The present invention relates to the constant bit-rate control of a picture signal compressed by variable-length coding, and more particularly, to a transmission bit-rate controlling apparatus for the high efficiency coding of a moving picture signal, which can maintain a high picture quality by dividing the picture signal into segments and allotting a target coded bit quantity according to segment complexity.
Owing to recent remarkable developments in the fields of microprocessors, semiconductors and digital signal processors, various digital techniques for storing and transmitting a picture signal have been rapidly popularized. These techniques take into consideration the fact that when a picture signal is digitized to be stored or transmitted, the information quantity becomes considerably larger than that of the original analog signal, so that data compression becomes necessary. Basically, the data compression removes the redundancy of the picture signal for the efficient utilization of transmission channels and storage media. However, when a picture signal which has been encoded by a discrete coefficient transform (DCT) technique is compressed by variable-length coding, the thus-generated picture signal data has an irregular bit-stream. Therefore, rate control is necessary in order to maintain a constant transmission bit-rate for the processing of a digitized picture signal.
The advantage of video coding with the DCT technique is that the information of an original picture signal can be expressed efficiently with very few bits by fully coding the low-frequency portion a DCT-coded picture signal, where the energy is concentrated. In variable-length coding, a given code word is allotted variably with reference to probability concentration of the picture information, so that a code word having a variable length is transmitted. This method is suitable for the case when motion picture signals to be processed are distributed unevenly in the frequency domain. Thus, a DCT variable-length coding method is widely used as a compression method for fully expressing motion picture signals having unevenly distributed information. Here, however, since the information quantity (bits) generated per unit time should be constant when the picture information is stored or transmitted, a buffer is installed and forward or backward rate control is executed using the buffer, thereby adjusting the step size of a quantizer and adjusting generated bit quantity accordingly.
The conventional forward rate control method has an advantage in that errors due to a storage or transmission medium are propagated only within a given fixed-length unit. However, it is difficult to exactly match the generated bit quantity with target bit quantity. Without an exact match, the coding efficiency is lowered.
Also, the conventional backward rate control method has an advantage in that the generated bit quantity to be stored or transmitted is easily matched with target bit quantity since the step size of the quantizer is determined by the fullness of the buffer, by periodically checking the buffer state. However, since the quantization step size is determined irrespective of human visual characteristics, the picture quality is likely to be poor.
In order to improve the poor picture generated by the backward rate control method, there has been recently proposed a final tuning method which discriminates the local complexity of a picture through forward estimation, with reference to an updated quantization step size. The use of this method is illustrated in FIG. 1.
Referring to FIG. 1, an encoder 1 includes a data formatter 10, a DCT circuit 20, a rate controller 30, a quantizer 40 and a variable-length coder (VLC) 50. Essentially, encoder 1 compresses input picture data for output to an encoder buffer 2. Also, rate controller 30 includes a reference quantization coefficient calculator 31 for receiving a buffer fullness value d.sub.j from encoder buffer 2 and outputting a reference quantization coefficient Q.sub.j, and an activity calculator 32 for receiving the output of data formatter 10, estimating current screen activity in a unit of a macroblock, performing an operation on the estimated activity using the reference quantization coefficient Q.sub.j, and outputting a new quantization coefficient Q'.sub.j, and maintains the output of encoder buffer 2 at a constant bit-rate.
As described above, according to the conventional rate control method, in the case of a system having a relatively low transmission bit-rate, since the reference quantization coefficient Q.sub.j does not reflect the local characteristics of a picture at all, the picture improvement effect of the forward estimation is reduced.