This same signal is converted by the transmitting antenna into electromagnetic waves that will be sent to a receiving antenna. The electromagnetic waves resulting from the transformation of the electrical signal produced by the microphone travel at the speed of light, reflect on the ionosphere to end up in a receiver antenna.
Terrestrial relays are used to ensure that the waves reach receivers located far from the transmitter. Satellites can also be used.

Once the electromagnetic waves reach the receiver, the receiving antenna transforms them into an electrical signal. This electrical signal is then transmitted to the receiver via a cable. It is then transformed into an audible signal by the receiver elements.
The sound signal obtained in this way is reproduced by the loudspeakers in the form of sounds.

The transmitter is an electronic device. It ensures the transmission of information by emitting radio waves. It essentially consists of three elements : the oscillation generator which ensures the conversion of the electric current into the radio frequency oscillation,
the transducer which ensures the transmission of information through a microphone, and the amplifier which, depending on the frequency chosen, ensures the amplification of the force of the oscillations.

The receiver is used to pick up the waves emitted by the transmitter. It is composed of several elements : the oscillator, which processes the incoming signal, and the outgoing one, and the amplifier, which amplifies the electrical signals captured.
the demodulator that ensures the exact retransmission of the original sound, the filters that ensure the elimination of signals that could spoil the proper perception of the messages, and the loudspeaker that serves to convert the electrical signals into sound messages so that they can be perceived by humans.

We sometimes hear about "carrier" (carrier in English) or "HF carrier" without really knowing what it is. A carrier is simply a signal that serves as a medium to carry the useful signal (the one you want to transmit such as voice, music, analog or digital data).
When we stay in the field of analog transmissions, the carrier is a simple and unique sinusoidal signal. In the field of digital broadcasting (DTT and DTT for example) there are a multitude of carriers who share the information to be transmitted.
We will not talk here about the case of these multi-carriers. The particularity of a carrier is that it oscillates at a much higher frequency than the maximum frequency of the signal to be transmitted. Suppose you want to transmit a spoken or sung speech for 10 km around (or in the black if the speaker speaks quickly).
A single transmitter is used that "emits waves" that several receivers can pick up simultaneously.

But physics cannot be invented. If you want to transmit the speaker's voice by simply connecting a wired loop or a huge antenna to the output of the LF amplifier, it will work but not very far (count a few meters or even tens of meters).
In order for transmission to take place over a comfortable distance, a carrier wave must be used, which acts as an intermediary and which has less difficulty in crossing distances. The choice of the frequency of this carrier wave depends on :

- the type of information to be transmitted (voice, radio, news or digital HD TV),

- expected performance;

- the distance you want to travel,

- the relief of the terrain between transmitter and receiver (from 50 MHz, the waves propagate more and more in a straight line and fear obstacles),

- the price you agree to pay to your electricity supplier or battery reseller,

- authorisations that the competent authorities are willing to grant us.

Because you can imagine the problems of the waves that collide if no one came to put a little order in this ! All of this is highly regulated, and frequency ranges have been reserved for this or that type of transmission (CB, radio broadcasting, television, mobile phones, radars, etc.).
In addition to these frequency range reservations, fairly strict technical characteristics are required of the transmitting circuits to limit as much as possible the risk of interference with other equipment that does not necessarily operate in the same frequency ranges.
Two neighboring transmitter circuits that work at very high frequencies and close to each other can very well jam a receiver working in a much lower frequency range. Especially true if the devices are homemade and they are insufficiently filtered in HF output.
In short, before venturing into the field of broadcasting, it is better to have some knowledge of the risks of interference involved.

In this mode of transport, we have a carrier whose amplitude remains constant regardless of the amplitude of the modulating signal. Instead of changing the amplitude of the carrier, its instantaneous frequency is changed. In the absence of modulation (amplitude of the modulating signal equal to zero), the frequency of the carrier remains at a perfectly defined and stable value, which is called the center frequency.
The value of the carrier frequency shift depends on the amplitude of the modulating signal : the greater the amplitude of the modulating signal, the farther the carrier frequency is from its original value. The direction of the frequency shift depends on the polarity of the alternation of the modulating signal.
For a positive alternation the frequency of the carrier is increased, and for a negative alternation the frequency of the carrier is decreased. But this choice is arbitrary, we could very well do the opposite ! The amount of variation in the carrier frequency is called the frequency deviation.
The maximum frequency deviation can take different values, e.g. +/-5 kHz for a carrier frequency of 27 MHz or +/-75 kHz for a carrier frequency of 100 MHz.
The following graphs show a modulating signal with a fixed frequency of 1 kHz modulating a carrier of 40 kHz (the horizontal scale is well dilated to better see what is happening on all the variations).

If we replace the fixed modulating signal of 1 kHz with a real audio signal, this is what it looks like.
This second set of curves is quite telling, at least for the green curve for which the maximum frequency deviation is very clear because it is "well adjusted". If we make the correspondence between the modulating signal (yellow curve) and the modulated carrier (green curve), we can see perfectly that the variations in the amplitude of the carrier are slower
- which corresponds well to a lower frequency - when the modulating signal is at its lowest value (negative peak).
On the other hand, the maximum frequency of the carrier is obtained for the positive peaks of the modulating signal (a little less easy to see on the curves, but we feel it with the most "filled" parts).
At the same time, the maximum amplitude of the carrier remains perfectly constant, there is no amplitude modulation related to the modulating source signal.

