0.2 Dsp laboratory: time and frequency displays  (Page 2/2)

• With the CD Audio input selected, run the program and examine the spectrum of the music signal. Now change the input tomicrophone and examine the frequency spectrum of your voice as well. Stop the program when done.
• The frequency spectrum is always displayed from 0 Hz. (DC) to Fs/2. For voice signals, which have most of the energy<2000 Hz, the voice frequencies only cover a small portion of the display(assuming Fs = 48000 Hz.). To get a more detailed view of your voice spectrum, change the sampling frequency to 8000 Hz. (makingsure to change both the Anaog Input and Analog Output nodes). Now, your voice spectrum will cover a wider range of the display. Runthe VI.
• Speak into the microphone using a single held-out note (like AHHHHHHHHHH or EEEEEEEEEEE). Examine both the time-domain andfrequency domain displays. If necessary, change the Chart History Length so that about 4-5 periods of the signal are on thetime-domain display. Because a note like this is periodic, you should notice a fairly steady time signal as well as a well-definedfrequency spectrum consisting of large peaks at your note’s fundamental frequency and its harmonics. Change the frequency(pitch) of this note and examine how it affects the frequency spectrum. Also try experimenting with various notes. Once you get afairly steady display in both the time- and frequency-domains, hold your note and stop the program to freeze the displays. You can nowzoom into various parts of the spectrum to examine it closely.

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• We are now ready to use the third input – the function generator. Begin by going to the Surround Mixer and changing theactive playback device to Line-in. Make sure that the volume is set to 100%. Locate the black cable (with an RCA-type connector on theend) coming out of the computer – this cable is connected to the Line-in port of the sound card. Connect the other end of this cableto the Func Out port of the function generator using the RCA to BNC adapter. Turn on the function generator and set the followingparameters. You need to press EXEC after setting each one.
  • FREQ: 400 Hz
  • AMPL: 0.2 V
  • OFST: 0 V (DC offset)
  • MODE: CONTINUOUS (use the  keys to cycle through the options if needed)
  • FUNC: SINE (use the  keys to cycle through the options if needed)
• After all of these are set, press the OUT/ON button (bottom middle) and use the  keys to select OUTPUT ON. If you now run your VI again, you should hear a 400 Hz tone. You should also see the sine wave in the time- and frequency-domain Charts on your Front Panel. Use the Zoom tools to view the signals in detail.

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• Now change the amplitude of the sine wave to 1 V and observe the resulting frequency spectrum. Is this expected?

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• When you’re done, stop the program and save the VI.