Multiple frequency-shift keying (MFSK) is a variation of frequency-shift keying (FSK) that uses more than two frequencies. MFSK is a form of M-ary orthogonal modulation, where each symbol consists of one element from an alphabet of orthogonal waveforms. M, the size of the alphabet, is usually a power of two so that each symbol represents log2M bits. M is usually between 4 and 64 Error correction is generally also used Fundamentals In a M-ary signaling system like MFSK, an "alphabet" of M tones is established and the transmitter selects one tone at a time from the alphabet for transmission. M is usually a power of 2, so each tone transmission from the alphabet represents log2 M data bits. MFSK is classed as an M-ary orthogonal signaling scheme because each of the M tone detection filters at the receiver responds only to its tone and not at all to the others; this independence provides the orthogonality. Like other M-ary orthogonal schemes, the required Eb/N0 ratio for a given probability of error decreases as M increases without the need for multisymbol coherent detection. In fact, as M approaches infinity the required Eb/N0 ratio decreases asymptotically to the Shannon limit of −1.6dB. However this decrease is slow with increasing M, and large values are impractical because of the exponential increase in required bandwidth. Typical values in practice range from 4 to 64, and MFSK is combined with another forward error correction scheme to provide additional (systematic) coding gain. Spectral efficiency of MFSK modulation schemes decreases with increasing of modulation order M: Like any other form of angle modulation that transmits a single RF tone that varies only in phase or frequency, MFSK produces a constant envelope. This significantly relaxes the design of the RF power amplifier, allowing it to achieve greater conversion efficiencies than linear amplifiers. 2-tone MFSK It is possible to combine two MFSK systems to increase the throughput of the link. Perhaps the most widely used 2-tone MFSK system is dual-tone multi-frequency (DTMF), better known by its AT&T trademark of "Touch Tone". Another is the Multi-frequency (MF) scheme used during the 20th century for in-band signalling on trunks between telephone exchanges. Both are examples of in-band signaling schemes, i.e., they share the user's communication channel. Symbols in the DTMF and MF alphabets are sent as tone pairs; DTMF selects one tone from a "high" group and one from a "low" group,