Sinjalet e Radios mund te perdoren per te transmetuar informata.
Informacioni, i cilat mund te jene zeri, te dhenat ose forma te tjera perdoret per te modifikuar nje frekuenc te vetme te quajtur si bartes/transportues. Informacioni bashkohet me transmetuesin/bartesin dhe e krijojn nje forme nje sinjal radius i cili transmetohet deri te pranuese.Ky funksion njihet si modulimi ose pershtatja e sinjalit per dergim. Pra Modulimi eshte proces i ndryshimit te nje ose me shume vlerave tek nje sinjal konstant/periodik
Ketu pranuese e terhjek informacionin nga sinjali i radios dhe e rikonstrukton sinjalin ne formen baze te njohur si demodulim. Njihen shume lloje te modulimeve por te gjitha hyna ne tri kategori :
1. Modulimi me amplitud
2. Modulimi me frekuenc
3. Modulimi me faze edhe pse modulimi me frekuenc dhe faze jane pothuajse te njejt ne esenc.
Nje modulator eshte nje paisje qe ka aftesi me modelu sinjalin ose informaten.
Nje demodulator eshte nje paisje qe e ka aftesin me rikthy sinjalin ne formen qe ishte
Ne rastin kur e perfundojm me modulimin ateher nepermjet multipleximit mund te bashkohen frekuencat dhe te dergohen paralelisht nepermjet nje mediumi te perbashket.
Nje Modem (qe rrjedh prej fjaleve modulator–demodulator) mund te performoje te dy funksionet njekohesisht.
Radio carrier
Padyshim baza e nje sinjali te radio or dergese eshte vet transmetuesi. Ai perbehet nga nje vale sikur ne figuren 1-0.
Ky sinjal gjenerohet nga transmetuesi dhe nese e ka(rrezaton) ne kete forme ateher nuk ka fare informat dhe tek pranuese shifet sikur sinjal konstant.
Figure 1-0. An alternating waveform.
Amplitude modulation
Figure 3-2. An amplitude modulated signal.
Figure 3-3. A simple diode detector circuit.
Figure 3-4. Synchronous AM demodulation.
A simple example: A telephone line is designed for transferring audible sounds, for example tones, and not digital bits (zeros and ones). Computers may however communicate over a telephone line by means of modems, which are representing the digital bits by tones, called symbols. If there are four alternative symbols (corresponding to a musical instrument that can generate four different tones, one at a time), the first symbol may represent the bit sequence 00, the second 01, the third 10 and the fourth 11. If the modem plays a melody consisting of 1000 tones per second, the symbol rate is 1000 symbols/second, or baud. Since each tone (i.e., symbol) represents a message consisting of two digital bits in this example, the bit rate is twice the symbol rate, i.e. 2000 bits per second. This is similar to the technique used by dialup modems as opposed to DSL modems.
