1.  Field of the Invention
The invention relates to an apparatus for highly efficient coding of a digital image signal which is applied to a digital VTR or the like. 
2.  Description of the Prior Art
There have been known several highly efficient video signal coding methods whereby a mean bit length per pixel or a sampling frequency is reduced in order to fit the encoded signal into a narrower transmission band.  The applicant of the present invention has already proposed highly efficient coding apparatuses in which a dynamic range which is specified by the maximum and minimum values of a plurality of pixels which are included in a two-dimensional block or a three-dimensional block is obtained and an encoding adapted to the dynamic range is executed (refer to JP-A-61-144989, JP-A-62-92620).  Further, as disclosed in JP-A-62-128621, there has been proposed a variable length coding method in which a bit length used to represent data is changed in accordance with a dynamic range so that a maximum distortion due to quantization is set to be constant. 
According to the adaptive dynamic range coding method (hereinafter, referred to as ADRC) which has been proposed above, the dynamic range DR (difference between the maximum value MAX and the minimum value MIN) is calculated for every two-dimensional block comprising, for instance, 64 pixels (=8 lines.times.8 pixels).  The minimum level (minimum value) in the block is subtracted from each of the input pixel data.  Each of the pixel data after the minimum value was subtracted is converted into a representative quantized level.  The above quantization relates to processes for dividing the dynamic range DR which has been detected into four quantization level ranges corresponding to a bit length such as two bits which is smaller than the original unquantized bit length, for detecting the quantization level range to which each pixel data in the block belongs, and for generating a code signal indicative of the level range. 
A highly efficient coding apparatus in which a three dimensional ADRC and a frame dropping process are combined in order to further raise a compression ratio has also been proposed by the applicant of the present invention.  According to the above apparatus, motion between two areas of a three-dimensional block is detected, three-dimensional ADRC is executed in the block having motion, and in the stationary block, the transmission of one of the areas is omitted and two-dimensional ADRC is executed with respect to a block comprising a mean value of the two areas. 
In the above apparatus in which the three-dimensional dimensional ADRC and the frame dropping process are combined, there is need for a memory capacity of two frames for a block segmentation process to form a three-dimensional block.  When a buffering process is applied to the above apparatus, an additional memory of two frames is necessary to delay input data for a time until the number of bits to be assigned is decided by a buffering process.  A memory capacity of two frames is also needed for a block desegmentation process in a decoding apparatus.  As mentioned above, there is a problem in that the necessary memory capacity is extremely large.