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1. (WO1991006171) DIGITAL MODULATION TECHNIQUE
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WHAT IS CLAIMED IS:-

1. In a method for producing an output image signal to drive a raster print engine from an multiple bit image input signal by generating in a signal generator a pulse having a width corresponding to the input image signal, the improvement comprising processing the multiple bit image input signal in a digital pulse width modulator which functions to reduce the number of bits in the multiple bit image input signal by separating from the input image signal at least one of the least significant bits in the input image signal; processing the remaining bits in the input image signals to provide an output signal pulse corresponding to acceptable number of grey scale levels; and simulating one of the separated bits by a delay signal which modifies the output signal pulse width in response to the value of the separated bit.
2. A method according to claim 1 in which the two least significant bits are separated, the most insignificant bit is discarded, and the value of the other separated bit is used to modify the output signal pulse width.
3. A method according to claim 1 in which the input image signal is an 8-bit signal.
4. A digital pulse width modulator which comprises in combination:
(i) a means to receive a multiple bit input image signal from an
image signal generating means;
(ii) a means to separate from the input image signal at least one bit including the most insignificant bit;
(iii) a signal generator means including a counter means controlling the width of the pulse generated by the signal generator in response to the value of the remainder of the input image signal after separation of at least one bit;
(iv) a clock means controlling the counter means; and
(v) a delay means which receives one of the separated bits from the input image signal and which extends the width of the pulse generated by the signal generator in response to the value of the second separated bit.

5. A digital pulse width modulation which comprises in combination:
(i) a means to receive a multiple bit input image signal from an
image signal generating means;
(ii) a means to separate and discard from the input image signal a first bit consisting of the most insignificant bit;
(iii) a means to separate from the input image signal a second
bit consisting of the least significant bit int the remaining signal ;
(iv) a signal generator means including a counter means controlling the width of the pulse generated by the signal generator in response to the value of the input image signal remainder of the input image signal, after separation of the two bits;
(v) a clock means controlling the counter means; and
(vi) a delay means which receives the first separated bit from the input image signal and which extends the length of the pulse generated by the signal generator in response to the value of the second separated bit.
6. A digital pulse width modulator according to claim 4 wherein the input image signal is an 8-bit signal.
7. A digital pulse width modulator according to claim 5 wherein the input image signal is an 8-bit signal.
8. An apparatus for generating a printed image containing at least 63 grey scale levels comprising in combination:
(a) an image input signal generating means which provides a multiple bit input image signal comprising at least 8 bits and corresponding to an image to be printed;
(b) means to separate at least one least significant bit from the input image signal;
(c) a clock means substantially synchronized with a print engine pixel clock means; wherein the frequency of the clock means is a whole number multiple of the frequency of the pixel clock means having a value of 2n, wherein n is the number of bits remaining in the image signal after at least one bit has been separated from it;

(d) a signal generating means including a counter means actuated by the clock means and which controls the width of the signal generator output pulse in response to the value of the input image signal;
(e) a delay means receiving one of the separated bits in the image input signal which lengthens the signal generator output pulse in response to the value of the received separated bit to provide a pulse width modulated signal;
(f) means to apply the pulse width modulated signal to a Raster scanning point engine for generating a series of successive scan lines, to cause the print engine to generate each scan line as a succession of printed segments the size of each of which is determined by the pulse width of the pulse width modulated signal, thereby to provide a variable density printed image comprising a plurality of printed segments corresponding to an image including a grey scale having at least 64 levels.
9. An apparatus according to claim 8 wherein the multiple bit input image signal comprises 8 bits.
10. An apparatus according to claim 8 wherein the separation means separates one least significant bit from the input image signal which bit is received by the delay means.11. An apparatus according to claim 8 wherein the separation means separates two least significant bits; the most insignificant bit is discarded; and the remaining least significant bit is received by the delay means.