Before the proliferation of vacuum tube amplifiers in the 1920s, carbon microphones were the only practical means of obtaining high-level audio signals. They were widely used in telephone systems until the 1980s, while other applications used different microphone designs much earlier. Their low cost, inherently high output and frequency response characteristic were well suited for telephony. For plain old telephone service (POTS), carbon-microphone based telephones can still be used without modification. Carbon microphones, usually modified telephone transmitters, were widely used in early AM radio broadcasting systems, but their limited frequency response, as well as a fairly high noise level, led to their abandonment in those applications by the late 1920s. They continued to be widely used for low-end public address, and military and amateur radio applications for some decades afterward.