Specifications

Rasterizing
The result of the formatting process is a detailed set of commands defining the exact placement of
every character and graphic on each page of the document. In the final stage of the data interpreta-
tion process, the controller processes the formatting commands to produce the pattern of tiny dots
that will be applied to the page. This process is called rasterization. The array of dots typically is stored
in a page buffer while it awaits the actual printing process.
The efficiency of this buffering process depends on the amount of memory in the printer and the res-
olution of the print job. On a monochrome printer, each dot requires 1 bit of memory, so a letter-size
page at 300dpi requires 1,051,875 bytes of memory {[(8 1/2×11)×300
2
]/8}, or just over 1MB. At 600dpi,
the memory requirement jumps to 4,207,500 bytes—more than 4MB. Some printers have sufficient
memory to buffer an entire page while the formatting of the next page proceeds. Others might lack
enough memory to store even one full page and use what are called band buffers instead.
Printers that use band buffers divide a page into several horizontal strips, or bands. The controller ras-
terizes one band’s worth of data at a time and sends it to the print engine, clearing the buffer for the
next band. This way, the printer can process a page gradually, with the entire array coming together
only on the photosensitive drum in the print engine. The band buffer method is cheaper than a full-
page buffer because it uses less memory, but it is also slower and more prone to errors. In recent years,
the price of memory has dropped so much that band buffers are rarely used in laser printers.
Band buffers are used primarily by inkjet printers, which convert each line of text or graphics into a
band.
Tip
Some printer drivers enable you to control whether graphics are sent to the printer in vector or raster form. In general, vec-
tor graphics provide better speed, but if you experience problems with the placement of the graphics on the page, you
can switch to the raster option. Most printer drivers that offer this feature place the control on the Graphics page of the
printer’s Properties dialog box. However, some drivers might place the control elsewhere or not provide it at all.
A common reason for switching to raster graphics is when a multilayer graphic doesn’t print properly. This can be a prob-
lem with PCL 5 laser printers and some presentation programs, such as Microsoft PowerPoint or Lotus Freelance Graphics.
Laser Scanning
After the rasterized image of a page is created by the controller and stored in memory, the processing
of that page passes to the print engine, the physical part of the printing process. Print engine is a col-
lective term used to refer to the actual imaging technology in the printer, including the laser scanning
assembly, the photoreceptor, the toner container, the developer unit, the corotrons, the discharge
lamp, the fuser, and the paper transport mechanisms. These components often are treated as a collec-
tive unit because the print engine is essentially the same hardware that is used in copy machines.
Most printer manufacturers build their products around a print engine they obtain from another
manufacturer, such as Canon. A PC printer differs from a copy machine primarily in its data-acquisi-
tion and processing procedures. A copier has a built-in scanner, whereas a printer receives and
processes digital data from the PC. After the raster image reaches the print engine, however, the pro-
cedure that produces the actual document is very similar.
Figure 2 illustrates the laser writing process.