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 comprises a configuration in which a condition not to make a receiving buffer overflow can be controlled on the transmitting side, and a plurality of quantizers are prepared and a proper quantizing characteristic is selected and used according to the sequence of transformed coefficients and the size of a motion vector. Therefore, it is not necessary to prepare a receiving buffer of a large capacity or a high speed decoding circuit, so that the apparatus can be manufactured at a low cost and the stability of operation is improved and also a signal can be compressed efficiently owing to the realization of quantization corresponding to a statistical characteristic of an input signal.

Description: The invention relates to a picture signal encoding and decoding apparatus comprising an encoding section for converting a series of picture elements, whose number is predefined (one picture frame or one line etc.), of a digitized picture signal series to a variable-length code-word series for transmission; and a decoding section for receiving a transmitted encoded signal and converting the signal to a fixed- length signal and thereafter decoding the signal to a digital picture signal series; said picture signal encoding and decoding apparatus having a transmitting buffer for performing speed conversion by storing data temporarily in order to transmit a sequence of said variable-length code-word series at a constant transmission rate. Fig. 1 is a simplified block diagram of such a conventional picture signal encoding and decoding apparatus 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 smoothes out 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. Fig. 2 is an illustrative drawing showing the operation of the transmitting buffer 3. Fig. 3 is an illustrative drawing showing the operation of the receiving buffer 8. The operation is explained in the following. The digitized picture signal series 101 is converted to a code-word series 102 in the encoding section 1. In this step, each code word has a fixed length. Next, the variable-length encoding is performed by utilizing the deviation of occurrence probability of each code word in the variable-length encoding section 2; thus the code-word series 102 is converted to a variable-length code-word series 103. In the result, the sum of the length of the variable-length code-word series 103 becomes smaller than a code quantity, the sum of the length of the code-word series 102, so that transmission efficiency can be improved. As the code quantity becomes variable, to send out these data to a channel at a constant transmission rate smoothing out of the speed is performed in the transmitting buffer. The output 109 of the transmitting buffer 3 has a constant rate (a code quantity in a unit time) corresponding to the transmission rate. The operation of the transmission buffer 3 is explained referring to Fig. 2. In Fig. 2 (a), the axis of abscissa represents time and the axis of ordinate represents a buffer accumulation quantity. The series to be input to the transmitting buffer 3 are of variable-length and the output has a constant rate; therefore the accumulation quantity varies as shown in Fig. 2 (a). Fig. 2 (b) is a buffer output corresponding to Fig. 2 (a). The decrease in the buffer accumulation quantity means a state where data are read from the buffer more than the data are written to the buffer, and when the accumulation quantity becomes zero nothing is left to be read in the buffer so that a dummy signal is output. In contrast with this, when more is written into the buffer than read out from the buffer accumulation quantity increases. As the buffer capacity is limited, if the buffer accumulation quantity is kept increasing, the buffer may overflow in time. To prevent the overflow, when the buffer accumulation quantity becomes large, the creation of code words is suppressed by controlling the operation of the encoding section 2 by using a feedback control signal 108. The output 109 read from the buffer has a constant rate. In a framing section 6 framing is performed at fixed intervals for the transmitting buffer output 109 and the data are output to a transmission line. On the receiving side deframing is performed for an input signal series in a deframing section 7, and the data are once stored in a receiving buffer 8. The output of the buffer 8 is a variable-length code series 111, and the series 111 is converted to the fixed-length code series 112 in the variable-length decoding section 9. The fixed-length code series 112 is decoded in the decoding section 10 and the digital picture signal series 113 is obtained. In the above-mentioned process of receiving and decoding of a signal, the decoding section 10 can only decode a certain section of a picture signal series (for example, a sequential picture signal series such as a picture signal frame or a line) at a fixed speed. Therefore, the speed of the fixed-length code series 112 cannot exceed the speed in which the decoding section 10 is capable of decoding. In the similar way, there is a speed limit in the operation of the variable-length decoding section 9. On the transmitting side, the transmitting buffer 3 located near the output port smoothes out the speed of a signal; in correspondence to this, the receiving buffer 8 on the receiving side adjusts the speed. The operation of the receiving buffer 8 is explained in the following referring to Fig. 3. It is assumed that the input signal series 110 for the receiving buffer 8 is given as shown in Fig. 3 (a). As the signal series is a variable-length code-word series, when the series is separated, for example, into picture frames, the intervals are not uniform. T in Fig. 3 (b) is a time interval in which the variable-length decoding section 9 and the decoding section 10 can process code words corresponding to the frame. When the quantity of the code words corresponding to a frame in Fig. 3 (a) is smaller than the time interval in Fig. 3 (b), the code words corresponding to the next frame sent in a time difference mentioned in the above are accumulated in the receiving buffer 8. If a condition under which data accumulation being done continues long, there is a fear that data may overflow the buffer memory, so that the capacity of the receiving buffer is designed to have some redundancy. In a picture signal encoding and decoding apparatus of the conventional type as described above, it is necessary to gather the operating speed of a variable-length decoding section and a decoding section as fast as possible. Because