|
A vocoder (, short for ''voice encoder'') is a category of voice codec that analyzes and synthesizes the human voice signal for audio data compression, multiplexing, voice encryption, voice transformation, etc. The earliest type of vocoder, ''channel vocoder'' was originally developed as a speech coder for telecommunications applications in the 1930s, the idea being to code a speech for reducing bandwidth (i.e. audio data compression) for multiplexing transmission. On the channel vocoder algorithm, among the two components of analytic signal, only consider the amplitude component and simply ignore the phase component, and it tend to result in the unclear voice. For the improvement of this issue, see phase vocoder. In the encoder, the input is passed through a multiband filter, each band is passed through an envelope follower, and the control signals from the envelope followers are transmitted to the decoder. The decoder applies these (amplitude) control signals to corresponding filters for re–synthesis. Since the control signals change only slowly compared to the original speech waveform, the bandwidth required to transmit speech can be reduced. This allows more speech channels to share the single communication channel such as a radio channel or a submarine cable (i.e. multiplexing). By encrypting the control signals, voice transmission can be secured against interception. Its primary use in this fashion is for secure radio communication. The advantage of this method of encryption is that none of the original signal is sent, but rather envelopes of the bandpass filters. The receiving unit needs to be set up in the same filter configuration to re–synthesise a version of the original signal spectrum. The vocoder has also been used extensively as an electronic musical instrument (see #Uses in music). The decoder portion of the vocoder, called a voder, can be used independently for speech synthesis (see #History). ==Theory== The human voice consists of sounds generated by the opening and closing of the glottis by the vocal cords, which produces a periodic waveform with many harmonics. This basic sound is then filtered by the nose and throat (a complicated resonant piping system) to produce differences in harmonic content (formants) in a controlled way, creating the wide variety of sounds used in speech. There is another set of sounds, known as the unvoiced and plosive sounds, which are created or modified by the mouth in different fashions. The vocoder examines speech by measuring how its spectral characteristics change over time. This results in a series of signals representing these modified frequencies at any particular time as the user speaks. In simple terms, the signal is split into a number of frequency bands (the larger this number, the more accurate the analysis) and the level of signal present at each frequency band gives the instantaneous representation of the spectral energy content. Thus, the vocoder dramatically reduces the amount of information needed to store speech, from a complete recording to a series of numbers. To recreate speech, the vocoder simply reverses the process, processing a broadband noise source by passing it through a stage that filters the frequency content based on the originally recorded series of numbers. Information about the instantaneous frequency (as distinct from spectral characteristic) of the original voice signal is discarded; it wasn't important to preserve this for the purposes of the vocoder's original use as an encryption aid, and it is this "dehumanizing" quality of the vocoding process that has made it useful in creating special voice effects in popular music and audio entertainment. Since the vocoder process sends only the parameters of the vocal model over the communication link, instead of a point by point recreation of the waveform, it allows a significant reduction in the bandwidth required to transmit speech. Analog vocoders typically analyze an incoming signal by splitting the signal into a number of tuned frequency bands or ranges. A modulator and carrier signal are sent through a series of these tuned bandpass filters. In the example of a typical robot voice the modulator is a microphone and the carrier is noise or a sawtooth waveform. There are usually between 8 and 20 bands. The amplitude of the modulator for each of the individual analysis bands generates a voltage that is used to control amplifiers for each of the corresponding carrier bands. The result is that frequency components of the modulating signal are mapped onto the carrier signal as discrete amplitude changes in each of the frequency bands. Often there is an unvoiced band or sibilance channel. This is for frequencies outside of analysis bands for typical speech but still important in speech. Examples are words that start with the letters s, f, ch or any other sibilant sound. These can be mixed with the carrier output to increase clarity. The result is recognizable speech, although somewhat "mechanical" sounding. Vocoders also often include a second system for generating unvoiced sounds, using a noise generator instead of the fundamental frequency. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「vocoder」の詳細全文を読む スポンサード リンク
|