Amplitude Shift Modulation (ASK) is a form of amplitude modulation that represents digital data as variations in the amplitude of a carrier wave. In an ASK system, the binary symbol 1 is represented by transmitting a fixed amplitude carrier wave and a fixed frequency for a bit duration of T seconds. If the signal value is 1, then the carrier signal will be transmitted; otherwise, a signal value of 0 will be transmitted.
Any digital modulation scheme uses a finite number of distinct signals to represent digital data. ASK uses a finite number of amplitudes, each assigned a unique pattern of binary digits. Generally, each amplitude encodes an equal amount of bits. Each bit pattern forms the symbol which is represented by the particular amplitude. The demodulator, which is designed specifically for the symbol set used by the modulator, determines the amplitude of the received signal and reassigns it to the symbol it represents, thus recovering the original data. The frequency and phase of the conveyor are kept constant.
Like AM, an ASK is also linear and sensitive to atmospheric noise, distortions, propagation conditions on different routes in PSTN, etc. Both ASK modulation and demodulation processes are relatively inexpensive. The ASK technique is also commonly used to transmit digital data via fiber optics. For LED transmitters, binary 1 is represented by a short pulse of light and a binary 0 by the absence of light. Laser transmitters typically have a fixed "bias" current that causes the device to emit a low light level. This low level represents the binary 0, whereas a light wave of greater amplitude represents the binary 1.
The simplest and most common form of ASK functions as a switch, using the presence of a carrier wave to indicate a binary and its absence to indicate a binary zero. This type of modulation is called a key-on-off (OOK), and is used in radio frequencies to transmit the Morse code (known as continuous wave operation)
More sophisticated coding schemes representing data in groups using additional amplitude levels have been developed. For example, a four-level coding scheme can represent two bits with each change in amplitude; an eight-level scheme can represent three bits; and so. These amplitude shift coding forms require a high signal-to-noise ratio for their recovery, since by their nature much of the signal is transmitted at reduced power.