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This work analyzes the operation of a dithered quantizer of picture luminance. It is shown that the addition of dither before quantization restores some of the pictorial information which a coarse quantizer would otherwise discard. Two-dimensional ordered dither patterns are described which are considerably more effective for this purpose than random distributions of the same dither samples. The patterns of dither can also be designed so that noise artifacts on the output picture are less visible than with equivalent random dither and so that (contrary to the random case) it is not necessary to subtract the same dither pattern at the receiver for substantially best results.