The FEC adds redundancy to the data, such that all of a message may be successfully recovered even if some bits are not received by the receiver. Like the other modes, multiple-frequency shift keying is employed unlike the other modes, messages are transmitted as atomic units after being compressed and then encoded with a process known as forward error correction (or “FEC”). It can decode signals many decibels below the noise floor in a 2500 Hz band (note that SNR in a 2500 Hz band is approximately 28 dB lower than SNR in a 4 Hz band, which is closer to the channel bandwidth of an individual JT65 tone), and can often allow amateurs to successfully exchange contact information without signals being audible to the human ear. JT65, developed and released in late 2003, is intended for extremely weak but slowly varying signals, such as those found on troposcatter or Earth-Moon-Earth (EME, or “moonbounce”) paths.
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