To make an FM receiver, you can get by with a few transistors or with a single integrated circuit (a TDA7000 for example). But in this case we get a standard listening quality. For a "high-end" listening, you have to go all out and know the subject well. And this is even more true when it comes to decoding a stereo audio signal.
And yes, without a stereo decoder, you have a mono signal where the left and right channels are mixed (if the radio program is broadcast in stereo of course). From a high-frequency point of view, the source signal is not visible in the amplitude of the carrier and you can't be satisfied with a rectifier/filter like the one used in an AM receiver.
As the useful signal is "hidden" in the frequency variations of the carrier, a way must be found to transform these frequency variations into voltage variations, a process that is the opposite (mirror) of the one used for transmission.

The system that performs this function is called an FM discriminator and basically consists of an oscillating (and resonant) circuit whose frequency/amplitude response is in the shape of a "bell". For the discrimination function, discrete components (small transformers, diodes and capacitors) or a specialized integrated circuit (SO41P for example) can be used.

In its simplest application, a digital transmission gives the carrier the possibility of having two possible states that correspond to a high logic state (value 1) or a low logic state (value 0).
These two states can be identified by a different amplitude of the carrier (obvious analogy to be made with amplitude modulation), or by a different value of its frequency (frequency modulation).
In AM mode, for example, we can decide that a modulation rate of 10% corresponds to a low logic state and that a modulation rate of 90% corresponds to a high logic state.

In FM mode, for example, you can decide that the center frequency corresponds to a low logic state and that a frequency deviation of 10 kHz corresponds to a high logic state.
If you want to transmit a very large amount of digital information in a very short time and with strong protection against transmission errors (advanced error detection and correction), you can transmit several carriers at the same time and not just one.
For example, 4 carriers, 100 carriers, or more than 1000 carriers.
This is what is done for digital terrestrial television (DTT) and digital terrestrial radio (DTT), for example.

In old remote controls for scale models, a very simple digital transmission function could be used : activation or deactivation of the transmitter's HF carrier, with a receiver that simply detected the presence or absence of the carrier (without a carrier we had a lot of breath so "BF" of high volume,
and in the presence of a carrier, the breath disappeared, the signal "BF" disappeared).
In other types of remote control, a principle of "proportionality" was implemented which made it possible to transmit several pieces of information in a row, simply using monostable ones producing slots of varying duration. The duration of the pulses received corresponded to very precise "numerical" values.

The transmission of speech does not require great sound quality, as long as it is a question of conveying an informational message. The main thing is that we understand what is being said. On the other hand, we expect more from the quality of transmission when it comes to a singer's voice or music.
For this reason, the transmission methods used for a pair of intercoms or walkie-talkies and those used for broadcasting are not based on strictly identical rules. We cannot say that we have a necessarily better sound with frequency modulation transmission than that transmitted in amplitude modulation (AM in French, AM in English).
Even if it is obvious that your hifi tuner gives better results on the FM band 88-108 MHz. If you want to, you can do quite well in AM and you can do very badly in FM. Just like you can do very good analog audio and very bad digital audio.
If you want to transmit music from one room to another in your house or from the garage to the garden, you can build a small radio transmitter that can transmit on the FM band or on the small wave band (PO in French, MW in English), in which case a commercial receiver can do the complement.
In FM you will get better sound results, simply because the broadcasting standards provide a much different bandwidth than that available in the AM (GO, PO and OC) bands. The higher sensitivity of an AM receiver to ambient interference (atmospheric and industrial) also has a lot to do with it.

Here, it is a question of transmitting an analogue value such as a temperature, a current, a pressure, a quantity of light, etc., which will first be transformed beforehand into a direct voltage that is proportional to it.
There are several methods and of course each has its advantages and disadvantages, you can use amplitude modulation or frequency modulation. The term amplitude modulation or frequency modulation is somewhat exaggerated since if the analogue value to be transmitted does not vary,
The carrier retains its amplitude and frequency characteristics that correspond to the value to be transmitted in progress. But we must speak of the greatness that varies. In fact, it is no more difficult to transmit information that varies little (if at all) than information that varies rapidly.
But you can't always use a classic AM or FM radio transmitter (available commercially made or in kit form) because the latter may very well have a low-pass filter at the input which limits slow voltage variations.

And if a link capacitor is implanted in the path of the input signal, then the operation is simply impossible ! Modifying such an emitter to make it "compatible" is not necessarily always easy...
which may involve the design of a specialized transmitter/receiver assembly for the operation.
But if we look at the problem from the side, we realize that we can very well transmit a signal whose amplitude, depending on the value of the continuous voltage to be transmitted, itself causes the carrier to vary. And if the intermediate modulating signal is within the audible band (e.g. between 100 Hz and 10 kHz), then the use of a conventional radio transmitter can be considered again.

As you can see, a simple voltage/frequency converter on the transmission side and its complement a frequency/voltage converter on the receiver side is one solution among other examples.

Be careful not to confuse "digital transmission" and "digital data transmission". We can transmit analog information with a digital transmission mode, just as we can transmit digital data with an analog transmission mode, even if for the latter case we can discuss it.
To transmit digital data with an analogue transmission mode, it can be assumed that the electrical levels of the digital signals correspond to the minimum and maximum of an analogue signal.
However, be careful with the shape of the digital signals, which if they are fast and square, can contain a high rate of harmonics that cannot necessarily be digested by the transmitter.
It may be necessary to transmit the digital data with signals having an "analog form" such as sine. If the digital data to be transmitted is very important (secure access with access code, for example), a few precautions must be taken.

In fact, in no case can it be considered that the transmission from one point to another will be free of defects, and part of the information transmitted may very well never arrive or arrive distorted and unusable.
The information transmitted can therefore be supplemented by control information (CRC for example) or simply be repeated two or three times in a row.
https : //onde-numerique.fr/la-radio-comment-ca-marche/