ASK transmits 1's and 0's by representing each one as a different amplitude at the same frequency. Therefore, the amplitude of a 1 is larger than the amplitude of the 0.

This time the amplitude does not change, but instead the frequency is altered. A 1 has a lower frequency than a 0. These frequencies are defined by the CCITT standard. The transmitter transmits a 0 at 1180Hz and a 1 at 980Hz. The receiver transmits a 0 at 1850Hz and a 1 at 1650Hz. (and the transmitter listens at these freqeinces)

Phase modulation or Phase Shift Keying (PSK) & Differential PSK (DPSK)

In this case, the frequency and amplitude are both kept constant. However, the phase of the sinewave is shifted to signify logic 0 and logic 1. Phase Coherent PSK uses two fixed signals, and a logic 0 is represented by a 180 degree phase shift and a logic 0 by a 0 degree phase shift. This is a complex method to demodulate for the receiver because the phase shifts are from an absolute value. Another method called Differential PSK exists (DPSK), which is a phase shift relative to the previous logic bit transmitted. Binary 0 is therefore a 90 degree phase change from the previous logic bit and binary 1 is a 270 degree phase change from the previous logic bit. In this case, the receiver only needs to detect the phase change which occured from the previous bit rather than off an absolute value that it has to know.

To acheive higher speed data communication a combination of PSK and AM can be used, which produces Quadrature Amplitude Modulation (QAM). This makes use of 0, 90, 180, 270 degree phase shifts together with ASK. These are used to produce bit rates of 2400, 4800 & 9600 bps.

PSK data signal