Transform domain quantization technique for adaptive predictive coding

#The present invention relates to digital signal transmission systems, and more specifically to digital signal transmission systems using adaptive predictive coding techniques.

#Adaptive predictive coding (APC) methods are widely used for high quality coding of speech signals at 16 kbit/s. An adaptive predictive coder digitizes an input signal by performing two basic functions: adaptive prediction and adaptive quantization. The adaptive prediction function removes the redundancies inherent in any information carrying signal such as speech. The residual nonredundant signal is then quantized by the adaptive quantization function. Various realizations of the above basic concept are possible, differing mainly in the method of residual quantization. In the most common approach, the residual nonredundant signal is quantized in the time domain, within a feedback loop. This arrangement will be referred to as the conventional APC or the APC with noise feedback (APC-NFB).

#FIGS. 1 and 2 show block diagrams of the conventional encoder and decoder respectively. Since input signals such as speech have time varying characteristics, the predictor and quantizer circuits included in the adaptive predictive coder must adapt to match the time varying input signal. The conventional APC schemes are block adaptive in that the signal is processed in blocks, or frames, of samples and optimal predictor and quantizer parameters are computed for each block (frame). These parameters are also quantized and transmitted to the decoder at the receiving end of the transmission system.

#The present invention will be more clearly understood from the following description in conjunction with the accompanying drawings, wherein:

#FIG. 1 shows a conventional encoder using a noise feedback quantization technique

#FIG. 2 shows a conventional decoder corresponding to the encoder of FIG. 1;

#The present invention is a method of quantizing the residual signal, and is intended to replace the noise feedback quantization method used in the conventional APC encoder. The methods used for short term and long term prediction shown in FIGS. 1 and 2 are not affected. Actually, the quantization technique according to the present invention is independent of the particular approaches employed for short term and long term predictor parameter computation. Hence, the following description will focus on the quantization technique only. FIG. 3 is a block diagram of the encoder used in conjunction with the quantization technique according to the present invention. FIG. 4 is the block diagram of the associated decoder. Circuit elements identical to those in the conventional APC encoder and decoder are numbered in FIGS. 3 and 4 with the same reference numerals used in FIGS. 1 and 2, and no independent discussion of these elements will be set forth here, in order to avoid repetition.

#In the embodiment shown in FIG. 3, the output of the long term predictor 1 is fed as an input to frequency domain coefficient calculator 91 where the time domain residual signal r[i]output from the long term predictor 10 is transformed to a frequency domain signal by calculating corresponding frequency domain coefficients by a known method, such as the Discrete Cosine Transformation (DCT). Quantization circuit 93 receives the calculated coefficients and quantizes them. An output of quantization circuit 93 is sent to multiplexer 20 for transmission. The quantization circuit 93 also receives an input from a noise spectral shaping circuit 92 which determines how many quantization bits should be used in quantizing each frequency coefficient according to an algorithm which will be discussed later.

#It is desirable that the frame size (i.e., the number of samples held in frame buffers 1 and 7) be an integer power of 2 to obtain a computation efficient realization. For a 16 kbit/s coding rate, 128 samples/frame was found suitable. For generality, however, the frame size will be denoted by N in the following discussion.