The vocoder goes was originally designed to act as a form of voice encryption for secure telephone communication during World War II. Vocoder is a portmanteau of the term “Voice Encoder.” Mainstream use of the vocoder didn’t develop until the 1970s, when musicians began to experiment with the effect in their music. It remains a common effect in popular and independent music, particularly electronic music. Highly advanced forms of vocoders are still in use as security devices to this day.
The vocoder effect is the result of two distinct audio signals being combined in a mathemetically precise way through the use of multiple filters and modulation envelopes. One audio signal is designated the “carrier,” the other is designated the “modulator.” Vocoding’s special cross-filtration method causes the carrier signal to take on the frequency characteristics of the modulator. For example, a synthesizer used as a carrier combined with a vocal track used as a modulator results in something resembling a talking synthesizer.
The difficulty of achieving a true vocoder effect in the absence of an actual vocoder lies in the precision with which the bandpass filters that produce the effect must be interwoven. The more filters there are, the more clarity the vocoding effect will have. Using only five or six bandpass filters is only capable of producing the most rudimentary vocoder effect. Correctly interweaving the frequency ranges of dozens of bandpass filters by hand is most likely impossible. Further, the modulator signal must be converted into a modulation source through the use of envelope followers precisely attuned to the incoming signal.
A simple yet convincing vocoder effect can be achieved using a comb filter and pitch shifting plug-in in your DAW or other audio plug-in host. Comb filters produce a layered delay effect, producing interference that often resembles the characteristic graininess of the vocoder effect. Applying pitch shifting produces the other characteristic effect of the vocoder: its ability to adjust the pitch of the modulator signal on the fly. A bitcrusher plug-in will further enhance the effect by reducing the bitrate of the resulting signal, producing a “crunchier” sound.