What is the amplitude modulation and frequency modulation?
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For plate printing, prepress production is always an important part of the quality of the print. Screen printing must be used for image printing in printing. In the traditional image color separation screening and plate making technology, the production unit mostly adopts the amplitude modulation network technology. It has been practiced and improved over 100 years and is very mature. However, people also found that there are some inevitable defects in AM screening, which is not a barrier to print quality. Nowadays, a new screening technology, FM screening, has begun to spread in printing houses, which can effectively avoid the defects of AM screening.
First, Characteristics and defects of AM screening
Now printing mainly uses halftone screening to achieve continuous tone image effects. This kind of screening is to divide the original image part into a black and white dot in a grid form, and reproduce the effect of shadow, color and the like by changing the size of the dot in the mesh. If we look at these outlets as a whole, the images are continuously adjusted. This technique is called amplitude modulation (Amplitude).
Modulated) Screening. Before the appearance of the color prepress system, all the screening methods are basically the same. The distance between the network points in the AM network is fixed, that is, the frequency of the network points is fixed, and only the size of the network points is changed. In the AM screening method, the hanging method used by the electronic color separation machine is generally IS (Irrational) developed by Hell Company of Germany.
Tangent Screening) dot technology (irrational tangent dot) and RT (Rational Tangent Screening) dot technology (rational tangent outlets). PostScript screening technology is used in the color desktop publishing system. Although the AM screening technology is more mature, there are some inevitable defects.
1) Loss of detail Because the distance between the AM screens is fixed, the blank area in the middle will cause the image details to be lost.
2) Grid conflicts and screening to create moiré
The mesh shape of the original and the mesh shape of the amplitude modulation dot will cause interference and a moiré will appear. In addition, due to the different colors, the shape of the screen is different. When overlapping, there will be a moiré when there are regular lines in the image.
3) Rose spot
Due to the different angles of the four colors, when the color is copied, small rose spots appear. If the rose spots appear in important tonal areas, or are caused by inaccurate color registration, it will cause the phenomenon of rose spots.
4) visual collision
In the process of expanding the tone, with the expansion of the AM network, the network will eventually touch each other, so it should be a gradual change but suddenly a black tone. Although chain-type meshes can be used to avoid problems as much as possible, in general, this situation is still difficult to avoid.
Second, Characteristics and advantages of FM screening
The network points of FM screening are also called random network points. The size of the network points is the same. The image density is represented by the density of the network points, and the network points are randomly distributed. The advantages are as follows:
1) Improve the output quality of the film
Low-resolution imagesetters use FM technology to achieve high-quality images, while AM technology requires high-resolution imagesetters. For example, the FM image output by the 1200dpi imagesetter is significantly better than the 175 mesh AM image output of the 3000dpi imagesetter. Since the distribution of the dot points of the FM screening is irregular, as long as the dots in the FM screening are small enough, the printed matter can be sufficiently approximated to the original.
2) No moire and rose spots
FM screening does not produce moiré that is prone to color overprinting. Because it has no screen angle problem, it is randomly distributed randomly, and there is no fixed direction and the distance between the dots is not fixed, which can avoid the mesh conflict and the moiré caused by the screening. In addition, because there is no halftone screen line number and angle, the structure that causes the rose plaque is eliminated from the root. FM technology prints are more elaborate than AM technology prints. No matter how good the AM screening technology is, it can only avoid the big wrong network, but it can't avoid the rose spot.
3) The print is clear and stable
In the AM network technology, the quality factor determines the sharpness, and the larger the quality factor, the smaller the output dot area and the better the resolution. However, it is often difficult to make the quality factor too large, so the output image produced by the AM screening technology should be emphasized at a subtle level. Because the area of the dot used by FM screening is particularly small, it is equivalent to the use of a considerable quality factor in the amplitude modulation technology, so it is clearer. When using the FM network technology, you can use less subtle level of emphasis, even if you don't use it, the effect is quite good. The FM network is expressed in micron-sized dot size. Generally, it can reach 21~25 microns in single-sheet carousel printing. The ink volume and color reflectivity are better than the 133~200lpi amplitude modulation, such as 21 micron FM. The details of the net are equivalent to 400lpi, which can effectively reduce the color pollution from paper.
