length coding component contains the average level of the block and is called the d.c. component. The others are a.c. components. The use of variable length codes (VLCs), also referred to as Huff- The DCT is the basis of the JPEG standard (ISO, 1991) for coding man coding, or entropy coding, offers compression when the input of continuous-tone still images. The basic coder structure is shown data does not have a uniform probability distribution (Huffman, in Figure 57.25. After rearranging the input pels into 8 by 8 blocks 1952). Short code words are allocated to frequently occurring they are transformed. The resulting coefficients are quantised, each events and longer codes to infrequent ones. The indexes from a with a different uniform law; the stepsize reflects the visibility of DPCM coder typically have a peaked distribution centred on zero the quantising errors in that coefficient after inverse transformation. magnitude prediction errors and worthwhile compression can be The quantised d.c coefficients are DPCM coded from block to block obtained from variable length coding. This coding is completely with variable length codes. The quantised a.c. coefficients are rear- reversible in that the original input to the VLC coder can be recon- ranged into a zigzag order, F(0,1), F(1,0), F(2,0), F(1,1), F(0,2), structed exactly in error free conditions. Errors can cause significant F(7,6), F(7,7). Each non-zero a.c. coefficient is coded in combina- problems as the boundaries between code words can only be found tion with the run length of the preceding zero value coefficients from inspection of the received bit stream. A single bit in error can using variable length codes. make one transmitted code appear to be two shorter ones at the The coefficient weighting matrix and the VLC tables for the d.c. decoder or vice versa. Not only does this corrupt the corresponding and a.c. coefficients are downloadable to the decoder and SO can be data but the following code words, even if decoded to the correct optimised for each pic