Real-Time All-Digital FFT Vocoder

Abstract

In this paper, we describe a real-time all-digital spectrum channel vocoder implemented on a programmable signal processor. The vocoder system efficiently utilizes a radix four, 64-point FFT algorithm for spectral analysis, while the pitch extractor selects the pitch period using decision logic on the average magnitude difference function (AMDF) of the input speech. The receiver linearly interpolates both the pitch period and the synthesized impulse response which is generated by using the inverse Fourier transform. The vocoder frame updating rate is 22 msec, and the input speech sampling rate is 7040 samples/sec. The configuration of the vocoder and the logic of the AMDF pitch extractor is discussed and the system demonstrated with audio tapes.