Dot printer for reproduction of halftone images

A dot printer adapted to receive a gray scale input signal comprising a recording head having a tendency to become inoperative in a range below a threshold level, means for controlling said recording head in response to a clock pulse to record a dot with varying proportions of high and low discrete levels as a function of said gray scale input signal on a recording medium, means for relatively moving said recording head and recording medium to each other so that the recording head scans along a line path and is successively shifted to the next by the width of a line path, means for generating a random signal of which the instantaneous value has a probability density function of its value and the maximum amplitude has a predetermined value with respect to said threshold level, and means for modulating the amplitude of said gray scale input signal with said random signal so that the modulated signal oscillates randomly above and below said threshold level.

In a dot printer adapted to receive a gray scale input signal and including a recording head inoperative for signals having an amplitude lower than a threshold level,

• a threshold extending control apparatus for said recording head operative to cause said recording head to print dots of varying density levels for gray scale input signals having an amplitude lower than said threshold level, comprising:
• modulating means receiving said gray scale input signals and connected to said recording head for modulating the gray scale input signal and for providing to said recording head a modulated input signal varying above and below the threshold level for values of said gray scale input signal lower than said threshold value, and
• generating means connected to said modulating means for generating a varying signal and providing said varying signal to said modulating means for modulation of said gray scale input signal.