JPH0712193B2 - Modulation signal generation method in optical scanning device - Google Patents

Description

TECHNICAL FIELD The present invention relates to a modulation signal generating method in an optical scanning device.

(Prior Art) Optical scanning in which a light beam modulated by a modulation signal is deflected by a deflecting means, for example, a rotating polygon mirror, and scanning is performed on a photosensitive recording medium, for example, a photoconductive photoconductor to perform optical writing. Devices are well known in the context of optical printers and the like.

Optical writing methods include a negative-positive method in which a portion exposed by a light beam is visualized and a positive-positive method in which a portion not exposed by a light beam is visualized.

There is a problem of thickening and thinning of characters in connection with such an optical scanning device. Thickening of characters is a negative-positive type optical writing, and when writing characters, the visible image of the written characters is made bolder than the originally intended character. It is a phenomenon. Further, the thinning of a character is a phenomenon in which a character written and visualized by a positive-positive type optical writing is subcultured compared to an intended character.

A thick character means that a thin line becomes thick in the recorded visible image, and a thin character, on the contrary, may cause a thin line of a certain thickness or less to be cut or disappear in the recorded visible image. Therefore, whether the character is thick or thin, the reproducibility of the thin line in the recorded visible image is deteriorated.

Conventionally, a method of changing the width of a modulation signal has been known as a method for eliminating such thickening and thinning of characters (Japanese Patent Laid-Open No. Sho 61-187).
56-8112 publication).

However, since this conventional technique uses the one-shot circuit, it is easily affected by environmental changes.

On the other hand, a scanning method has been proposed which realizes uniform scanning speed by changing the pixel clock period without using an expensive fθ lens for optical scanning (Japanese Patent Laid-Open No. 58-87965).

Here, it is as follows that the constant velocity of scanning is realized by changing the period of the pixel clock without using the fθ lens.

That is, as is well known, the fθ level has a function of accelerating the movement of a light spot (converging a deflected light beam having a constant angular velocity) on a recording medium.
Called function.

When the fθ lens is not used, the moving speed of the light spot is not constant on the recording medium and changes according to the position of the light spot according to the characteristics of the lens that focuses the deflected light beam as the light spot. . That is, the functional relationship of the speed of the light spot with respect to the position of the light spot is determined according to the characteristics of the lens.

The optical scanning in which the moving speed of the light spot is not constant on the recording medium in this way is called non-uniform optical scanning.

When the optical scanning is non-constant, if the period of the pixel clock is constant, the distance for scanning the recording medium per pixel becomes large at the place where the moving speed of the light spot is high, and conversely Where the spot moving speed is low, the distance for scanning the recording medium per pixel becomes small.
The distance over which the recording medium is scanned changes per pixel.

Therefore, when the moving speed of the light spot is high, the period of the pixel clock is made small, and when the moving speed of the light spot is small, the period is made large so that one pixel can be displayed regardless of the fluctuation of the moving speed of the light spot. The distance that the light spot scans the recording medium per hit can be made substantially constant.

In this way, by changing the period of the pixel clock,
The fact that the distance that the light spot scans the recording medium per pixel is substantially constant means that the optical scanning corrects the non-uniform velocity, and the period of the pixel clock is changed without using the fθ lens. This means that uniform scanning speed can be achieved.

By the way, the technique disclosed in Japanese Patent Laid-Open No. 56-8112 has a uniform change width of the modulation signal, and there is no problem in constant velocity scanning using an fθ lens. However, this technique is described in Japanese Patent Laid-Open No. 58-87965. When applied to the scanning method of No. 2, there arises a problem that the fluctuation range of the duty ratio in the pixel becomes large.

The duty ratio of a pixel is a ratio of a time for turning on a light source in one pixel of optical scanning: τ to a period of a pixel clock that defines the size of the pixel: τ / T, which is generally a design condition. Is set to a desired value (usually 50 to 70%), but in the invention of JP-A-58-87965, since τ is constant, it is applied when the period of the pixel clock: T changes. , Τ / T greatly fluctuates according to the cycle: T.

