.427 hours.

Note: When saving data to long DDisk files, the raw (undecimated) data is saved by default. This can result in extremely large file sizes, but it has the advantage that you can change Decimate Factor as desired during later file viewing. If you don't need this ability, you can record with the Decimate Lock option selected in the DDisk Controls dialog. This will save only the decimated data, just the way it appeared on-screen during the recording.

See also Slow (Decimate) Controls, X-Axis Control Dialog


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