1915: First Audio Filter/Bandpass EQ Circuit

George Ashley Campbell joined AT&T’s engineering department in 1897. His initial research was in improving the performance of long-distance telephone lines. He later focused his research on audio filter development, creating the classic L-C (inductor and capacitor) filters that are widely used in analog tone control and filtering circuits.
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George Ashley Campbell

George Ashley Campbell joined AT&T’s engineering department in 1897. His initial research was in improving the performance of long-distance telephone lines. He later focused his research on audio filter development, creating the classic L-C (inductor and capacitor) filters that are widely used in analog tone control and filtering circuits.

Campbell’s concept of combining simple lowpass and highpass filters resulted in a more complex bandpass filter (ideal for emphasizing voice intelligibility over telephone lines), documented in Campbell’s 1915 Electric Wave-Filter patent. Beyond simple analog filtering chores, the Wave Filter also paved the way for multiplexing—the ability to run multiple voice conversations over a single phone wire. Ironically, this foundation of the modern audio equalizer was laid years before sound systems or even electrical recording existed. But the basics of audio filtering had been established and the groundwork was ready to become part of a revolution yet to come.