DCS stands for Desktop Color Separation. It is a file format that is based on the EPS file format. In fact, you could regard DCS files as a collection of EPS files.
DCS files are mainly used to exchange bitmap images between prepress applications. Occasionally DCS files are also used for vector data or text. The main advantage of DCS over its parent EPS file format is that it adds a kind of OPI-functionality to the file format. Since DCS files contain separate EPS-files for each plate, an application can generate and print colour separations faster when the DCS format is used. This was a valid approach when when Macs, PCs and software were not as powerful as they are today and when all output was done from the lay-out application. In today’s world with the increased popularity of in-rip separations as well as improved support of EPS-files within applications like QuarkXPress, DCS can be a very inefficient file format.
As already noted, DCS-files are actually EPS-files that have to follow the Adobe specifications (Appendices G and H of the PostScript Language Reference Manual, 2nd ed.). The only differences are some changes in the header comment area as well as additional comment lines in the main section of the file that describe the separation data. DCS-files contain a preview image, just like EPS-files.
There are two different versions of the DCS file format: version 1 and the more recent version 2.0. Both formats are covered in more detail below.
DCS 1 was developed by Quark to add a file format that could easily and efficiently be separated to their main application, QuarkXPress. This file format is usually referred to as DCS.
A DCS 1 file is composed of 5 separate files. Below you see such a file: the main file has the extension .eps while the 4 other files have an extension that marks the color data they contain. The file size shows that the main file does not contain any actual image data but only a preview image and pointers to the other 4 corresponding highres files.
Because the main file has lines in it that refer to the other files, you cannot simply rename DCS-files in the Macintosh Finder or Windows Explorer. If you want to change the name of a DCS-file, it is best to open it in Photoshop and use a SAVE AS to save the file using a different name.
The image data in the four CMYK-files can be compressed using JPEG compression. This has often posed problems with OPI-systems and older RIPs sometimes also choke on the decompression.
Development on the DCS-2 format started in 1993 and it became available in 1995 or so. These are 2 new features in DCS 2.0:
- The option to choose a multiple- or single-file version. DCS originally required that the separation files be separate. With DCS 2.0, these files may now be combined into one. Please note that this does not turn DCS-2 files into true composite files, just look at single file DCS-2 files as a collection of separate files that are glued together to form one big file.
- The ability to specify additional plate colors. DCS 2.0 can point to spot color plates in addition to the standard cyan, magenta, yellow, and black. This ability makes DCS-2 the ideal file format for hexachrome images. These contain 6 colors: cyan, magenta, yellow and black as well as orange and green.
Early design applications did not support DCS-2 files. This includes versions of QuarkXPress prior to 3.32 and PageMaker 6.5. Because DCS-files are a kind of EPS-files, they can be imported in these applications but the output on film or plate is incorrect.
DCS must die
DCS made a lot of sense 10 years ago but in todays world it has become a real nuisance. The main problem is that popular applications like QuarkXPress and InDesign (before the CS release if I am correct) do not properly support DCS-files when they print composite PostScript files (files that are not yet separated, with the separation option switched ‘off’ in the PRINT-menu). Instead of reading the high-res data in the DCS-file, they only include the lowres screen preview in their composite printfiles. If you fail to notice this, the output will contain ugly 72-dpi images. Some more recent lay-out applications can merge DCS-data when generating their output file.
Although there are extensions on the market to solve this problem (SmartXT for XPress) and most OPI-servers can merge the highres data when OPI is used, DCS is simply a nuisance and deserves to disappear, at least for pure CMYK images.
If you need more than 4 colors in an image (e.g. hexachrome) or you wish to process copydot files, then DCS is still a valid file format. Using native Photoshop files is probably a better alternative if your design program supports this. For simple CMYK images DCS should never be used anymore.
Name: EPS DCS
Release date: ?
Type of data: bitmap
Number of colors: ?
Color spaces in practice: DCS-1: CMYK or 1-bit copydot, DCS-2: CMYK, hexachrome or copydot
Compression algorithms: JPEG
Ideal use: exchange of images between prepress applications
Extension on PC-platform: .DCS, .C, .M, .Y and .K for the separate colour files.
Macintosh file type: ?
Special features: can be troublesome in composite workflows