In the first diagram (broadcast AM) "the message" is a sinewave of frequency \$\omega_m\$. In the second diagram (full AM suppressed carrier modulation) "the message" is also a sinewave of frequency \$\omega_n\$. OK so far? - "the message" signal or modulating signal is just a plain ordinary sinewave.
Either of these two methods produce two sidebands at either side of the carrier frequency. So if the carrier were 1 MHz and the modulation ("the message") were 2 kHz, you would see frequencies of 998 kHz and 1002 kHz on a spectrum analyser. These are the upper and lower sidebands.
If you filtered out the upper sideband you would be left with only the lower sideband and if this was mixed (in a receiver) with a sinewave of 1MHz, you'd recover the original modulating frequency (aka "the message").
Please explain in TIME domain with diagram. Please focus on conceptual
treatment rather than individual techniques of modulation and
demodulation.
No I'm not going to do that. If you don't understand my words then please let me know.
What you call envelope detector, consists of the detection diode and a low-pass filter. This is one way of demodulating an AM signal, more precisely, it is called asynchronous demodulation.
If you do not connect the diode, and only you process the AM signal with a low pass filter, you get the modulating signal, to which is added the carrier signal. Watch the third oscillogram well. It is the component of the modulating signal to the signal shaping, while is added a proportion of the carrier signal, attenuated by the filter.
The correct operation, is to use the envelope detector. The problem we are having is that the modulating signal is too high (amplitude), so when you make the detection and filtering, unwanted components appear, causing the distortion you see in the second oscillogram.
In the first oscillogram, one can see that the modulation index is extremely high; you should decrease the amplitude of the modulating signal and use the envelope detector to demodulate the transmission.
Best Answer
I don't understand how you could have studied AM demodulation and not understand the purpose of an envelope detector. Something is lacking in your studies. In any case, in AM (the M stands for modulation) all of the information is contained in the amplitude variations of the carrier. That amplitude variation is usually called the envelope of the resultant AM signal due to its appearance on an oscilloscope. Hence, to recover the signal information, the envelope of the AM signal must be extracted. This is the job of the envelope detector, hence the name. For analog AM, such as ordinary broadcast radio, an envelope detector is as simple as a diode and a low pass filter. Any communications textbook can provide more information.