GMDSS Standards of Training, Certification and Watchkeeping for Seafarers (STCW) General Operator's Certificate (GOC) FCC Element 7 Practice Test

Two-tone digital encoding is what defines J2B mode. In this mode, data is carried by switching or presenting two discrete audio frequencies, one representing a binary state and the other representing the opposite state. The receiver detects which of the two tones is present at any moment and reconstructs the data from that pattern. This two-tone scheme is a hallmark of J2B, allowing binary information to be sent over the SSB channel in a way that’s distinguishable even in noise or signal distortion.

If someone suggested there’s only a single tone, zero tones, or many sidebands without distinct tones, those descriptions don’t match how J2B carries information. The essence of J2B is precisely the use of two tones to encode data.