Vocoders

Ø  Vocoders are an analysis and synthesis system, used to reproduce human speech in limited bandwidth.

Ø  Vocoders produce unnatural sounding speech and generally used for recorded information such as wrong number messages, computer output signals and educational games.

Ø  The purpose of vocoder is to encode the minimum amount of speech information necessary to reproduce a perceptible message with fewer bits than those needed by conventional encoder/decoder.

Idea behind Vocoders

The voice consist of sound made by human being using vocal folds for talking, singing, laughing, crying etc. The human voice is specifically a part of human sound production in which vocal folds (vocal cords) are primary sound source. The mechanism for generating the human voice can be subdivided into three parts, lungs, vocal folds within larynx and articulators( the part of vocal tract above the larynx consist of tongue, palate, cheek, lips etc.

Lungs: The lung (the pump) must produce adequate airflow and air pressure  to vibrate vocal folds (the air pressure is fuel of voice).

The vocal folds (vocal cords) are a vibrating valve that chops up the airflow from lungs into audible pulses that form the laryngeal sound source. The muscles of larynx adjust the length and tension of vocal folds to fine tune pitch and tone. 

Articulators: The articulators articulate and filter the sound emanating from larynx and to some degree can interact with laryngral airflow to strengthen it or weaken it as a sound source.

                The tone of sound may be modulated to suggest emotions such as anger, surprise or happiness. The vocal fold size of men and women are different so they have different pitched voices.

    Mechanism for Generating Human Voice

Basic working of Vocoders

A vocoder require two inputs a ‘carrier’ wave, and a ‘modulator’ input to function properly. The carrier is the sound you want to vocode through, and the modulator is your voice. The modulator takes your voice, finds the fundamental frequencies (important bits) of it, and converts them into levels of amplitude on a series of band pass filters (this is why some vocoders have different numbers of bands) – in general, the more bands available the more understandable your speech will be. These band pass filter signals are then passed onto the carrier wave where your final sound is created.