Amplitude shift keying
Amplitude shift keying (ASK), is a digital modulation technique in which the amplitude of a carrier is altered to convey information. Although theoretically any two (or more) amplitudes could be used, the most widely implemented forms of ASK represent a one (1) as a sine wave with a specified amplitude, and a zero (0) as no signal at all (i.e., a sine wave with zero amplitude).
ASK is a simple technique, but its noise susceptibility in the electronic space limits it to low-speed applications over telephone lines, which have become essentially obsolete. However, ASK is commonly used over optical fiber with light-emitting diode (LED) and LASER transmitters. In simple systems, the presence of light represents a one and the absence of light represents a zero. Such systems may be prone to synchronization problems, however. SONET solves this by running the transmission stream through a mathematical formula that scrambles the bits and ensures a rich mixture of ones and zeroes. At the receiver, the scrambling is reversed to restore the original bit stream.
Other optical systems use one of many techniques for mapping a defined set of bits onto a larger set of signals. For example, 8B10B maps a set of eight bits onto a set of ten signals. Because there are 256 possible 8-bit combinations, but 1024 10-signal combinations, the mapping can be made to signal patterns that are rich in their mixture of ones and zeroes, and omit signal patterns that have long sequences of ones or zeroes. The resulting bit rate is 80% of the signaling rate, making them slightly inefficient. However, there are longer strings that minimize this inefficiency while retaining the synchronization. For example, 64B66B maps 64 bits to 66 signals, making it 97% efficient.
|<mp3>http://podcast.hill-vt.com/podsnacks/2008q2/ask-fsk.mp3%7Cdownload</mp3> | Amplitude shift keying (ASK) and frequency shift keying (FSK)|