while MF selects its two tones from a common set. DTMF and MF use different tone frequencies largely to keep end users from interfering with inter-office signaling. In the 1970s, MF began to be replaced by digital out-of-band signaling, a conversion motivated in part by the widespread fraudulent use of MF signals by end users known as phone phreaks. These signals are distinctive when received aurally as a rapid succession of tone pairs with almost musical quality. The simultaneous transmission of two tones directly at RF loses the constant-envelope property of the single tone system. Two simultaneous RF tones is in fact the classic "stress test" of an RF power amplifier for measuring linearity and intermodulation distortion. However, two audio tones can be sent simultaneously on a conventional, constant-envelope FM RF carrier, but the noncoherent detection of the FM signal at the receiver would destroy any signal-to-noise ratio advantage the multitone scheme might have. MFSK in HF communications Skywave propagation on the high frequency bands introduces random distortions that generally vary with both time and frequency. Understanding these impairments helps one understand why MFSK is such an effective and popular technique on HF. Delay spread and coherence bandwidth When several separate paths from transmitter to receiver exist, a condition known as multipath, they almost never have exactly the same length so they almost never exhibit the same propagation delay. Small delay differences, or delay spread, smear adjacent modulation symbols together and cause unwanted intersymbol interference. Delay spread is inversely proportional to its frequency-domain counterpart, coherence bandwidth. This is the frequency range over which the channel gain is relatively constant. This is because summing two or more paths with different delays creates a comb filter even when the individual paths have a flat frequency response. Coherence time and Doppler spread Fading is a (usually random and undesired) change in path gain with time. The maximum fade rate is limited by the physics of the channel, such as the rate at which free electrons form and are recombined in the ionosphere and charged particle cloud velocities within the ionosphere. The maximum interval over which the channel gain does not appreciably change is the coherence time. A fading channel effectively imposes an unwanted random amplitude modulation on the signal. Just as the bandwidth of intentional AM increases with the modulation rate, fading spreads a signal over a frequency range that increases with the