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Lý thuyết về điều chế tín hiệu cơ bản

http://encyclopedia2.thefreedictionary.com/Amplitude+Modulation
Amplitude modulation
Varying the voltage of a carrier or a direct current in order to transmit analog or digital data. Amplitude modulation (AM) is the oldest methodof transmitting human voice electronically. In an analog telephone conversation, the voice waves on both sides are modulating the voltageof the direct current loop connected to them by the telephone company.

Modulate a Carrier
Amplitude modulation (AM) is also widely used to alter a carrier wave to transmit data. For example, in AM radio, the voltage (amplitude) ofa carrier with a fixed center frequency (the station's channel) is varied (modulated) by the analog audio signal.

AM is also used for digital data. In quadrature amplitude modulation (QAM), both amplitude and phase modulation are used to createdifferent binary states for transmission (see QAM). AM is also used to modulate light waves in optical fibers. See modulation and carrier.

Vary the Amplitude
In AM modulation, the voltage (amplitude) of the carrier is varied bythe incoming signal. In this example, the modulating wave impliesan analog signal.


Digital Amplitude Shift Keying (ASK)
For digital signals, amplitude shift keying (ASK) uses two voltage levels for 0 and 1 as in this example.

Tóm lại:
- Các khái niệm AM, FM, PM dùng cho điều chế tín hiệu tương tự. Ví dụ, khi muốn truyền tín hiệu âm thanh đi xa (đài radio, tín hiệu điện thoại cố định...)
- ASK, FSK, PSK dùng để điều chế tín hiệu số. Dạng tín hiệu số là các bit 0,1 (mức điện áp cao và thấp)

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