Data AcQuisition And Real-Time AnalysisScope - Spectrum - Spectrogram - Signal Generator
Software for Windows
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Sound Card Burst Overview
When you turn a pure tone on or off suddenly, the clicking sound you hear is "spectral splatter" that adds many other frequencies both higher and lower than the desired tone. In effect, you are multiplying the original wave by a rectangular wave from the on/off switch. The rectangular wave has many harmonics, and the multiplication gives sum and difference frequencies of each harmonic with the original tone frequency.
These spurious components can be greatly reduced by turning the tone on and off slowly. The burst modulator fades the tone up and down with smooth rise and fall times instead of toggling it instantly on and off, analogous to using a light dimmer instead of a toggle switch. Slower changes cause greater reductions in the splatter components farthest from the tone frequency, so you can tailor the effective width of the splatter by controlling the rise and fall times.
The burst modulator allows you to control the shape and slope of the rise and fall times, as well as the durations of the on and off intervals. You can also create "trains" of bursts, like "Beep, beep, beep, (pause), beep, beep, beep, (pause)...".
The overall burst cycle time can be up to 2^32 - 1 samples, which is over 27 hours at a 44100 Hz sample rate.
In addition, you can further modulate any of the durations to produce trains with variable timing. For example, you can cause the length of the "beep" to increase and decrease in duration on subsequent repetitions, or to move earlier or later in the overall cycle. You can have trains with a variable number of bursts between pauses, or a variable pause duration after each train. Modulation control dialogs are invoked by the small buttons that hold the timing control labels.
Besides "normal" burst operation, you can use Burst in unusual ways. See Jitter-Free Square Waves - Burst Method under Square Wave for a way to create ultra-low-frequency square and sine waves with high frequency resolution. Creating Complex Arb Waves and Creating Rectified-Wave Arbs discuss some additional alternative uses.
Note that Daqarta also includes an overall "burst"-type option that smoothly fades the output up when the Generator is toggled on, and back down when toggled off, at the rate you specify. See Generator Fade Up/Down for details, including macros to fade the output down or up at arbitrary times and rates while running.
See also Burst Button, Basic Burst Operation, Modulating Burst Values, Burst Controls, Waveform Stream Controls, Burst Application: Auditory Evoked Responses, Burst Rise/Fall vs. Spectral Width, Fundamental Time - Frequency Domain Concepts
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