Image signals may be compressed by motion-compensated interframe discrete cosine transform (DCT) coding such as is defined by a MPEG (Moving Picture Expert Group) international standard.  This form of signal compression has attracted much attention in the field of high definition television (HDTV). 
FIG.  1 is a block diagram of such a conventional motion-compensated interframe DCT coder.  In the shown coder, an image signal is divided into a plurality of sub-blocks.  The sub-blocks are all of the same size, for example 8×8, 16×16, . . . .  A motion estimator 40 produces a motion vector, defined by the difference between the current image signal and a one-frame delayed image signal, output by a frame memory 30.  The motion vector is supplied to a motion compensator 50 which compensates the delayed image signal from the frame memory 30 on the basis of the motion vector.  A first adder 8a serves to produce the difference between the present frame and the delayed, motion compensated frame.  A discrete cosine transform portion 10 processes the difference signal, output by the first adder 8a, for a sub-block.  The motion estimator 40 determines the motion vector by using a block matching algorithm. 
The discrete cosine transformed signal is quantized by a quantizer 20.  The image signal is scanned in a zig-zag manner to produce a runlength coded version thereof.  The runlength coded signal comprises a plurality of strings which include a series of “0”s, representing the run length, and an amplitude value of any value except “0”. 
The runlength coded signal is dequantized by a dequantizer 21, inversely zig-zag scanned and inversely discrete cosine transformed by an inverse discrete cosine transforming portion 11.  The transformed image signal is added to the motion-compensated estimate error signal by a second adder 8b.  As a result the image signal is decoded into a signal corresponding to the original image signal. 
Refresh switches RSW1, RSW2 are arranged between the adders 8a, 8b and the motion compensator 40 so as to provide the original image signal free from externally induced errors. 
The runlength coded signal is also supplied to a variable length coder 60 which applies a variable length coding to the runlength coded image signal.  The variable length coded signal is then output through a FIFO transfer buffer 70 as a coded image signal. 
In motion-compensated adaptive DCT coding, the interframe signal can be easily estimated or coded by way of motion compensation, thereby obtaining a high coding efficiency, since the image signal has a relatively high correlation along the time axis.  That is, according to the afore-mentioned method, the coding efficiency is high because most of the energy of a discrete cosine transformed signal is compressed at the lower end of its spectrum, resulting in long runs of “0”s in the runlength coded signal. 
However, the scanning regime of the aforementioned method does not take account of differences in the spectrum of the motion-compensated interframe DCT signal with time. 
A method is known wherein one of a plurality of reference modes is previously selected on the basis of the difference between the present block and that of a previous frame and the image signal is scanned by way of a scanning pattern under the selected mode and suitably quantized.  With such a method, however, three modes are employed to compute the energies of the intermediate and high frequency components of the image signal in accordance with the interframe or the intraframe modes in order to determine the appropriate mode.  This mode determining procedure is undesirably complicated.