History
Early experiments with stereo AM radio involved two separate stations (both AM or sometimes one AM and one FM) broadcasting the left and right audio channels. This system was not very practical, as it required the listener to use two separate receivers. Synchronization was problematic, often resulting in "ping-pong" effects between the two channels. Reception was also likely to be different between the two stations, and many listeners used mismatching models of receivers.
After the early experiments with two stations, a number of systems were invented to broadcast a stereo signal in a way which was compatible with standard AM receivers.
FM stereo was first implemented in 1961. In the United States, FM overtook AM as the dominant broadcast radio band in the late 1970s and early 1980s.
Read more about this topic: AM Stereo
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