NI LabVIEW DSP is a block diagram-based DSP
development platform that allows the user to quickly set up complexDSP algorithms. The true power of LabVIEW lies in its ability to
interface with external DSP devices and/or internal sound cardsthat are installed on the PC. The developed algorithms are
downloaded to the DSP board, which then runs the algorithm in areal-time environment. In this lab, we will only scratch the
surface regarding LabVIEW DSP’s capabilities. For starters, we willlook at how LabVIEW DSP interfaces with the A/D and D/A operations
of the DSP board. Specifically, we will simply connect the A/Dconverter to the D/A converter so that the DSP system plays back
audio signal sent to it.
A/d and d/a conversion
NI LabVIEW DSP is a block diagram-based DSP
development platform that allows the user to quickly set up complexDSP algorithms. The true power of LabVIEW lies in its ability to
interface with external DSP devices and/or internal sound cardsthat are installed on the PC. The developed algorithms are
downloaded to the DSP board, which then runs the algorithm in areal-time environment. In this lab, we will only scratch the
surface regarding LabVIEW DSP’s capabilities. For starters, we willlook at how LabVIEW DSP interfaces with the A/D and D/A operations
of the DSP board. Specifically, we will simply connect the A/Dconverter to the D/A converter so that the DSP system plays back
audio signal sent to it.
Simple sampling/reconstruction system
Keeping the Surround Mixer open, start LabVIEW Embedded
Edition. On the startup screen from the Execution Target dropdownmenu select your hardware device. Refer to Figure 2 below for
details. Execution target determines where your application willexecute when you run it.
Note: All the screenshots of LabVIEW windows are taken with
the SPEEDY-33 selected as the execution target. However, theinstructions still apply for any other target unless otherwise
specified.
Switch Execution Target to your DSP hardware device
Start with a new blank labview vi
Select File»New VI to start a new LabVIEW application. (Note:
A LabVIEW application is called Virtual Instrument or VI). AnyLabVIEW VI consists of two windows; a Front Panel where you will
design your user interface and a Block Diagram where you will addvarious block components or VIs and wire them together to design
your DSP system. On starting a new VI you will see the Front Paneland Block Diagram windows for your application appear on the
screen.
Switch over to the Block Diagram window by left clicking on
it and start developing your application. You will now develop asimple VI to read in an analog input from the A/D channel and play
it back using the D/A channel. From the menu select Window»ShowFunctions Palette. The Functions pallet contains functions or VIs
that you will wire together to develop the application. Click onthe Elemental I/O palette and bring out the Analog Input Elemental
I/O node and place it on the block diagram. This node is used toread in analog input from the A/D channel. Similarly place the
Analog Output Elemental I/O node on the block diagram. Refer toFigure 3 below for details.
Analog Input and Analog Output Elemental I/O
nodes.
The Analog Input and Output nodes control the input/output
flow of sound waves/data between the analog input channels and theDSP. The Analog Input node controls the A/D converter, which
converts the analog input signal (from the microphone, functiongenerator, or any other analog sound source) to a sequence of
numbers (digital data) and passes the data on to the DSP. Theanalog Output node controls the D/A converter, which takes the
digital data from the DSP and converts it back to an analogsignal.
Now we need to configure the Analog Input and Analog Output
nodes. Double click on the Analog Input node to bring up itsconfiguration dialog. In this dialog specify the following
setting:
General Tab:
Resource: 2 channel multiple samples
Configuration Tab
Sample Rate in Hz: 48000
Framesize: 256
Click OK when done.
Double click on the Analog Output node. On its Configuration
dialog specify the parameters as follows:
General Tab:
Resource: 2 channel multiple samples
Configuration Tab
Sample Rate in Hz: 48000
Before running any DSP algorithm, the Analog Input and Output
nodes need to be wired together. Hover your mouse over the Leftchannel output of the Analog Input node, the mouse curser will
change to a wire spindle. Left click and drag the wire to the Leftchannel input of the Analog Output node.
We will now place a While Loop so that the process can be run
in a loop. Bring out a While Loop from the Functions»StructuresPalette. Place the While Loop around the two nodes by left clicking
on the top left corner and dragging it to the bottom right. Thefinal application will look like Figure 4. Notice a stop button
appears which is wired to the conditional terminal of the loop. Youwill also see a stop button on the front panel. Once the
application is run you will press this button to stop theapplication.
Completed Application
Save your VI by selecting File»Save As and storing it in the
directory you created earlier. You can give the file any name youwant (such as lab1setup), but use the default extension of
vi.
Now you need to prepare an audio input for your system.
First, go to the Surround Mixer and activate the Wave/mp3 input.Place a CD in the CD player in the PC, select the Windows Media
Player program, and press play. (Note: The program may
automatically start when you insert the CD). You should not hearanything yet, because you haven’t compiled and downloaded the
simple playback program to the DSP yet.
Run labview code on the dsp
You’re finally ready to run this very simple DSP system. (If
using the SPEEDY-33 make sure that the Jumpers are in the Lineposition). To run the application left click on the run arrow on
the toolbar as shown in Figure 4. Notice on the LabVIEW DSP ModuleStatus Monitor window comes up and displays the Status as the
application is compiled and downloaded onto the DSP. The MemoryUsage bar at the bottom displays the amount of memory being used.
Once Status displays Running on Target you should hear the musicthrough the headphones.
LabVIEW DSP Module Status Monitor
To use the microphone input instead of the CD Audio input,
simply mute the Wave input button on the Surround Mixer andactivate the Microphone input instead. Give it a try. You can very
quickly toggle between (or even combine) different inputs by usingthe Surround Mixer.
Stop the program by pressing the Stop button on the Front
Panel.
Effects of Sampling Frequency on Sound Quality
Now we’ll briefly examine the effects of sampling frequency
on the sound quality of both music and voice signals.
Run the program again with CD audio input (using the original
sampling frequency of 48000 Hz.) and note the sound quality.
Stop the program and change the sampling frequency (in both
the Audio Input and Audio Output nodes) to 18000 Hz. Run theprogram again and listen to the sound quality. Repeat with a
sampling frequency of 8000 Hz.
Repeat steps 1-2 but use the microphone input instead of the
CD input.
Answer these questions
Describe how sampling frequency seems to affect the sound quality for both speech and music. Based on your observations, which seems to be most affected by sampling frequency, music or speech?
