Amplitude Shift Keying ASK
In amplitude shift keying (ASK), the strength of the signal is varied to represent binary 1 or 0. Both frequency and phase remain constant while the amplitude changes. Which voltage represents 1 and which represents 0 is left to the system designers. A bit duration is the period of time that defines one bit. The peak amplitude of the signal during each bit is constant and its value depends on the bit (0 or 1). The speed of transmission using ASK is limited by the physical characteristics of the transmission medium. Figure 4.3-3 gives a conceptual view of ASK.
Unfortunately, ASK transmission is highly susceptible to noise interference. The term noise refers to unintentional voltages introduced onto a line by various phenomena such as heat or electromagnetic induction created by other sources. These unintentional voltages combine with the signal to change the amplitude. Some types of noise, for example thermal noise, are constant enough not to interfere with the intelligibility of the signal. Impulse noise, however, is a sudden surge of energy that can wipe out an entire section of a transmission by inserting high-amplitude spikes where low amplitude was intended. In that case, a section of the signal that was intended to be received as one or more 0s will read as 1s. You can see how surges in voltage would be especially problematic for ASK, which relies solely on amplitude for recognition. Noise usually affects the amplitude; therefore, ASK is the encoding method most affected by noise.
A popular ASK technique is called on-off-keying (OOK). In OOK one of the bit values is represented by no voltage. The advantage is a reduction in the amount of energy required to transmit information.
- Fig. 4.3-3 ASK encoding
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