Why do we need a vocoder?

[Hanzhang X]

I am reviewing Lecture 5, getting lost in all the details, and just realized that I don’t get the point of using a vocoder in unit selection. Since we concatenate units of waveform, and we compute the target costs and join costs based on these units, why do we need to convert the waveform into another representation and convert it back to waveform?

[Simon]

(Lecture 5 was about pitch tracking and pitchmarking. Do you mean Lecture 6?)

Unit selection systems do necessarily not require a vocoder, but could optionally use one so that joins could be smoothed. Alternatively, we might only do time-domain processing (i.e., direct waveform concatenation).

The RELP coding used in Festival could be thought of as a type of vocoding, although the need for the residual (which is itself a waveform) would make this a rather inconvenient vocoder for other purposes, such as statistical parametric speech synthesis.

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