AM and FM

For printed editions, prepress production is always an important part of the quality of printed products. Both printing and graphic printing must use screening technology. In traditional image color separation screening and platemaking technologies, production units mostly use AM network technology. It has been practiced and improved for over 100 years and is already very mature. However, people have also found that there are some unavoidable defects in the amplitude modulation and screening, which cannot but be a barrier to the quality of printing. Nowadays, a new screening technology, FM screening, has begun to be popular in the printing industry. It can effectively avoid the defects of AM screening.
1. Characteristics and Defects of AM Screening Screen Printing is now mainly performed using halftone screens to achieve continuous tone image effects. This kind of screening is to divide the original image part into a black and white dot in the form of a grid. By changing the size of the dots in the grid, the effects such as shadow and color are reproduced. If we look at these outlets as a whole, the images will be continuously adjusted. This technique is called Amplitude Modulated screening. Prior to the emergence of the color prepress system, all screening methods were basically the same. The distance between the dots in the AM screening is fixed, ie, the frequency of the dots is fixed, and only the dot size is changed. In the AM screening method, the screening system used by the electronic color separation machine is generally an IS (Irrational Tangent Screening) network technology (an irrational tangent network point) and an RT (Rational Tangent Screening) network technology (a rational tangent screen point) developed by the German Hell Corporation. ). Postscript screening technology is used in the color desktop publishing system. Although the AM screening technology is relatively mature, there are some unavoidable defects.
1) Loss of detail Because the distance between the AM screening screens is fixed, the middle blank area will cause the image details to be lost.
2) Grid collisions and screening The generated moiré manuscript mesh shape and the grid shape of the amplitude modulation dot will generate interference phenomena and appear moiré. In addition, due to different colors, the shape of the screen is not the same. When overlapping, there are moire patterns when there are regular lines in the image.
3) Rose spots Because of the different angles of screening with four colors, when the colors are copied, small rose spots appear, such as rose spots appearing in important tonal areas, or due to inaccurate overprinting of certain colors, which may cause rose spots. Interference phenomenon.
4) Visually impinging on the net In the process of tune-up, with the enlargement of the amplitude-increasing screen dots, the dots will eventually contact each other. Then it should be a gradual change that becomes a sudden black tone. Although it is now possible to use chained outlets to avoid problems as much as possible, in general, this situation is still difficult to avoid.
Second, the characteristics of FM screening and superiority FM network screening network, also known as random outlets, the same size of its outlets, the use of network density to show the image level, and outlets randomly distributed, its advantages are:
1) An image-setter that improves the quality of film output with low resolution can also obtain high-quality images using frequency modulation technology, while AM ​​technology requires high-resolution imagesetters. For example, the output of a 1200 dpi imagesetter, the quality of the FM image is significantly better than the 175-dpi image-setter output of the 3000 dpi image quality. The network distribution of FM screening is irregular, as long as the screen points in the FM screening are small enough, the printed products can fully approach the original.
2) The absence of moiré and rose mottle frequency screening will not produce moiré that is easy to appear in color overlay. Because it does not have the problem of screen angle, it is randomly distributed without rules, and there is no fixed distance between the fixed direction and the network point, which can avoid grid conflicts and the moiré brought by screening. In addition, because it does not have halftone screen lines and angles, it has eliminated the structure of the rose spots. FM technology prints are more sophisticated than AM technology prints. No matter how good the AM screening technology is, it can only avoid the big mistakes, but it cannot avoid the rose spots.
3) In the clear and stable amplitude modulation network technology of printed products, the quality factor determines the sharpness, the larger the quality factor, the smaller the output dot area, and the better the clarity. However, it is usually difficult to make the quality factor too large. Therefore, the output image produced by the AM screening technology should be emphasized at a subtle level. The area of ​​the network used for FM screening is particularly small, which is equivalent to the fact that the amplitude modulation technique uses a relatively large quality factor and is therefore clearer. When using FM network technology, you can use less detail to emphasize, even if not used, the effect is quite good. FM network with micro-scale dot size performance, in the single-cassette rotary printing, generally up to 21 ~ 25 microns, the use of ink and color reflectance than 133 ~ 200lpi of the AM screening is better, such as 21 micron FM plus The details represented by the net are equivalent to 400 lpi, which can effectively reduce the color pollution from the paper.
