Shortwave radio is radio transmission using shortwave radio frequencies. Radio Netherlands Worldwide operated a shortwave relay station at . SSB is the predominant voice mode on shortwave radio other than shortwave broadcasting. CBC Radio also operated two shortwave services. There are also five shortwave radio stations. Foreign stations can be picked up on shortwave radio. Amateur radio operators are credited with the discovery of skywave propagation on the shortwave bands. Coded bursts of data sent by a shortwave radio transmitter were also used. Most telegrams were transmitted by cables or by shortwave radio. Radio Netherlands, heard worldwide via shortwave radio since the 1920s, is also based there. The crystals operated in the shortwave radio range. Amateur radio operators also discovered that long-distance communication was possible on shortwave bands. Five years later, two RCA shortwave transmitters were installed. Amplitude modulation is the simplest type and the most commonly used for shortwave broadcasting.