Analog demodulation challenge - labAlive experiment

A composite signal consists of several message channels carrying music, speech, text, and noise signals. To extract the essential information, demodulation techniques like AM, FM, QAM, and SSB must be utilized.

4 differently modulated channels are combined into one signal

SSB demodulation

The real box is used to convert a complex signal into a reel-valued signal.

For the single-sideband (SSB) modulation the phasing method is used. The phasing method uses complex IQ processing to resolve the superposition of lower and upper sideband. The Hilbert transformer filters the Q component and shifts it by 90ï¿½, to select a certain sideband.

For more information: SSB modulator

Analog QAM demodulation

The Analog QAM (Quadrature Amplitude Modulation) is a technique in which the modulation is a combination of AM and phase shifting.

QAM is available in "analog" and "digital" form. In this certain case we use the analog QAM.

This example shows an analog QAM signal generated with the program. Here you can see two signals in inphase and quadrature. The quadrature signal has a 90 degree phase rotation to the real signal. The resulting combined modulated signal (complex) looks like a conventional double-sideband suppressed carrier of AM, but in the case of QAM each of the two sidebands carry different information.

AM demodulation

For the AM demodulation a receive filter has to be used. Therefore you can open the properties for the filter by clicking on the marked box.

For more information: AM modulator

FM demodulation

For the FM demodulation the FM demodulator has to be used. Therefore you can open up its properties by clicking on the marked box.

For more information: FM modulator

The received signal consists of four channels, your task is to demodulate the different channels to get its audio signal. Its is emphasized that one channel can also consist of two subchannels. When the app is started, one of the three signals that need to be demodulated is already set as input. Furthermore, a sample file from server can also be imported via the signal generator menu shown below.

Channel Selection

There are one or more channels displayed. To select a certain channel mix a carrier to the base band and cut off other signals. We do that with the local carrier "cos(ωt)+jsin(ωt)" and the low pass filter.

Example

First of all we select a certain channel. In this case we have chosen the channel around -16.54kHz and use the receive filter with 5.5kHz to filter the other signals. To take a closer look on the channel, open the spectrum analyzer after the receive filter.

In this case you can use the complex oscilloscope to identify the modulation type. Now it is visible that it is a frequency modulation.

To monitor the results of your chosen demodulation, open the spectrum analyzer and the audio player at the wire of the related end point.

If the appropriate demodulator is selected, you will be able to hear the audio clearly. However, if you hear whistling sounds or crackling noises instead, it indicates that the demodulation method used is not suitable for that particular channel.

Best of luck in finding the right demodulation approach!

Solution

Solution for "Analog demodulation transmission signal 1".

Channel	Kind of modulation, dish	Start
Channel 1: -16.54kHz	FM, "sunday roast"
Channel 2: -5.51kHz	AM, "sticky toffee pudding"
Channel 3: 5.51kHz	QAM, "chicken tikka masala"
Channel 3: 5.51kHz	QAM, "bangers and mash"
Channel 4: 16.54kHz	SSB, LSB="Yorkshire pudding"
Channel 4: 16.54kHz	SSB, USB= "afternoon tea"

Solution for "Analog demodulation transmission signal 2".

Channel	Kind of modulation, dish	Start
Channel 1: -16.54kHz	SSB, LSB="Porridge"
Channel 1: -16.54kHz	SSB, USB="apple pie"
Channel 2: -5.51kHz	FM, "sausages"
Channel 3: 5.51kHz	SSB, LSB="sandwiches"
Channel 3: 5.51kHz	SSB, USB="pancakes"
Channel 4: 16.54kHz	AM, "fish and chips"