Daqarta for DOS Contents
The upper (blue) image in the above trace shows a noisy input wave, and below it in yellow is the result of averaging 1024 sweeps of that signal. Note that this is true sweep-synchronous averaging, which unlike "smoothing" or "moving window" averaging can faithfully recover signals of any shape, even those buried far below the noise. To use this, there must be a way to trigger each sweep at the exact same point on the desired signal... level triggering on the noisy input won't work.
In typical applications a stimulus signal is applied to a system under test, and the response is observed, possibly at a much lower level and contaminated with system noise. The idea is that each stimulus produces the same response, whereas the noise is random and has an average value of zero. The trigger can thus be derived from the stimulus, and in most situations (which don't involve a changing delay, like a moving sound source) synchrony with the response will be assured.
If the stimulus signal is generated by Daqarta (either via STIM3A or STIM3 on a lab-type board or the SB16, or the OPL2/3 synthesizer on any sound card), then the Stim trigger source is available internally. If the signal originates from some external source, that source must provide a TTL pulse for synchronization by Daqarta. (Due to Daqarta's generous trigger delay range, the pulse may come well before or even well after the desired response... all that is needed is that it be synchronized with some feature of the stimulus.) Alternately, Daqarta can generate a TTL trigger pulse which may be used to initiate an external stimulus, or be used as the stimulus itself.
Below are the spectra of the raw and averaged waves:
The averaged (yellow) trace shows about 30 dB of noise reduction compared to the raw (blue) one. This demonstrates the fact that random noise is reduced by about 3 dB for each doubling of the number of sweeps, since 1024 = 2^10 or 10 doublings, and 3 dB * 10 = 30 dB.
Note that this image shows the spectrum of a waveform average. This is NOT the same as spectral averaging, which would show the average noise level instead of reducing it. Daqarta can perform either type of average, depending upon whether you initiate the average during waveform or spectrum display.
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