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When the Demod button is active in Decimate mode, the waveform display shows half of the difference between the maximum positive and negative raw data points that are included in each decimated display point.
Note that this means the displayed waveform will always be positive.
This mode is included to support simple schemes for using a standard AC-coupled sound card to record DC or very slow signals, like temperature or atmospheric pressure.
If your measurement system (such as a strain-gage force or pressure transducer) uses an "AC bridge", you apply an AC "excitation" signal to the bridge. The output of the transducer is an AC signal whose amplitude encodes the measured value.
Similarly, you can apply an AC signal across a linear potentiometer, and the amplitude of the signal at the wiper indicates the wiper position.
An AC signal can drive an LED light source, and a photodiode can provide an AC signal whose amplitude is proportional to the amount of light from the LED that hits it. This can be used to measure light transmission, or the position of an interruptor vane that partially blocks the light.
An intrinsic DC signal (like thermocouple output or battery voltage) can be applied to a "chopper" circuit that simply switches the raw input on and off at (say) a few hundred or a few thousand cycles per second, producing a unipolar rectangular wave at a frequency the sound card can easily deal with. The exact switching frequency is unimportant, since the DC signal is encoded into the amplitude of the waveform.
Note, however, that since the reported amplitude is half of the peak-to-peak difference, the resultant trace will be only half the size of the raw input. This can easily be corrected via the External Gain controls.
Alternatively, the chopper can switch between the normal input signal and an inverted version, turning it into a bipolar rectangular wave. This chopper is only slightly more involved than the unipolar version, but gives the correct amplitude with no gain changes. The External DC-to-AC Modulator circuit (actually designed to support Decimate Signed mode) supports this as an option.
To use Demodulate with a chopper, you set Decimate on with a Decimate Rate that is less than twice the chopper switching rate. For example, if the chopper switches at 250 Hz, you should set Decimate Rate to less than 500 Hz. Since the external switch rate is typically subject to component tolerances, it's best to have a safety factor, such as setting Decimate Rate lower than the switch rate.
Whether you use a unipolar or bipolar chopper, Demodulate will work properly only with unipolar (positive or negative only) raw input signals, since only the peak-to-peak amplitude of the wave is recovered. (Negative-only signals will appear to be positive-only.) See the more-complex Signed option for bipolar raw input signals.
Decimate Demodulate can be used to demodulate a conventional amplitude-modulated waveform (such as a bridge, potentiometer, or LED as mentioned earlier), with some qualifications. First, assuming that the carrier (the excitation frequency being modulated) is a sine wave (instead of the rectangular wave used by a chopper), the Decimate Rate should be set lower than the carrier frequency (not twice the carrier).
Second, the resulting demodulated waveform will be the effective amplitude of the carrier at any moment in time. In other words, if it is multiplied by the carrier waveform using a true signed multiply, the result will be the original modulated signal.
This would typically be used for "envelope extraction", such as to obtain the envelope of speech or music for later application to a completely different "carrier" waveform or random noise. (You'd save the demodulated waveform as a file, then load it as a Play Wave in one stream, and use that stream to modulate another stream containing the other waveform or noise.)
(You might imagine that Envelope mode would do this, but although it shows the envelope nicely it is not recommended here. It has separate positive and negative values that make it inappropriate as a modulator.)
However, if you apply Demodulate to a conventional AM waveform, such as a carrier sine wave modulated by a lower-frequency sine wave, the demodulated waveform will not swing about zero like the original modulating sine.
For example, if there is no modulation (depth = 0), the demodulated wave will be a constant value, equal to the peak amplitude (half the peak-to-peak value) of the carrier wave. As depth increases, the demodulated wave positive peaks will stay at this value, and the waveform will grow downward from there.
At 100% depth the waveform will swing down to 0, as expected, but at 25% depth it will swing down to 25% below the peak (75% of the peak). Thus, the modulating waveform swings about a baseline that is dependent on depth according to:
Baseline = Amplitude - (Depth / 2)
If the modulation depth exceeds 100%, the demodulated waveform will "bounce back" from the zero line where the original modulating waveform goes negative and changes the sign of the carrier.
As with all Decimate modes, you will probably want to run with Trigger off whenever the effective sample rate is below about 500 Hz. This will give a scrolling waveform like a chart recorder or data logger, so the trace remains "live" without long waits for trigger events.
But if you do use Trigger with Demodulate (presumably at higher effective sample rates), note that the triggering operation is applied to the decimated data, not to the raw data. The Trigger Source label becomes a button marked 'Source Decimate', which (unlike in Envelope mode or plain Decimate mode) can't be toggled to 'Source Raw'
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