4) Good copying effect
The FM network replicates the best reflection of the midtone level, and can eliminate the tone jump of the middle tone, making the tone transition natural and harmonious. In addition, its color transition is fine, which is good for super four-color printing, and can record color separation films with lower resolution.
5) Better visual stability
When the AM screening is not accurate, the visual mesh structure will change. FM screening, no matter whether the version is not allowed, there will be no change. The printer also found that the FM network could better represent the details of the prints when the plates were not on time. In terms of color stability, when the amplitude modulation is not correct or the roller of the rotary machine is slightly offset, the color tone and color are shifted, and the FM network does not have this problem.
6) Not limited by screen angle
In the AM screening technology, various parameters such as the number of meshes, the mesh angle, and the mesh shape are important, and the technician must understand. The frequency modulation is only one parameter of the dot size, so its control technology is quite easy. FM screening is not carried out according to the fixed screening angle and grid. Therefore, the screen has no direction and is not restricted by the angle of the screen. The variable distance between the dots is not fixed, so there is no number of cables. Printing special reproduction conditions, such as: metal color, jewelry color, Chinese painting, etc., can also be printed by four colors or more, to expand the color reproduction range of printed matter, high-fidelity copy printing.
7) Avoid wrong network
The wrong network is the enemy that has long plagued the color plate industry. The distribution of the network points of FM screening is irregular, and there are no wrong nets when several color plates are superimposed. Even if the textiles in the printed matter or the original are copied, the wrong network will not be generated. In addition, the FM network is not sensitive to the pattern error in the printing plate making process, while the traditional AM screen may generate a wrong network due to the plate error.
8) Better printing
First of all, it is faster on the press without adjustment, the drying time of the ink is shortened, the working time is reduced accordingly, the ink balance is easier to maintain, the thicker ink coverage can obtain a higher dynamic range, and the ink distribution in the thinner layer is more. Evenly. In multi-color overprinting, the registration problem does not affect the color balance, and the screen color of the screen and the original color version will not appear.
Third, the improvement of FM screening technology
However, the FM screening itself is not perfect, and there are also deficiencies, such as:
1) It has a graininess due to the equal size of the dots, especially in the highlights and around 25%.
2) The latitude of the whole production process is smaller than that of amplitude modulation and screening, so more detailed process control and monitoring techniques than amplitude modulation screening are needed. For example, the tolerance of FM screening deviation is small, and many changes in film and development processing are final. The impact of the results is not significant, the operator will have to adopt a new level of precision throughout the production process;
3) Because the effect of increasing the thickness of the ink film on the size of the frequency modulation dot is not as significant as that of the amplitude modulation network, it is difficult to improve the quality by adjusting the ink on the printing machine by the image with poor color separation effect;
4) The minimum micro-dot size is too small for many printers to reproduce correctly;
5) At present, FM screening images still have certain difficulties in proofing, copying, printing, etc., which will seriously hinder the film combination;
6) At present, the difficulty of imaging FM screens on printing plates is relatively large, and it is also unstable. As long as any changes occur in the traditional plate making process, it will affect the final FM screening and reduce the reliability of the screening.
The above problems can be said to be a major aspect affecting the popularity of FM screening technology. In addition, since the AM screening technology has been continuously improved and improved over 100 years, it is now familiar and has its own advantages. With the direct engraving of computers and the flexible and stable digital proofing technology gradually promoted in the printing system, it will inevitably greatly reduce the variable factors affecting FM screening, and make it embark on a wide and practical road. This technology is now used in the fields of sheetfed and rotary, packaging, and magazine printing. The FM network is just like other printing technology innovations. To popularize and apply, it is necessary to continuously improve and improve the software and hardware technologies, and must have corresponding training, testing and other supporting facilities, and establish a sound production control system. One thing is certain, FM screening is not likely to completely replace AM screening in the current and future years. A new technology can replace traditional technology unless it has absolute advantages in all aspects. FM screening technology has not yet reached this level.