There are very different ways, information can be superimposed to a RF carrier signal. In the early years of wireless signal transmission, morse code has been transmitted by the means of simply switching on and off the carrier, in the thirtieas, ways have been found to use wireless transmission for telephony. Spoken information or music has to be modulated on the carrier signal.
In modern transmitters, the modulation type can be selected, in the receiver one has to select the corresponding reception mode to demodulate the signal correctly.
On the transmitter side, this „modulation“ is easily made: the morse code signal is generated by simply interrupting or keying the carrier signal. In advanced transmitters, keying is done by leding a negative voltage to a grid of the transmitting valve and thus interrupting the flow of electrones from the kathode to the anode or plate of the tube.
For reception of these signals, the signal of a „beat frequency oscilator“ or BFO has to be superimposed to the reception signal, in a simple AM broadcasting receiver, only a slight noise in the rhythm of morse might be heard from the speaker when no BFO is available.
So only receivers which have a switch for „BFO“ or a position CW or „telegraphy“ on the reception mode switch can be used to demodulate CW- or A1 - transmissions in morse code. Usually, there is a BFO note control to set the BFO to a BFO note pitch as desired.
Also TRF receivers with regeneration („regenerative sets“) can be used to make CW transmission audible, if you use the regeneration control to set the radio to oscillation.
Telegraphy modulated or modulated tone telegraphy is an operation mode in which the RF carrier is modulated with a tone frequency in the rhythm of morse code (in the very early years of wireless telegraphy, often the frequency of the power generator has been used for this).
In contrast to the unmodulated carrier of A1-transmissions, which is not audible in a standard AM radio, these tone modulated signals can be heard with a receiver in standard AM mode or even with a very simple crystal detector set.
For broadcast reception on mediumwaves and longwaves and also for telephony on early military and commercial wireless equipment, „amplitude modulation“ has been used. These signals can be heard with simple crystal detector sets as well as with advanced modern superheterodyne receivers - without having to make use of a beat frequency oscillator. Nowadays, this operation mode is known as A3 or A3E.
In AM mode, the signal strength of the carrier is modulated by the audio waveform; the low frequency audio signal can be recognized in the envelope of the radio frequency carrier. The modulation index gives the intensity of modulation of the carrier and is calculated from (a-b)/(a+b) * 100%.
A simple crystal detector or a valve or semiconductor diode is sufficient to demodulate such signals.
The requirements conncerning energy and bandwidth is quite high, when amplitude modulation is used. A relevant fraction of the transmitter output is used for the carrier and not carrying any information, the audio information is transmittet twice in both sideband with the same content. The bandwidth used is relatively wide as it covers twice the bandwidth of the highest frequency transmitted i.e. a signal with high audio quality uses more bandwidth or a wider frequency channel.
Signals using amplitude modulation are subjected to propagation disturbances („Fading“), static discharges and man made noise from sparks in motors and generators.
For a more efficient use of the transmitter output and bandwidth, single sideband modulation is used: Only one sideband containing the audio or voice information ist transmitted, and not two identical sideband or the carrier signal which does not contain any audio information. All the output of the transmitter valve can be used for transmitting informations and less bandwith is used.
In a single sideband transmission, the upper or the lower sideband can be transmitted, if on the receiver not the correct setting (USB or LSB) is used, the audio is not intelligible.
For military and commercial communication, always USB (Upper Sideband) is used, for amateur radio, on frequencies below 10 MHz LSB and on higher frequencies, USB mode is used.
For correct demodulation, in the receiver an auxiliary carrier signal instead of the carrier signal from the transmitter has to be added to the signal. High quality receivers use an oscillator with a fixed frequency below or above the reception frequency, on simpler receivers, a BFO is used. For correct tuning, a finetuning mechanism is mandatory, as when the auxiliary carrier signal is not exactly at the position of the omitted carrier, the voice sounds far too low or too high, like the voices in a Walt disney comic film.
A special transmission mode is ISB („independent side band“). In one signal two sidebands containing not identical information are sent from the transmitter. This technology has been used in shortwave feeder links of international shortwave services, so at the same time and from the same transmitter, a programme in french and english language could be transmitted.
On higher frequencies above 30 MHz, signal bandwidth or channel spacing is less important. FM signals have much better audio quality and are easier to understand and a „squelch technology“ to mute the receiver when no communication is on the air, can be used. Fading does not affect the transmission quality.
In a transmitter using frequency modulation, the transmitting frequency is altered according to the audio information transmitted. In the receiver, demodulation is far more demanding as a limiter (to eliminate unwanted residual amplitude modulation) and a discriminator (for the demodulation) stage is necessary.
In contrast to FM broadcasting, in which channel spacing of 100 kHz (or in some countries 50 kHz) is used, in military and commercial communication, a channel spacing of 50 kHZ, in new generation sets 25 kHz or even 12,5 kHz is in use. THen this kind of „Narrow band FM“ is used, audio bandwidth is smaller and the audio quality is poorer then with FM broadcasts, but still similar to AM signals.
For radioteletype, usually FSK („Frequency Shift Keying“, older designation F1, more modern F1B) is used. In the most simple form, the transmitter frequency is shifted between the states of „Mark“ and „Space“ (like 0 and 1 in computers) for a certain amount („Shift“, this amount can be selected in the radioteletype decoder).
As an alternative, a single sideband signal can be used to transmit an audio signal consisting of two tones for transmitting radioteletype messages, this mode is called „Audio Frequency Shift Keying“.
Quite similar is mode to transmit image data in a signal with frequency modulation, the designation for this „Faksimile“ mode is F4 or nowadays F3C.
A completely different approach is the HELL radioteletype mode. This has taken it's name from the German engineer Rudolf Hell, the developer of the Hellschreiber. In fact, this is a faksimile mode, in which all letters are changed into a 7 x 7 dot matrix, and this information is transmitted as audio signal over an amplitude modulated transmitter.
In the side of the receiving station, the Hell -signals are fed to the Hell - printing machine, a spiral writing head is electromechanically moved on a colour ribbon and the text is printed on paper tape which can be glued on the telegram form.
In Switzerland, the Army used commercially available Hell systems made by Siemens on high power stations on behalf of the Army or armed corps commands. The Hell System never has seen tactical use, as the Feld Hell in Germany during World War II.