Now you are going to add two more FIR filters
that cover the rest of the frequency range DC – Fs/2 and thencombine these FIR filters into a filter bank that will allow you to
selectively boost/attenuate the various frequency ranges. A filterbank is nothing more than a parallel combination of different
filters. This is basically what a graphic equalizer does on atypical stereo system, except that many equalizers are analog
filters, rather than digital filters.
Open LabVIEW (not Embedded Edition) and open the Filter
Design VI built in Section II.
Design a bandpass FIR filter to meet the following
specifications:
Type: bandpass
Design Method: Kaiser Window
Stop Edge Frequency: 400Hz / 2000Hz
Passband Edge Frequency: 800Hz / 1600Hz
Passband ripple: 1 dB (doesn’t matter because stopband specs
will determine passband ripple)
Minimum stopband attenuation: 40 dB
Sampling Frequency : 8000 Hz
After entering the parameters, press OK. Run the VI and save
the filter coefficients as BPFIR800_1600.fds.
The filter length for this BP filter was the same as for the LP filter. Why did this happen?
Design a highpass FIR filter to meet the
following specifications:
Type: Highpass
Design Method: Kaiser Window
Stopband Edge Frequency: 1600 Hz
Passband Edge Frequnecy: 2000 Hz
Passband ripple: 1 dB (doesn’t matter because stopband specs
will determine passband ripple)
Minimum stopband attenuation: 40dB
Sampling Frequency: 8000 Hz
Run the VI and save the coefficient file as
HPFIR2000.fds
You should have noticed that all 3 filters had (nearly) identical filter lengths. For an FIR-based filter bank, this is a very important feature to have. Why? Hint: Think about the phase responses of FIR filters.
You should have noticed that the length of the highpass filter was slightly different than the other filters. What is the reason for this? Hint: Think about the 4 basic types of FIR filters (Type I – Type IV).
Open the lab3setup VI and change the filter coefficient’s
file in the DFD filter VI to the bandpass filter file. Save the VIwith a different name such as lab3filters.vi. Run the VI and make
sure it works fine.
Change again the filter file for the highpass filter file. Do
not change the name of the file. Save it and run it.
Once you have verified that your 2 additional FIR filters
work, add two more DFD Filter VIs to the block diagram and two Addfunctions.
Wire the output of the Add function that adds the Analog
Input node to the two new DFD filters. Configure the three DFDFilter VIs with the Lowpass, Highpass and Bandpass filter
files.
Put three Multiply functions and wire the output of the
filters to them. Place three numeric controls in the Front Paneland set them to 1.
Wire a numeric control to the other input of the Multiply
functions. Do this for each of the DFD Filter VIs. With this we aregoing to control the gain for that filter.
Wire the output of the Multiply functions to the Add
functions to add the three multiplications or gain sets. Wire theoutput of the last Add to the Analog Output node and the signal
input of the Spectral Measurements Express VI.
The resulting configuration is a 3-band filter bank that
covers frequencies in the range DC-4000 Hz. Note that the frequencyranges of these 3 filters were selected so that the transition
regions overlap perfectly (i.e. the edge of the passband of onefilter is the edge of the stopband of the prior filter). This type
of overlap yields the best frequency coverage and provides a nearlyconstant gain in each transition region. (i.e. As one filter starts
attenuating, the next filter starts amplifying).
Start your CD. Save and Run the VI. You should hear the music
basically unfiltered, since all frequencies in the range 0-4000 Hzare passed with a gain of 1. Start playing with the individual
Gains now to boost/attenuate the various frequency ranges byselecting gains<1 and>1. Don’t select gains too large
(greater than 3 or 4) or else you may start clipping the outputsignal. Call your TA over to verify that you have completed this
part of the lab.
When you are done with this lab, exit LabVIEW and LabVIEW Embedded Edition and log off the computer. Unplug the SPEEDY-33 board and turn off the headphone volume booster. Turn in your completed worksheet to the lab TA.
