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As a newbie I am trying to understand I and Q modulation and demodulation. So from what I read the I and Q components of a signal separately mix with RF signals that are 90 degrees out of phase with each other and then the summation is transmitted as an RF signal.

So let's say I am listening to a voice of a reporter via FM radio. The reporter's sound wave gets converted to electricity with a microphone and then presumably gets converted to EM wave with an antenna. I am kind of lost where modulation happens and how would his sound voice gets divided to I and Q before modulation. Is it done mathematically and digitally?

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  • $\begingroup$ @MBaz thank you I read them, but I am still confused about how the I and Q signals are generated from the baseband signals. $\endgroup$ – Jack Aug 3 '16 at 14:32
  • $\begingroup$ Can you please edit your question and add more detail about what your exact question is? The I and Q signals are baseband signals, so I don't know how to proceed from here. $\endgroup$ – MBaz Aug 3 '16 at 18:59
  • $\begingroup$ @MBaz m(t) is the baseband signal( information) which gets divided to I and Q for further processing (modulation) I was just wondering how we go from m(t) to Iand Q. $\endgroup$ – Jack Aug 3 '16 at 19:02
  • $\begingroup$ If this is related to your other question, the only way to answer is by defining the baseband processor. In other words, there is not a single answer. Usually, the input to the processor is not a continuous signal but a sequence of bits, which are divided into two streams, one of which is transmitted in-phase and the other in quadrature. $\endgroup$ – MBaz Aug 3 '16 at 22:34
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Analog QAM as an example:

https://en.wikipedia.org/wiki/Quadrature_amplitude_modulation#Analog_QAM

same idea for other IQ modulations schemes.

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For FM modulation of a real audio signal to a complex IQ signal at baseband, you would create a complex IQ signal (of constant magnitude) such that the derivative of the phase of your complex IQ waveform, d(unwrapped.atan2(Q,I))/dt, was proportional to the amplitude of your audio input signal. The proportionality constant will be related to your desired FM modulation index if/when you later use your IQ signal to modulate a carrier (via complex multiplication).

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  1. Reporter talks into the microphone
  2. Microphone converts real sound into a "real" signal.
  3. Real signal goes into an encoder that chops up the signal and converts this chopped real signal into a chopped complex signal.
  4. The encoder's output is fed into an RF channel
  5. The RF channel carries the complex signal on its back and flies over the air to your radio (or repeaters in between, and finally to your radio).

Let's talk about #3

Your analog voice is first converted to digital audio. Then this audio signal, will be grouped into chunks, or blocks, of binary digits. Then a modulation is chosen. The chosen modulation will determine how many binary bits can be encoded into each one symbol of that modulation scheme. For 4-QAM, its two bits per symbol, for 64 QAM its 8 bits per symbol. Each symbol in these modulations is a complex number represented as I-Q.

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