Picture signal encoding and decoding apparatus

Abstract: A picture signal encoding and decoding apparatus which transmits a digital picture signal by encoding, and receives and decodes an incoming encoded signal. The apparatus comprises a plurality of quantizers having different characteristics, and a proper quantizing characteristic is selected and used according to the sequence of transformed coefficients and the size of a motion vector. The apparatus can be manufactured at low costs and the stability of operation is improved in that a signal can be compressed efficiently owing to the realization of quantization corresponding to a statistical characteristics of an input signal.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a picture signal encoding apparatus which encodes a digital picture signal for transmission and decodes a received encoded signal. Description of the Prior Art Fig. 1 is a simplified block diagram of a configuration of a picture signal encoding and decoding apparatus of a conventional type, for example, as shown in "Interframe Encoding apparatus for video conferencing", a technical report IE84-4, 1984 of The Institute of Electronics and Communication Engineers of Japan. In the figure, 1 is an encoding section which encodes a digitized picture signal series; 2 is a variable-length-encoding section which variable-length-encodes code words encoded in the encoding section 1; 3 is a transmitting buffer which smooths out a speed; 6 is a framing section; 7 is a deframing section; 8 is a receiving buffer; 9 is a variable-length-decoding section; 10 is a decoding section. 101 is a digitized picture signal series; 102 is a code-word series encoded in the encoding section 1; 103 is a code-word series variable-length-encoded in the variable-length encoding section; 108 is a feedback control signal; 109 is a code-word series whose speed is smoothed out by the transmitting buffer 3; 110 is a received code-word series; 111 is an output read from the receiving buffer 8; 112 is a code-word series converted to a fixed-length code-word series in the variable-length decoding section; 113 is a digital picture signal series decoded in the decoding section 10. The encoding of a signal in an encoding section of a picture signal encoding and decoding apparatus is, to be precise, performed by a transform coding method. Transform coding is a process wherein a digital picture signal is transformed to a sequence corresponding to a spatial frequency by orthogonal transformation like a method in which a discrete cosine transform is used as shown, for example, in the following reference. W. H. Chen. "Scence Adaptive Coder". (IEEE. Transactions on Communications, Vol. COM 32, No. 3, March, 1984.) Fig. 2 is a block diagram showing the configuration of a transform coding apparatus as shown in Kato, et al., "A proposal for an encoding control method in MC - DCT encoding system" (No. 203 all-Japan meeting of information and system branch of The Institute of Electronics, Information and Communication Engineers of Japan, 1987.) In the figure, 31 is a subtracter which performs subtraction of an interframe predictive signal 202 for movement compensation from an input signal 201; 21 is an orthogonal transformation section which performs an orthogonal transformation for an interframe differential signal 203; 22b is a quantizing section which threshold-processes and quantizes a transformed coefficient 204 obtained by orthogonal transformation according to a buffer accumulation quantity 211; 23 is an inverse orthogonal transformation section which creates a decoded interframe differential signal 207 by performing inverse orthogonal transformation for a quantized output signal 206; 32 is an adder; 24 is a frame memory which creates the interframe predictive signal for movement compensation 202; 25 is a movement compensation section; 26 is a variable-length encoding section; 27 is a transmitting buffer. The operation is explained in the following. A differential signal 203 from which a redundant component is removed is created by getting the difference between the digitized input signal 201 and the interframe predictive signal for movement compensation. In the orthogonal transformation section 21, the transformed coefficient 204 is created by transforming the interframe differential signal to a spatial frequency domain through orthogonal transformation. In the quantizing section 22b, the transformed coefficient Ci 204 undergoes threshold processes as mentioned below based on a buffer accumulation quantity to be described later. Buffer accumulation quantity: large → threshold value Th: large, buffer accumulation quantity: small → threshold value Th: small, Th < Ci → Ci: a significant coefficient, Th ≥ Ci → Ci: an insignificant coefficient. In the result of a threshold process, a transformed coefficient 204 which is classified to be a significant coefficient is quantized in the quantizing section and is output as a quantized output signal 206. On the other hand, the transformed coefficient 204 which is classified to be an insignificant coefficient is output as a zero quantized output signal 206. The quantized output signal 206 is is encoded into a variable-length signal together with a moving vector 205, which is explained later, in the variable-length encoding section 26 and is output as an encoded data 210; on the other hand the signal 206 is converted to a decoded interframe differential signal 207 in the inverse orthogonal transformation section 23. In the adder 32, the decoded interframe differential signal 207 is added to the interframe predictive signal for movement compensation 202 to create a decoded signal 208. The decoded signal 208 is stored temporarily in the frame memory 24, and in the result of movement compensation the interframe predictive signal for movement compensation 202 is output. In the movement compensation section 25, the movement quantity of the input signal 201 is detected by using a foreframe decoded signal 209 from the frame memory 24 to output a movement vector 205. The transmitting buffer 27 stores the encoded data 210 temporarily and outputs them as transmitting data 212 at a constant bit rate, and at the same time it outputs a buffer accumulation quantity 211 as a feedback signal to prevent buffer overflow. In addition, US-A-4 797 742 discloses a block transform coding system in which the quantization of the coefficients depends on their order or sequency. Finally, JP-A-61 046 685 discloses a prediction encoder in which coarse quantization is applied to the predicted error signal in discriminated still areas of the image. The encoding section by a transform coding method of a conventional picture signal encoding and decoding apparatus is constituted as described in the above, so that the adaptive quantization of an input signal according to its statistical characteristic is difficult, and it has been a problem to be solved to compress a signal effectively. SUMMARY OF THE INVENTION The object of the present invention is to provide a picture signal encoding apparatus which has an encoding section by a transform coding method in which effective quantization according to the statistical characteristic of a signal to be encoded is possible. To accomplish this object, the picture signal encoding apparatus according to the present invention adaptively selects a specified quantizing characteristic for a signal to be encoded according to the detected movement quantity and the sequency of a transformed coefficient as set out in claim 1. The above-mentioned objects and other objects and new features of the present invention will be understood clearly with reference to the following detailed description and accompanying drawings. The drawings are, however, solely for explanatory purpose, and do not limit the scope of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a block diagram showing the configuration of a conventional picture signal encoding and decoding apparatus. Fig. 2 is a block diagram showing the configuration of an encoding section by a transform coding method of the conventional picture signal encoding and decoding apparatus. Fig. 3 is a block diagram showing the configuration of an encoding section of a picture signal coding and encoding apparatus according to an embodiment of the present invention. Fig. 4 is a drawing showing the relations among the sequency of transformed coefficients, the size of a movement vector and a quantizing characteristic to be used, in the case where three kinds of quantizing characteristics of different quantizing step sizes are prepared in another embodiment mentioned in the above. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The preferred embodiments according to the present invention will be explained in detail referring to the drawings. Fig. 3 is a block diagram showing the configuration of a decoding section in a picture signal encoding and decoding apparatus in an embodiment of the present invention. In the figure, 22a is an adaptive quantizing section which selects and uses a proper quantizer out of a plurality of them according to the spatial frequency which corresponds to a movement vector 205 of an input signal 201 and a sequency of a transformed coefficient. The other component parts are similar or equivalent to those of conventional apparatuses, so that the explanation of them is omitted. The operation is explained in the following. The transformed coefficient 204 and the movement vector 205 obtained by similar processes to the conventional ones are input to the adaptive quantizing section 22a. In the adaptive quantizing section 22a as shown in Fig. 4, according to the sequency of the transformed coefficient 204 and to the size of the movement vector 205, a proper quantizing characteristic is selected and quantization is performed for the transformed coefficient 204 which is judged to be significant through a threshold process based on the buffer accumulation quantity 211. In the case of the selection of a quantizing characteristic, the movement vector 205 is transferred to the decoding side and the sequency of of the transformed coefficient is also known to the decoding side, so that there is no need to send an overhead information. The other processes are similar to those of conventional apparatuses. In the above-mentioned embodiment, a case is shown in which quantizers are switched according to a spatial frequency component corresponding to the sequency of transformed coefficients 104 and the size of a movement vector 205, but in the case where the movement vector becomes zero, such a control as to interrupt the transformed coefficient whose corresponding spatial frequency is high is effective to decrease the temporal direction noise in a static region, for example, in a background. In the above-mentioned embodiment according to the present invention, a plurality of quantizers of different characteristics are prepared and a quantizing characteristic is selected based on the sequency of transformed coefficients and the size of a movement vector, so that the quantizing corresponding to the statistical characteristic of an input signal can be realized and also a signal can be efficiently compressed.

