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28. Modem Modulation |
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28. Modem Modulation
There are 3 basic types of modulation used in modems:
- FSK - Frequency Shifted Keying
- QPSK - Quadrature Phase Shifted Keying
- QAM - Quadrature Amplitude Modulation
Modern modems use a combination of the above basic modulation techniques and compression to achieve the high data transfer rates (14.4 Kbps and up).
Frequency Shift Keying or FSK is the frequency modulation of a carrier to represent digital intelligence. For Simplex or Half Duplex operation, a single carrier (1170 Hz) is used - communication can only be transmitted in one direction at a time. A Mark or 1 is represented by 1270 Hz, and a Space or 0 is represented by 1070 Hz. The following diagram shows the Voice Channel with Simplex/Half Duplex FSK.
Simplex/Half Duplex FSK
28a. FSK - Frequency Shift Keying