The earliest type of vocoder, the channel vocoder, was originally developed as a speech coder for telecommunications applications in the 1930s. The development of a vocoder was started in 1928 by Bell Labs engineer Homer Dudley. A vocoder works by analysing the sound of a modulator signal. The modulator signal is split into many frequency bands. The level of each band is sent as a signal to a corresponding band-pass filter. The filter is set to the same frequency that was analyzed. A sound source, called the carrier, is sent through the bank of filters. The level of each band-pass filter is automatically adjusted to that of the corresponding frequency in the modulator signals. In this way, the carrier is filtered so the harmonic content that passes through is similar to that of the modulator signals. In music a synth is often used to carry the frequency, this allows it to be pitched to specific notes.