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I have a fundamental question. Why do we modulate a signal before the transmission? What is the obstacle for transmitting a signal in its typical form?

mohammad rezza
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    I think you are trying to ask the same thing as in [this other question.](https://electronics.stackexchange.com/questions/598767/what-is-the-purpose-of-a-carrier-signal-in-communication-technology) Is that correct? – JRE Oct 19 '22 at 12:41
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    Transmitting through what medium? Coax cable? Optical fiber? Over-the-air radio transmission? Also, have you read the [Wikipedia article](https://en.wikipedia.org/wiki/Modulation)? – Solomon Slow Oct 19 '22 at 13:13
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    Yes, my question is there. Thank you – mohammad rezza Oct 19 '22 at 13:14
  • OK. This is a duplicate, then. – JRE Oct 19 '22 at 13:17

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The signal can be in such media which doesn't carry the message far enough. Think, for ex. your speech. It's very tiny air particle movements and and air pressure changes bound together to make a soundwave. In otherwise full silence someone may hear you even over distance say 100 yards, but surely not if the distance is a mile and there's some traffic and wind noise. Thomas Edison invented in 1800s the carbon microphone which modulated DC current so that wires could carry the speech signal tens, even hundreds of miles without any amplifiers. At the far end of the wires a magnetic speaker recreated the audible speech.

It's well possible that you think "if the mic generates electricity from the speech, why to use wires? Shouldn't the electricity as well be able to jump into the free space from an antenna as a radiowave and be caught by another antenna and then be converted back to speech? -Maybe only some amplification should be needed to cover the loss which is obvious, because all of the transmitted wave cannot hit to the receiving antenna, but bypasses it and spreads to the space?"

That's perfectly possible, but the needed antenna would be hundreds of miles long. That comes from the physics. There one can easily prove that effective radiowave radiation (and also catching) happens in reasonable sized antennas only if the signal frequency is high enough. Signal got from a mic has far too slow voltage and current changes to cause any practically useful radiation from a practical size whip antenna.

Modulation inserts those slow changes to a high frequency sinewave so that the resulted signal can be radiated and caught by a practical antenna and the original mic signal is still possible to be reconstructed in the receiver. The speed of voltage and current changes of the modulated signal is determined mostly by the frequency of the sinewave and the speed variation caused by the modulating speech are so small that the radiation and catching abilities of the antennas can be calculated based on the sinewave (called often "the carrier") frequency.

Modulation gives another advantage: By modulating signals to different carrier frequencies they can occur at the same time in the same space without disturbing each other. Radio receivers have been more than 120 years designed to ignore the wrong carrier frequency signals by applying bandpass filters.

Hwacot Chickadwa
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    Hi Hwacot. This is a well written answer. But it probably won't get much attention, since this question has been closed due to being a duplicate. I'd advise that you also add this post as answer to the [original question](https://electronics.stackexchange.com/questions/598767/what-is-the-purpose-of-a-carrier-signal-in-communication-technology). – Velvel Oct 19 '22 at 14:01