(Object) The present invention has been made in view of the above-mentioned circumstances, and an object of the present invention is to use a lens having no fθ function and to change the period of the pixel clock so as to perform non-uniform speed of optical scanning. In an optical scanning device that corrects the property, the duty ratio in the pixel can be made substantially constant regardless of the change of the cycle of the pixel clock, and the device is simple in structure and low in cost because it is hardly affected by the environmental change. realizable. It is to provide a new modulation signal generation method.

(Structure) Hereinafter, the present invention will be described.

According to the present invention, a light beam modulated by a modulation signal is deflected by deflecting means such as a rotating polygon mirror or a holographic scanner, and f
A non-constant speed optical scanning is performed without using a special lens such as a θ lens, and the non-constant speed of optical scanning is corrected by changing the period of the pixel clock, while a photoconductive photoconductor or the like is used. , A method of generating pixel data in an optical scanning device that optically scans a photosensitive recording medium.

The period of the pixel clock is changed so that the size of the written pixel is substantially constant regardless of the writing position, regardless of the non-uniform speed of the optical scanning.

That is, the period of the pixel clock becomes short in the portion where the scanning speed of the optical scanning is high, and the period of the pixel clock becomes long in the portion where the scanning speed is low. The duty ratio of the pixel clock is approximately 50% and does not change and is constant.

By the way, in the present specification, two types of inventions are proposed as the modulation signal generating method. These two kinds of inventions are common in the above-mentioned objects, and the structure is also common in the main part.

Both inventions use first and second flip-flops.

In the first type of invention, the pixel data transferred by the pixel clock is synchronized with the pixel data by the first flip-flop, and further delayed by half the pixel clock by the second flip-flop.

Then, the logical product of the output of the first flip-flop and the output of the second flip-flop is obtained, and the result is used as the modulation signal.

Also in the second type invention, the pixel data is synchronized with the pixel clock by the first flip-flop, and
The flip-flop of the above delays the pixel clock by half the clock. The output of the first flip-flop, on the one hand, is delayed by a delay element by td. This time td is the delay time set in the delay element and satisfies the condition of O <td <t. Time t
Is the pulse width of the pixel clock.

The signal delayed by td by the delay element is ANDed with the output of the second flip-flop, and the result is used as the modulation signal.

Hereinafter, a specific description will be given with reference to the drawings.

FIG. 1 (A) shows one embodiment of the first type invention of the present invention.

In this embodiment, the light source is turned on when the pixel data is at L level (low level).

The pixel clock has a pulse width t (half the clock of the pixel clock) as shown in FIG. 2 (A), and is input to the CK input of the first flip-flop 10 (FIG. 1) and the inverter. An inverted signal is input via 14 to the CK input of the second flip-flop 12. The pixel clock has a fixed duty ratio of about 50% as shown in FIG.
have. Half the pixel clock, that is, pulse width: t
Is from the rising edge to the falling edge of the pixel clock pulse.

The pixel clock, of course, has its cycle changed, and in FIG. 2, the cycle is depicted as a constant cycle for the sake of simplicity of illustration, but the cycle is not actually constant.

For example, if the deflecting means is a rotary polygon mirror and the imaging lens is a normal lens, the scanning speed is low in the central portion of the scanning area, so the period of the pixel clock is long, and the scanning speed is high at both ends of the scanning area. Since it is large, the period of the pixel clock becomes short. The same applies to FIG.

On the other hand, the pixel data has H (high) level and L level as shown in FIG. 2 (A), and is input to the D input of the first flip-flop 10 as shown in FIG. 1 (A). input.
As a result, the Q output of the first flip-flop 10 becomes
The pixel data is synchronized with the pixel clock. This output is input to the D input of the second flip-flop 12.

The output of the first flip-flop 10 is an inverted signal of the Q output, which is indicated by reference numeral 1 in FIG.

On the other hand, since the inverted signal of the pixel clock, that is, the signal with a 180-degree phase delay of the pixel clock is input to the CK input of the second flip-flop 12, the output of the same flip-flop 12 is shown in FIG. As indicated by reference numeral 2, the signal 1 is further delayed by half the pixel clock.

