Data AcQuisition And Real-Time AnalysisScope - Spectrum - Spectrogram - Signal Generator
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Sound Card Pink Noise
This is similar to White noise, but has less energy at higher frequencies. Specifcally, the noise energy falls off as the square root of the frequency. If you plot a frequency response of this noise using ordinary spectrum averaging, the curve will slope down at -3 dB per octave of frequency increase.
As for White noise, the Pink name is in reference to light: Pink light has more energy at low frequencies (the red end of the spectrum) than at high (the violet end).
Pink noise is often used in audio testing because it roughly approximates the energy distribution of music and many other sounds. For a given power applied to the system under test, this is a more reasonable compromise than White. In testing loudspeakers, for example, White noise would tend to blow out high-frequency drivers (tweeters) before the low-frequency drivers (woofers) even got warmed up.
In a system tested with Pink noise, a perfect frequency response would show a an averaged spectrum that falls at the same -3 dB per octave as the test source. This is not the "flat" response we are used to looking at, and it makes it especially difficult to spot a slightly tilted response. To simplify this sort of testing, you can use the Tilt option of the Spectrum Curves dialog to tilt the spectrum upward at +3 dB per octave, to exactly compensate for the Pink source. Then you can evaluate the spectrum for flatness just as you would a conventional frequency response (say, from a frequency Sweep).
However, the Daqarta Generator Pink source is not perfect. If you observe the output directly with a long spectrum average using the above Tilt option, you will note several ripples of +/- 0.85 dB in the otherwise-flat spectrum. For critical frequency response measurements, you can create a custom mirror Curve file that will exactly match it. Used instead of Tilt, it will give perfect measurements.
Suppose you want to use the Pink source to control some other stream via Stream Modulation. Often, for control applications, you may be interested in very low (sub-audio) frequencies. The standard Pink source is normally best for frequencies in the range of the FFT analysis, but you can extend this to arbitrarily low frequencies using the Slow Timing option. Since that uses a linear interpolation to spread out (slow down) the output data, you can also use the Smooth option to smooth out the peaks formed by connecting straight lines. Typically, you would set Smooth TC to the same number of samples as the Slow Factor. (Set the Samples/Seconds button to Samples to do this.) This will give a good approximation to true low-frequency Pink noise.
If you are interested in "colored" noise, the other color you may want to experiment with is "brown" noise, where the energy is inversely proportional to frequency (-6 dB/octave), instead of the square root of frequency. There is no specific Wave selection for this, but you can create it easily from White noise by adding a low-pass filter. The -6 dB/octave curve is what you get from a simple RC filter, which is the same as the Smooth TC option available in the Timing dialog.
Note that the -6 dB/octave portion only applies to frequencies above the reciprocal of the Time Constant (TC), whether the TC of the simple low-pass filter, or the Smooth TC value. So if you set Smooth TC = 100 msec, the Brown spectrum will be correct for frequencies above about 10 Hz. To get to lower frequencies you must set larger TC values, but you will find that the overall level goes down as TC is raised. (This is just what you should expect from a low-pass filter!)
You might want to experiment with the Slow Timing option. Start by setting the Slow factor to the same number of samples as the Smooth TC. You will see a low-frequency peak when you use Slow, but you will greatly boost the flat part of the response at the upper frequencies. The reciprocal of the Smooth TC in seconds will determine how low the flat part of the spectrum extends before the peak begins.
L.1.Wave=Pink or L.1.Wave=9 sets Left Stream 1 Wave to Pink.
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