of this, there has been a problem that the size of an apparatus and the capacity of a receiving buffer have to be made large, which makes the delay time large. It is an object of the present invention to provide a picture signal encoding and decoding apparatus in which it is possible on the transmitting side to guarantee a condition under which the reception of a signal and the decoding of the signal are possible on the receiving side. Moreover, the the operation speed shall be kept low, that is, the size of the configuration can be made small. To accomplish the object of the present invention, a picture signal encoding and decoding apparatus as defined above is characterized by a reference timing pulse generator being adapted to create timing pulses having an adjustable fixed period; and a timing comparing and adjusting section for comparing the phase of head pulses of said variable-length coded-word series read from said transmitting buffer with said reference timing pulses for timing, and if said reference pulse timing is found to be later than the head pulses as the result of said comparison, the time difference between said reference pulse and said head pulse of said variable-length code-word series is detected, and if said time difference is larger than a predetermined threshold value, the reading from said transmitting buffer is stopped and after the insertion of dummy data the reading from said transmitting buffer is resumed for the compensation of said time difference, and if said reference pulse timing is found to be earlier than the head pulses as the result of said comparison, said reference pulse generator is reset for time adjustment at an arbitrary time in the range of a predetermined time difference from the read timing of a head pulse of said variable-length code-word series, whereby the encoded-data series to be transmitted is time adjusted based on the reference timing given by said reference timing generator. In a preferable embodiment, said reference timing length corresponds to a time interval necessary to decode a fixed section of a picture signal on said decoding section side. The above-mentioned object and 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. Fig. 1 is a block diagram showing the configuration of a conventional picture signal encoding and decoding apparatus. Fig. 2 is an explanatory drawing showing the operation of a transmitting buffer of the conventional picture signal encoding and decoding apparatus. Fig. 3 is an explanatory drawing showing the operation of a receiving buffer of the conventional picture signal encoding and decoding apparatus. Fig. 4 is a block diagram showing the configuration of an encoding and decoding apparatus according to an embodiment of the present invention. Fig. 5 is an explanatory drawing showing the operation of a transmitting buffer according to the embodiment of the present invention. Fig. 6 is an explanatory drawing showing the operation of a receiving buffer according to the embodiment of the present invention. The preferred embodiments according to the present invention will be explained in detail referring to the drawings. Fig. 4 is a block diagram showing the configuration of a picture signal encoding and decoding apparatus according to an embodiment of the present invention. In the figure, 4 is a reference timing pulse generator which can generate a pulse signal of a fixed period; 5 is a timing pulse comparator which compares the output of the reference timing pulse generator 4 with the timing pulses monitored by the transmitting buffer 3; 104 is a reference pulse train output from the reference timing pulse generator 4; 105 is a timing pulse train monitored by the transmitting buffer 3; 106 is a reset signal which resets the phase of the pulses output from the reference timing pulse generator 4; 107 is a control signal which controls the operation of the transmitting buffer 3 based on the comparison result by the timing pulse comparing and adjusting section 5. The other component parts are similar or equivalent to those of the conventional apparatus; therefore the explanation of them is omitted. Fig. 5 is an explanatory drawing showing the operation of the transmitting buffer 3 according to the embodiment of the present invention. Fig. 6 is an explanatory drawing showing the operation of the receiving buffer 8 according to the embodiment of the present invention. The operation is explained in the following. A digitized picture signal series 101 is converted to a coded-word series 102 in the encoding section 1; in this step each coded word has a fixed length. In the next step, variable-length encoding utilizing the deviation in the occurrence probability of each coded word is performed in the variable-length encoding section 2. In other words, the coded-word series 102 is converted to the variable-length coded-word series 103. In comparison with the code quantity, the sum of the length of the coded-word series 102, the sum of the length of the variable-length coded words becomes smaller, which improves the transmission efficiency. As the code quantity becomes variable, to send out the data to a channel at a constant rate, the smoothing out of the data speed is performed by the transmitting buffer 3. The operation of the transmitting buffer 3 is explained referring to Fig. 5. The axis of abscissa of Fig. 5 (a) expresses the time and the axis of ordinate expresses the buffer accumulation quantity. The basic operation of the buffer 3 is shown in the following. When more data are read from the buffer than those written to the buffer the accumulation quantity is decreased, and when more data are written to the buffer than those read from the buffer, the accumulation quantity is increased. The read speed from the buffer 3 is a constant rate (the number of codes per unit time) corresponding to the transmitting rate. When data are not read from the buffer 3 it outputs dummy pulses. There are two kinds of cases where reading is not performed. In a first case the buffer accumulation quantity is zero and there is no data to be read. Following is the explanation of a second case. In Fig. 5 (b), T is a time interval in which a certain section of a picture signal (a continuous, fixed length picture signal series such as one picture frame or one line etc.) can be decoded on a receiving side, which will be explained later. In this case, the section mentioned in the above is assumed to be a frame for explanation. The pulses having the period T are created in the reference timing pulse generator 4 as a timing pulse train 104. On the other hand, the