Eventually, the pixel data is synchronized with the pixel clock by the first flip-flop 10 (Q output, signal 1) and further delayed by t by the second flip-flop (signal 2).

The signals 1 and 2 are applied to the NAND gate 16, and the NAND gate 16 takes the logical product of the two, and as a result, a modulation signal is obtained as shown in FIG.

With this modulation signal, the light source is turned off earlier than the actual pixel data by the time t on the terminal side thereof. That is,
The lighting time of the light source is shorter than the pixel data length by the time t.
Of course, the time t is half the clock of the pixel clock, and the period of the pixel clock changes, so it goes without saying that the time t changes accordingly.

FIG. 1 (B) shows one embodiment of the method of the second kind in the present invention. Hereinafter, this embodiment will be described with reference to FIG. 2 (B). However, in order to avoid complication, it is assumed that there is no risk of confusion, as shown in Fig. 1 (B).
, The same reference numerals as in FIG. 1 (A) are used,
The outputs of the flip-flops 10 and 12 are also represented as 1 and 2 in FIG. 2 (B).

Also in this embodiment, the light source is L of pixel data.
It lights up in the state.

The difference from the embodiment shown in FIG. 1A is that one output 1 of the first flip-flop 10 is input to the delay element 18 and delayed by a predetermined delay time td (O <td <t). Input to the NAND gate 16.

The pixel data is synchronized with the pixel clock by the first flip-flop 10 (1, Q output), and delayed by the second flip-flop 12 by half the pixel clock (pulse width t) (2). , Signal 1 is
The signal is delayed by the delay element 18 by td, and the signal 2 is ANDed with the NAND gate 16 to obtain a modulated signal as shown in FIG. 2 (B). When modulated with this modulation signal,
The lighting time of the light source is shorter than the pixel data length by Δt. Δt = t−td. Although td is constant, since the time t changes according to the period of the pixel clock, Δt also changes according to the pixel position.

In the two embodiments described above, the pixel data structure is not detected, and the control is the same regardless of whether the black and white 1-dot structure or continuous data is used.

In the above-mentioned embodiment, the modulation signal for lighting the light source at the L level of the pixel data input is generated, but the embodiment for the case of generating the modulation signal for lighting the light source at the H level of the pixel data input It is shown in FIG. The same reference numerals as those in FIG. 1 are used for those which are considered to have no possibility of confusion.

FIG. 3A shows an embodiment of the first type of invention in which pixel data is synchronized with a pixel clock by a first flip-flop 10 and a half clock of the pixel clock by a second flip-flop 12. Delayed by minutes.

The Q outputs of the first and second flip-flops are logically ANDed by the AND gate 17, and the result becomes a modulation signal.

Each signal in the case of this embodiment is shown in FIG. The light source is lit according to the H level of the pixel data by the modulation signal, but its lighting time is shorter than the data length by the clock pulse width t.

FIG. 3 (B) shows an embodiment of the second type invention.

The Q output of the first flip-flop 10 is pixel data synchronized with the pixel clock as shown in FIG. 4 (B). By a predetermined delay time td (O <td <t) by the delay circuit 18 and the Q output of the second flip-flop 12 and the AND gate 17. Is ANDed with. The modulated signal obtained as a result turns on the light source according to the H level of the pixel data as shown in FIG. 4 (B), but the lighting time is Δt = t−td rather than the data length.
Just short. In the above embodiment, the modulation signal is obtained by the logical product, but the similar modulation signal can be obtained by the logical sum.

(Effect) As described above, according to the present invention, it is possible to provide a novel modulation signal generation method in an optical scanning device.

According to the present invention, at the boundary where the light source is turned on / off by the modulation signal, the lighting time is shorter than the data length by t (first invention) or t-td (second invention). Assuming that one period of the pixel clock is T, in the case of pixel data for each black and white dot, the lighting time is T-
Since t or T-t + td, the reproducibility of fine lines in a recorded visible image is improved in both the negative-positive method and the positive-positive method, and the thick and thin characters can be effectively reduced and improved.