Claims: A picture signal coding apparatus comprising: a subtractor (31) which eliminates a redundant component by subtracting a motion compensation interframe predictive signal (202) created according to a movement quantity that is detected by a motion detecting section from a digital picture signal (201); an orthogonal transformation section (21) which generates a transform coefficient (204) by performing an orthogonal transformation to an interframe differential signal (203) obtained by said subtractor (31); an adaptive quantizing section (22a) which quantizes said transform coefficient (204) according to a quantization characteristic determined on the basis of a movement quantity and sequency of the coefficient (204); an inverse orthogonal transformation section (23) which generates a decoded interframe differential signal (207) by performing an inverse transformation on a quantized output signal (206) inputted from said adaptive quantizing section (22a); an adder (32) which generates a decoded picture signal (208) by adding said decoded interframe differential signal (207) and said motion compensation interframe predictive signal (202); and a frame memory (24) which stores said decoded picture signal (208) temporarily and creates said motion compensation interframe predictive signal (202) according to a movement quantity; characterized in that said adaptive quantizing section (22a) adaptively selects a quantization characteristic among predetermined quantization characteristics such that the quantizing step size becomes larger as the frequency of the sequency of the transform coefficient (204) becomes higher and the movement quantity becomes larger. A picture signal coding apparatus according to claim 1, characterized in that said adaptive quantizing section (22a) includes a plurality of quantizers corresponding to respective quantization characteristics. A picture signal coding apparatus according to claim 1 or 2, characterized in that said adaptive quantizing section (22a) cuts off the high frequency transform coefficient (204) in case the movement quantity is zero.

Inventors: ASAI KOHTARO, MATSUZAKI KAZUHIRO, MURAKAMI TOKUMICHI, ASAI, KOHTARO, MATSUZAKI, KAZUHIRO, MURAKAMI, TOKUMICHI, ASAI, KOHTARO, C/O MITSUBISHI DENKI K. K., MATSUZAKI, KAZUHIRO, C/O MITSUBISHI DENKI K. K., MURAKAMI, TOKUMICHI, C/O MITSUBISHI DENKI K. K.
Applicants: MITSUBISHI ELECTRIC CORP, MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1989
Main classification: H04N7
IPC codes: H04N7, G06T9