transmitting buffer 3 monitors a timing pulse 105 at the head of coded words corresponding to each frame. Refer to Fig. 5 (c), the reference timing pulse 104 is compared with a timing pulse at a frame head 105 in the timing pulse comparing and adjusting section 5. When the timing of the reference timing pulse is earlier, at the later timing of the frame head timing pulse 105, a reset signal 106 is created to reset the reference pulse timing as shown by A ○ in Fig. 5. When the timing of the frame head timing pulse 105 is earlier, the time difference "d" between the reference timing pulse 104 and the frame head timing pulse shall be found. The time difference "d" is obtained by subtracting the lapse of time from the reference timing pulse of the prior period from T. If the time difference "d" is larger than a predetermined threshold value "D", a control signal 107 is created to stop the reading from the transmitting buffer 3 as shown by B ○ in Fig. 5. At the timing of delayed reference timing pulse 104, the control signal 107 is released to resume the reading from the transmitting buffer 3 as shown by C ○ in Fig. 5. The description in the foregoing explains the second case where data are not read from the buffer 3. The object of above-mentioned control is to secure a necessary time interval for decoding on the receiving side, which will be explained later. The output 109 from the transmitting buffer 3 (to be described later in detail) has a constant rate including dummy pulses. A framing section 6 performs framing at fixed intervals for the transmitting buffer output 109, and send them out to a transmission line. On the receiving side, deframing is performed in a deframing section 7 for an input signal series and the data are once stored in the receiving buffer 8. The output of the receiving buffer is a variable-length code series 111, and the series 111 is converted to a fixed-length code series 112 in the variable-length signal decoding section 9. Further the fixed-length code series 112 is decoded to obtain a digital picture signal series 113 in the decoding section 10. In the course of receiving and decoding of a signal the decoding section 10 requires a certain time to decode the code words corresponding to a certain section of a picture signal. Following the former example, assuming that the certain section is a frame and a minimum time interval for decoding is T, the operation of the receiving buffer 8 is explained in the following. Assume that the input signal series 110 is given as shown in Fig. 6 (a). The T is, as described in the above, the minimum time interval in which the decoding section 10 can decode the code words corresponding to a frame. When the timing of each frame head timing pulse precedes the timing of period T, the code words corresponding to the time difference are stored in the receiving buffer 8. Because of a slow decoding speed, if the accumulation quantity overflows the capacity of the receiving buffer 8, the decoding becomes impossible, but the upper limit of the accumulation quantity never exceeds "D" as described in the explanation of the transmitting buffer 3. "D" shall be decided corresponding to the capacity of the receiving buffer 8. If the two parameters "T" and "D" which represent the decoding capacity on the receiving side are known to the transmitting side there is no probability of the overflow of the receiving buffer 8. In the above embodiment the section of a picture signal series is explained assuming that the section is a frame but the same effect can be obtained even when a line or other sequential section is used in place of a frame. As shown in a preferred embodiment according to the present invention the apparatus is so constituted that the condition not to cause the overflow of the receiving buffer can be controlled on the transmitting side; it is not therefore an essential condition to have a large capacity receiving buffer or a high speed decoding circuit so that there are advantages that the apparatus can be constituted at a low cost and the stable operation is guaranteed.

Claims: A picture signal encoding and decoding apparatus comprising an encoding section (1, 2) for converting a series of picture elements (101), whose number is predefined (one picture frame or one line etc.), of a digitized picture signal series to a variable-length code-word series (103) for transmission; and a decoding section (9, 10) for receiving a transmitted encoded signal and converting the signal to a fixed-length signal (112) and thereafter decoding the signal (112) to a digital picture signal series (113); said picture signal encoding and decoding apparatus having a transmitting buffer (3) for performing speed conversion by storing data temporarily in order to transmit a sequence of said variable-length code-word series (103) at a constant transmission rate; characterized by a reference timing pulse generator (4) being adapted to create timing pulses (104) having an adjustable fixed period; and a timing comparing and adjusting section (5) for comparing the phase of head pulses of said variable-length coded-word series (103) read from said transmitting buffer (3) with said reference timing pulses (104) for timing, and if said reference pulse timing is found to be later than the head pulses as the result of said comparison, the time difference between said reference pulse (104) and said head pulse of said variable-length code-word series (103) is detected, and if said time difference is larger than a predetermined threshold value, the reading from said transmitting buffer (3) is stopped and after the insertion of dummy data the reading from said transmitting buffer (3) is resumed for the compensation of said time difference, and if said reference pulse (104) timing is found to be earlier than the head pulses as the result of said comparison, said reference pulse generator (4) is reset for time adjustment at an arbitrary time in the range of a predetermined time difference from the read timing of a head pulse of said variable-length code-word series (103), whereby the encoded-data series to be transmitted is time adjusted based on the reference timing given by said reference timing generator (4). A picture signal encoding and decoding apparatus according to claim 1, wherein said reference timing length corresponds to a time interval necessary to decode a fixed section of a picture signal on said decoding section (9, 10) side.

Inventors: ASAI KOHTARO C O MITSUBISHI DE, MATSUZAKI KAZUHIRO C O MITSUBI, MURAKAMI TOKUMICHI C O MITSUBI, 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