The Czechs first used ripple control in the 1950s. Early transmitters were low power, compared to modern systems, only 50 kilovolt-amps. They were rotating generators that fed a 1050 Hz signal into transformers attached to power distribution networks. Early receivers were electromechanical relays. Later, in the 1970s, transmitters with high-power semiconductors were used. These are more reliable because they have no moving parts. Modern Czech systems send a digital "telegram." Each telegram takes about thirty seconds to send. It has pulses about one second long. There are several formats, used in different districts.