Real-Time Formant Vocoder, Using a Filter Bank, a General-Purpose Digital Computer, and an Analog Synthesizer
Cecil H. Coker · The Journal of the Acoustical Society of America · 1965
A formant vocoder has been built as a combination of analog equipment and a digital computer. The time domain operations—spectrum filtering and synthesis—are done in analog form. The formant-extraction operations are done at modest sampling rates (about 100 scans/sec) with a DDP-24 computer. The program is a tightly coded form of an analysis method described earlier [J. Acoust. Soc. Am. 35, 1911 (1963)] and first written in FORTRAN to run slower than real time with a programmed synthesizer. Spectra from a 24-channel filter bank are scanned for a peak, a resonance curve of appropriate frequency and average bandwidth is subtracted, and the process repeated to obtain four maxima. The lower three of these, ordered in frequency, are transmitted to the synthesizer along with other control data. The analyzer, in minimal form, takes about 7 msec to run and 1 msec for input-output, leaving time for a few frills in a 10- to 12-msec scanning period. Tapes of the system are demonstrated.