Daqarta for DOS Contents
Waveform Averaging and flip to FFT mode. (You may need to set
the CTRL-A Spectrum Averager menu, since the values set for
the Waveform Averager menu are separate.) Initially, make
sure Y-log and X-log (ALT-X) are off. The trace will show a
line at the frequency and approximate amplitude you set in the
Virtual Source menu. There will also be a broad "floor" due
to the noise, at a much lower amplitude. It will be very
busy. Hit the Avg key and you will see it resolve to a
smoother line at the average value of all the original bouncy
values. With many sweeps averaged, this line will tend to
become perfectly flat.
Since the spectrum shows magnitudes, the average value is not zero. This makes sense, since if we imagine the FFT response at any given frequency to be due to an equivalent input sine wave at that frequency, then even if the next sweep contained that exact sine wave but with opposite polarity, it would still have the same frequency and magnitude... but the two waveforms would cancel out. To see this dramatically, set the noise to 0 and set Slope = Alt in the Trigger control menu. The trace will alternate between two sine waves of opposite polarity, so the waveform averages exactly to zero. But with spectral averaging there is no change!
More relevant to real applications, try toggling Trig off with the T-key. The waveform trace will show an unsynchronized, "rolling" Free-run sine wave, and waveform averaging will bring this to a very low value... just as it would for any unsynchronized noise. But spectral averaging shows hardly any difference between Trig and Free modes.
Think of the FFT spectrum as similar to what your ear does when listening to a tone: You detect the loudness of the tone and its pitch, but not (usually) its polarity. A guitar string sounds pretty much the same whether plucked on an upstroke or downstroke, and rapidly plucking up and down doesn't sound softer than plucking only down or only up.
The noise generator in Daqarta puts out equal energy at all frequencies, when averaged over enough sweeps. But this average level is much lower than the Noise level shown in the Virtual Source menu. Why is this? There are two reasons: First of all, the Noise level setting is a misleading value for these purposes. It reflects the peak waveform values that you actually see on the waveform trace, just like the main wave level shows the peak values of the sine wave. But whereas a sine wave hits its peak values each cycle, the same is not true for the noise. It hits its peaks only rarely, so the equivalent energy is not the same. (A better way to compare levels of different sources like sine versus noise would be to use RMS measurements.)
The second reason the noise level seems low is that the energy is evenly distributed across all frequencies, not concentrated into one like the sine wave. So of course any one of those frequencies will not have the same level as the sine.
Further, the average noise level changes if we use a different number of points N for the FFT. With more points, the width of each frequency band is narrower, so it holds less total energy... but there are more bands, so the total remains the same. See PSD for more thoughts about this.
Although spectral averaging doesn't actually reduce the average noise level, it makes the spectrum display look less noisy by reducing the trace-to-trace variablity. This can actually help detect a signal in a noisy background, if the trace-to-trace noise peaks are taller than the signal peak, yet the signal peak happens to be larger than the averaged noise floor. This ability may be important in cases where you can't synchronize to the signal in order to use the true noise reduction afforded by synchronous waveform averaging.
sweeps as you have specified. A special Continuous sweep setting allows this to proceed indefinitely.
A typical use for Peak mode is in determining how much noise a vehicle makes as it passes by at a fixed distance. You just start the "average", wait for the vehicle to pass, and then hit Pause to see the result. You don't usually need to worry about precise timing because the vehicle will be much louder than anything else at most frequencies, so the peak will only reflect that one source.
This is also ideal for looking at explosions, animal calls, music recordings, or anything else where you might want to know about spectral peaks but the source is not well-behaved or repeatable enough to use with a normal trigger.
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