due to multipath distortion and the nature of ionospheric propagation. The FCC approved the use of ASCII by Amateur Radio stations on March 17, 1980 with speeds up to 300 baud from 3.5 to 21.25 MHz and 1200 baud between 28 and 225 MHz. Speeds up to 19.2 kilobaud was authorized on Amateur frequencies above 420 MHz. These symbol rates were later modified: 12m band and below -- 300 bauds symbol rate -- 47 CFR § 97.307 (f)(3) 10m band -- 1200 bauds symbol rate -- 47 CFR § 97.307 (f)(4) 6m and 2m bands -- 19.6 kilobauds symbol rate -- 47 CFR § 97.307 (f)(5) 1.25m and 70cm bands -- 56 kilobauds symbol rate -- 47 CFR § 97.307 (f)(6) 33 cm band and above -- symbol rate not specified -- 47 CFR § 97.307 (f)(7) The requirement for Amateur Radio operators in the United States to identify their station callsign at the beginning and the end of each digital transmission and at ten-minute intervals using International Morse code was finally lifted by the FCC on June 15, 1983. Comparison with other modes RTTY has a typical baud rate for Amateur operation of 45.45 baud (approximately 60 words per minute). It remains popular as a "keyboard to keyboard" mode in Amateur Radio. RTTY has declined in commercial popularity as faster, more reliable alternative data modes have become available, using satellite or other connections. For its transmission speed, RTTY has low spectral efficiency. The typical RTTY signal with 170 Hz shift at 45.45 baud requires around 250 Hz receiver bandwidth, more than double that required by PSK31. In theory, at this baud rate, the shift size can be decreased to 22.725 Hz, reducing the overall band footprint substantially. Because RTTY, using either AFSK or FSK modulation, produces a waveform with constant power, a transmitter does not need to use a linear amplifier, which is required for many digital transmission modes. A more efficient Class C amplifier may be used. RTTY, using either AFSK or FSK modulation, is moderately resistant to vagaries of HF propagation and interference, however modern digital modes, such as MFSK, use Forward Error Correction to provide much better data reliability. Primary users Principally, the primary users are those who need robust shortwave communications. Examples are: All military departments, all over the world (using cryptography) Diplomatic services all over the world (using cryptography) Weather reports are transmitted by