When the data length of the light source to be turned on is n (> 1) dots, the lighting time is nT-t or nT-t + td, and the light source can be turned on continuously for approximately dots, and it is reproduced at the boundary of the background image. And the reproducibility of the background, solid areas, and solid black areas are both improved.

Further, in order to correct the non-uniform velocity of the optical scanning due to not using a special level such as the fθ level for the optical scanning,
Despite changing the period of the pixel clock, it is possible to improve the on / off ratio of the light source in one pixel.

For example, in the case of the black-and-white 1-dot configuration, in the case of the first type invention, the light source on / off ratio in the modulation signal is constant according to the duty ratio of the pixel clock. For example, if the duty ratio is 50%, the on / off ratio of the light source is also 50%.

In the case of the second invention (O <td <t), it is assumed that the cycle of the pixel clock changes in n steps of T _{1 to} T _n , the duty ratio of each pixel clock is 50%, and the pulse width is t _k. = (1/2) T _k
If (k = 1 to n), the on / off ratio of the light source is Given in.

Assuming that T ₁ = 400nS, T _n = 600nS, the on / off ratio of the light source is about 7
When 0% is achieved, td = 96nS, at which time T _{1 to} T
_{At n} , the light source on / off ratio is 70 ± 4%.

However, when this is configured by the conventional technique with a constant delay time, the delay time is 336 nS, and the ON / OFF ratio of the light source is 70 ± 14% in T _{1 to} T _n . As is clear from this comparison, in the present invention, even if the period of the pixel clock changes, the lighting time of the light source changes accordingly, so the accuracy of the on / off ratio of the light source becomes high. Also,
Since the circuit configuration is simple, it can be realized at low cost and is not easily affected by environmental changes.

[Brief description of drawings]

FIG. 1 is a diagram showing an embodiment of the present invention, FIG. 2 is a diagram for explaining the above embodiment, FIG. 3 is a diagram showing another embodiment of the present invention, and FIG. 4 is another embodiment described above. It is a figure for explaining. 10 ... First flip-flop, 20 ... Second flip-flop, t ... Pixel clock pulse width.

Claims (2)

[Claims] 1. A light beam modulated by a modulation signal is deflected, and a lens having no f.theta. Function is focused as a light spot on a photosensitive recording medium to perform optical scanning. The lens has an f.theta. Function. An optical scanning device that performs optical writing while correcting the change in pixel size due to the non-uniform velocity of the movement of the light spot due to the absence of the light spot by changing the period of the pixel clock according to the position of the light spot. In a method of generating a modulation signal based on pixel data, wherein pixel data transferred by a pixel clock having a duty ratio of about 50% is synchronized with the pixel clock by a first flip-flop, and Half a clock of the above pixel clock (from the rising edge to the falling edge of the pixel clock pulse) by the flip-flop of
And a logical product of the output of the first flip-flop and the output of the second flip-flop is used as the modulation signal. 2. A light beam modulated by a modulation signal is deflected, and a lens having no f.theta. Function is focused as a light spot on a photosensitive recording medium to perform optical scanning. The lens has an f.theta. Function. An optical scanning device that performs optical writing while correcting the change in pixel size due to the non-uniformity of the movement of the light spot due to the absence of the light spot by changing the period of the pixel clock according to the position of the light spot. In a method of generating a modulation signal based on pixel data, wherein pixel data transferred by a pixel clock having a duty ratio of about 50% is synchronized with the pixel clock by a first flip-flop, and Half a clock of the above pixel clock (from the rising edge to the falling edge of the pixel clock pulse) by the flip-flop of
Delay time, and the pulse width of the pixel clock is t, the delay time is
The delay element is set so that td satisfies the condition of 0 <td <t, and the output of the first flip-flop is td by the delay element.
A method of generating a modulation signal, characterized in that a logical product of a signal sent only by the above and an output of the second flip-flop is used as a modulation signal.