In some ways, digital presses are similar to desktop printers, but they are
physically larger and output documents at a higher speed than desktop printers.
The smaller desktop printers could also be considered digital presses because
some of the same technologies are used for both. Digital presses utilize an
image carrier that can be reimaged with every revolution of the press, if necessary.
Reimaging makes it is possible to print a different page of content on every
sheet of paper that passes through the press. Printing based on this technology
is known as "variable data printing" and is a unique feature of true
digital printing systems.
Nearly all digital presses use electrophotography
for imaging and there are a few models that use ink-jet
technology. Many of the presses can print on both sides of the paper in one
pass (duplex printing).
The output speed of a digital press is often determined by the type of application
that is printed. A job that has no variable data may print at the full speed
rating of the press because the RIP (Raster Image Processor, which converts
the elements on the document into a format that can be understood by the press)
processes the digital information only once and the press prints as many copies
as necessary. The press can run at full speed because the digital information
does not have to be reprocessed for every page that is printed.
When some of the elements on a document change for each printed impression,
the RIP must process the digital information for every sheet printed, which
slows the speed of the press. Since printers/RIPs are "page-oriented"
devices, (meaning that a full page is the smallest item that is processed rather
than individual elements on a page), the RIP must process the entire page even
though many of the elements from page to page may be identical.
PPML/VDX (Personalized Print Markup Language/Variable Data Exchange) is a language
developed for variable data applications that allows text and graphics (or objects)
that are common to every page, to be reused without the need for reprocessing.
Only the variable data is processed for each page, which increases the output
speed of the digital press. The PPML/VDX standard allows the RIP to be "object
oriented" rather than "page oriented".
Digital presses are part of the totally electronic workflow, also known as
computer-to-paper technology. The price per copy of an application printed on
a digital press is usually the same regardless if 1 is printed or 1000. The
cost per copy may be higher than the same job printed with conventional processes,
but the press time spent producing the job is less than what it would be if
printed by conventional means. Digital presses allow for more jobs to be printed
within the same amount of time required to produce one job with conventional
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