4) The copying effect is good. The copying of the FM network is the best for the middle tone level, and it can eliminate the tone shift of the middle tone, so that the tone transition is natural and harmonious. In addition, its color transition is delicate, which is conducive to ultra-four-color printing, and color separation films can be recorded with lower resolution.
5) Better visual stability When the AM screening is not accurate, the visual mesh structure will change. FM screening will not change whether or not the version is inaccurate. The printers also found that the FM network can better represent the details of the prints when the printing plate is not accurate. In terms of color stability, when the amplitude modulation screen is misaligned or the web offsets slightly, the hue and color will shift, and the FM network will not have this problem.
6) Not restricted by the screen angle In the AM screening and screening technology, various parameters such as the number of meshes, screen angle, and net shape are very important, and the technical personnel must understand. FM has only one parameter of network size, so its control technology is quite easy. FM screening is not performed according to the fixed screening angle and grid. Therefore, the screening screen has no direction and is not limited by the screen angle. The variable distance between the network points is not fixed, and there is no network cable number. Special conditions for creating special reproductions, such as metallic color, jewellery, color, Chinese painting, etc., can also be achieved by printing four or more colors, expanding the range of color reproduction of printed materials, and performing high fidelity copy printing.
7) Avoiding mis-networks Mis-networking is the enemy of the color plate making industry for a long time. There is no rule in the distribution of FM-screened networks, and there is no misplaced network with multiple color plates. Even if textiles in prints or manuscripts are copied, there will not be any mis-networks. In addition, the FM network is insensitive to overprinting errors in the process of printing platemaking, while the traditional AM dots may be misplaced due to overprinting errors.
8) The printing effect is better Firstly, it prints faster on the printing machine without adjustment, the ink drying time is shortened, the working time is reduced correspondingly, the ink and ink balance is easier to maintain, and the thicker ink coverage can obtain a higher dynamic range. Thinner inks are more evenly distributed. In multi-color overprinting, the problem of registration does not affect the color balance, and the color version of the screen plus the overprint of the primary color version does not appear to have a moiré pattern.
Third, the improvement of FM screening technology However, FM screening itself is not perfect, there are also deficiencies, such as:
1) Due to the equal size of the dots, there is a graininess, especially in the high-light part and the tone of about 25% is more obvious;
2) The latitude of the entire production process is smaller than that of the amplitude modulation screening. Therefore, a more detailed process control and monitoring technology than the amplitude modulation screening is required. For example, the latitude of the FM screening deviation is small, and many changes in the film and development processing are made. The result is not significant; operators will have to adopt new levels of accuracy throughout the production process;
3) Because the effect of increasing the ink film thickness on the size of the FM network is not as significant as that of the AM network, it is difficult to improve the quality of the image with poor color separation by adjusting the ink on the printing press.
4) The smallest micro-dot size is too small to allow many printers to reproduce correctly;
5) At present, FM screening images still have some difficulties in proofing, copying, printing, etc., which will seriously hinder the film combination;
6) At present, the difficulty of image formation on the printing plate by the FM network is still relatively large, and it is also unstable. As long as any change occurs in the traditional platemaking process, it will affect the final tuning of FM screening and reduce the reliability of screening.
The above problems can be regarded as one of the major aspects affecting the popularity of FM screening technology. In addition, the AM screening technology has been continuously improved for more than 100 years and is now familiar to people and has its own advantages. As computer direct plate making and flexible and stable digital proofing technology are gradually promoted in the printing system, it will inevitably greatly reduce the variable factors that affect FM screening and make it embark on an extensive and practical road. This technology is used in sheetfed and rotating, packaging, magazine printing and other fields. As with other printing technology innovations, FM networks must constantly improve and improve software and hardware technologies, as well as corresponding training, testing, and other supporting facilities. We must also establish a sound production control system. To be sure, FM screening cannot completely replace AM screening at present and in the coming years. A new technology can only replace traditional technologies unless it has absolute advantages in all aspects. FM screening technology has not yet reached this level.

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