Modes on the HF bands utilize frequency or phase modulation of a sub-carrier, which then in turn modulates a carrier using suppressed carrier SSB keying, or by directly modulating a carrier. Decoding can be done from the receiver AF or IF output (USB, LSB, CW or FAX demodulator). Examples of this type of modulation are ordinary two-frequency FSK, four-frequency FSK (Twinplex) and the STANAG PSK modes. M-ary designates the number of modulation levels. FSK designates common two-frequency FSK.
MFSK systems deviate from the classical binary transmission of '0' (Mark) and '1' (Space), as a single tone carries more information. This is the reason for a higher element period in MFSK, compared with binary transmissions having the same bit rate. This produces a substantial increase in the insensitivity to multi-path propagation and noise. Examples of MFSK systems are PICCOLO, COQUELET and CIS-36.
Due to intensive research and the availability of highly efficient hardware and software, the waveforms originally developed within the STANAG specifications of NATO have gained considerable influence in radio data communications. This has resulted in very efficient modems utilizing multi-phase modulated single-tones, combined with the use of adaptive equalization and sophisticated coding and demodulation and decoding.
Satellite TV broadcasting utilizes QPSK or even 8PSK.
A special instance of m-ary modulation is QAM (Quadrature Amplitude Modulation) in which phase and amplitude modulation are combined. An example of QAM is digital broadcasting in the MF and HF bands (DRM).