0.2 Dsp laboratory: time and frequency displays

Time and frequency displays

LabVIEW DSP gives you the ability to view the data in time domain or take its Fourier Transform and view itsfrequency content. In this section of the lab, we’ll learn how to set up and interpret these views. In the later labs, you will needto set up these displays on a regular basis, so you may want to refer to this part of the lab in the future.

Time domain

Whenever an analog signal is passed through an A/D converter, its samples are stored as a number (usually as aninteger). If we have a 16-bit system, there are 216 = 65,536 possible values that can be stored. These stored values can bedisplayed either as discrete samples or as a smooth line connecting the samples. The instructions below outline how to set up a basictime-domain display using a Waveform Chart.

Switch over to the Front Panel and right click on the Front panel window anywhere to bring out the Controls palette. From theGraph palette bring out the Waveform Chart and place it on the Front Panel, as shown in step 1 of Figure 1. Right click on theWaveform Chart and select Properties from the popup menu as shown in step 2 of Figure 6. In the Chart Properties window on theAppearance tab change the Label to Left Channel Time Domain. Specify the settings as shown in step 4 of Figure 1. DeselectingAutoscale will ensure that the Y-axis of the chart does not change with the data values. Minimum and Maximum values of -32000 and32000 will cover all the values for a 16-bit A/D channel.

The Waveform Chart maintains a buffer of values that are plotted on it, the number of points that are storedcan be adjusted by Right Clicking on the Chart and selecting Chart History Length… from the popup menu. By default the chart historylength is 1024. Right Click on Waveform Chart from the Visible Items sub menu and select X Scrollbar to show the toolbar for theX-axis. From the same sub menu (Visible Items) select Graph Palette. This palette gives you the ability to zoom in on thedata.

• Wire the Left output of the Analog Input Node to the Waveform Chart as shown in Figure 1 below.

• Un-mute the microphone Input, run the program and speak into the microphone. You should be able to “see” your voice, in additionto hearing it. Play around with the properties of the Waveform Chart (Right Click and select Properties form the menu) and becomefamiliar with the different options. Also use the zoom tools on the Graph Palette and become familiar with it.
• If you stop the program the currently displayed data will freeze on the Waveform Chart.
• Often, you’ll find it necessary to display less (or more) of the signal in the time-domain display. This is done by changing theChart History Length from the popup menu of the Waveform Chart. The time range that will be displayed will always be

Frequency domain magnitude display

• To display the frequency content of the audio signal we will take a Fast Fourier Transform (FFT) of the data and display it in aWaveform Chart. Add a second Waveform Chart Display to the Front Panel (as described in step 1 on page 7). Change the label of thisChart to Left Channel Frequency Domain and enable Autoscale for the Y-axis by checking the box in the Waveform Chart Properties windowon the Scales Tab. On the Scales Tab switch over to the X axis properties by selecting it in the dropdown menu and change theX-axis range to 0-255.
• Switch over to the Block Diagram and resize the while loop as shown in Figure 8. Place the Spectral Measurements Express VI fromthe Functions>>Signal Processing>>Frequency Domain Sub-Palette. Refer to Figure 8 for details.

• Wire the Block Diagram as in Figure 3.