Early linear predictive speech coding at Bell Telephone Laboratories and Electrical Communication Laboratory
Fumitada Itakura · The Journal of the Acoustical Society of America · 2006
At the 6th ICA in Tokyo (1968), two papers on the linear predictive coding of speech were presented by Manfred Schroeder and by me in the same session. The first paper was on the adaptive linear predictive waveform coding for medium band coding (time domain modeling) and the latter was on the LPC vocoder based on the autoregressive spectral estimation for narrow-band coding (frequency domain modeling). Although their approaches were very different in both concept and scope, there were dramatic common points: the covariance and autocorrelation methods were used to solve normal equation to determine the LPC parameters and also the pitch period was extracted to reproduce speech periodicity in both systems. Subsequently the multipulse (1982), stochastic (1984), and code (1985) excitation LP were proposed at AT&T BTL, whereas the transformation of LPC parameters to PARCOR (1969) and LSP (1975) and their efficient quantization were intensively studied at ECL of NTT. These efforts were eventually combined for practical implementations in the secure, cellular, and IP telephone in the 1980s. The brilliant ideas by Manfred Schroeder and his associates at BTL, in particular Bishnu Atal, have always been a helpful lighthouse to navigate the world-wide speech coding research in the right direction since 1980s.