|Red Hat Enterprise Linux 4: Introduction to System Administration|
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Before the advent of laser and inkjet technology, impact printers could only print standard, justified text with no variation in letter size or font style. Today, printers are able to process complex documents with embedded images, charts, and tables in multiple frames and in several languages, all on one page. Such complexity must adhere to some format conventions. This is what spurred the development of the page description language (or PDL) — a specialized document formatting language specially made for computer communication with printers.
Over the years, printer manufacturers have developed their own proprietary languages to describe document formats. However, such proprietary languages applied only to the printers that the manufacturers created themselves. If, for example, you were to send a print-ready file using a proprietary PDL to a professional press, there was no guarantee that your file would be compatible with the printer's machines. The issue of portability came into question.
Xerox® developed the Interpress™ protocol for their line of printers, but full adoption of the language by the rest of the printing industry was never realized. Two original developers of Interpress left Xerox and formed Adobe®, a software company catering mostly to electronic graphics and document professionals. At Adobe, they developed a widely-adopted PDL called PostScript™, which uses a markup language to describe text formatting and image information that could be processed by printers. At the same time, the Hewlett-Packard® Company developed the Printer Control Language™ (or PCL) for use in their ubiquitous laser and inkjet printer lines. PostScript and PCL are now widely adopted PDLs and are supported by most printer manufacturers.
PDLs work on the same principle as computer programming languages. When a document is ready for printing, the PC or workstation takes the images, typographical information, and document layout, and uses them as objects that form instructions for the printer to process. The printer then translates those objects into rasters, a series of scanned lines that form an image of the document (called Raster Image Processing or RIP), and prints the output onto the page as one image, complete with text and any graphics included. This process makes printed documents more consistent, resulting in little or no variation when printing the same document on different model printers. PDLs are designed to be portable to any format, and scalable to fit different paper sizes.
Choosing the right printer is a matter of determining what standards the various departments in your organization have adopted for their needs. Most departments use word processing and other productivity software that use the PostScript language for outputting to printers. However, if your graphics department requires PCL or some proprietary form of printing, you must take that into consideration as well.