The FDM telefone transmission uses a kind of modulation, like many other types of trasmission.(either by air or cable) There are amplitude modulation, frequency and phase modulation ( that represent the main types of modulation ). What is modulation?
Modulation consists in modifying the characteristics of a signal, called carrier, in accordance with another signal, called modulating signal.The resulting wave form is called modulated signal.
To adapt the signals to the transmitting channel
1. To adapt the signal to the receiving and transmitting circuits
2. To distinguish the various signals
There are two types of modulation:
• Analogic modulation
• Digital modulation
In my opinion the most important types of modulation are amplitude end frequency modulation. Infact these two tecnologies are used in analogue and digital modulation
FM modulators are devices used for generating a frequency modulated signal. They can be of two types:
• Direct modulators
• Indirect modulators
In direct modulators a frequency modulated signal is obtained by checking the carrier frequency directly through the modulating signal.
In indirect modulators a frequency modulated signal is obtained by acting indirectly on the carrier frequency.
Am modulators are devices used for generating an amplitude modulated signal. They can be of two types:
• Diode am modulators
• Transistor am modulators