More about the
Software D.U.T. mode.
See the attached Block Diagram.
You can view it while running the Analyzer, activating the "show Help" button (bottom right).
If you get another block diagram, it means that you are running another Analyzer mode.
Please select the appropriate Analyzer mode using the Mode Selector (top left).
Let's go back to the Block Diagram.
On the left side we select the signal that will be conveyed into the Software D.U.T. We get the choice between:
- exploiting the built-in
Generator- exploiting the sound card
Left In (this is the Left Analog-to-Digital converter of your sound card)
- exploiting the sound card
Right In (this is the Left Analog-to-Digital converter of your sound card)
The D.U.T. can thus be fed by digital-domain signals coming from the Generator, or Speech and Music realtime grabbed by the sound card.
The 3-to-1 selector output feeds :
- the Software D.U.T. input
- the ch1 Analyzer input
This way, the Analyzer gets his Reference signal. One need to tell the Analyzer that ch1 needs to be considered as Reference signal. This is done in the Analyzer control panel, setting the "reference" to ch1.
In this particular case, the Software D.U.T. consists on a BiQuad IIR Filter. For controlling it, we need to specify five parameters using five knobs :
- Fc is the resonance frequency to be set anywhere between 100 Hz and 10 kHz
- Q is the resonance factor (quality factor) to be set anywhere between 0.001 and 4 (a zero value would produce a Math overflow)
- LPF is the Lowpass contribution to be set anywhere between -1.0 and +1.0
- BPF is the Bandpass contribution to be set anywhere between -1.0 and +1.0
- BPF is the Highpass contribution to be set anywhere between -1.0 and +1.0
Double-clicking on a knob resets its to the median position.
Quite important to know is that the control box hosting the five knobs is relying on the
Bilinear Transform, defining the way the p-domain (analog BiQuad built using two integrator) gets mapped to the z-domain (digital BiQuad built using two delay cells).
A popular Bilinear Transform implementation for BiQuad IIR filters, is the one published by Robert Bristow-Johnson (the RBJ Audio EQ Cookbook) here
http://www.musicdsp.org/files/Audio-EQ-Cookbook.txt.
The BiQuad IIR Filter output feeds the ch2 Analyzer input.
One need to tell the Analyzer that ch2 needs to be analyzed and plotted. This is done in the Analyzer control panel, with ch2 attribute set to "Analyzer". Actually, as only ch2 needs to be plotted, all the other channels (ch1, ch3, ch4, ch5) need to have their attribute set to "Off".
One need to tell the Analyzer that ch2 Impulse Response needs to be computed and plotted. This is done in the Analyzer control panel, setting "impulse" to ch2.
Looking again to the block diagram, we see the sound card Digital-to-Analog section receiving two signals. Doing so you can listen what's happening through your PC headphone or speakers :
-
Left Out delivers the reference signal (D.U.T. input signal)
-
Right Out delivers the filtered signal (D.U.T. output signal)
Specifying a pure Lowpass (LPF = +1.0 while BPF and HPF are set to zero), you will realize the kind of error the
Bilinear Transform exhibits, with Fc set past 4 kHz.
Configure the Generator. Select the Pink Noise, with a -20 dB amplitude.
You should hear the Generator through your PC speakers or headphones.
Configure the IIR BiQuad filter used as Software D.U.T.
Select the Generator as source.
Set Fc to 4000 Hz and Q to 0.707.
Set LPF to +1.00, leaving BPF HPF to zero.
You should hear the IIR BiQuad filtered signal through your PC speakers or headphones.
Start the Analyzer, specifying a 0.50 sec. timebase and a 4096 samples FFT length.
Look the Gain asymptot plot. This is not -12 dB/octave anymore.
For restoring a proper -12 dB/octave asymptot, the HPF contribution needs to be set to -0.021.
This is the kind of knowledge the Analyzer can deliver to you. Pretty amazing, isn't?
Check the ch2 Phase plot also, selecting "Gain Phase" instead of "Gain" in the Analyzer control panel.
Mostly, I'm using the "True Segments" rendering for the Gain and Phase plots. I like it.
Check also the ch2 Impulse Response. For that one I'm using the "True bars" rendering.
The blue vertical bar showing on the Block Diagram is a mute line, setting the audio to zero when the Software D.U.T. mode is not in use. It has no effect on the measurements.