An inverting opamp is a perfect mixer because one signal does not affect the other. Many signals can be added.
You can add signals then use the added signals to modulate a carrier frequency. That is how radio stations play music and how television signals do it.
Thank you very much audioguru for your good explanation, this thread is very informative to me.
Can you explain the last paragraph?
As far as I know the AM transmitters transmit a modulating signal with a carrier so that the modulating signal is an audio signal. Is there any other signal inside the modulating signal which would be modulated with the carrier signal too??