Tektronix PHASER 380, Service Manual

Page: 128 / 346

5-26
Phaser 380 Color Printer
5
Theory of Operation
Printing
How the printhead deposits the image on the drum depends on the size of the
image. If the image is narrower than 20.12 cm (7.92 inches), then only a single
scan or pass of the printhead is required. Wider images require two scans of the
printhead; the two halves are interleaved, sometimes referred to as stitching,
where they join.
Single scan image deposition. To deposit the latent image on the drum, the printer
performs a “four-jet interlace.” As the drum reaches the correct speed of 112.8
rpm, the ink-jets begin to fire to deposit the image on the oiled portion of the
drum. As the jets fire, the printhead travels in the x-direction.
For 300 dpi printing, each jet lays down 28 pixel columns. Each jet lays down
one pixel column for each drum rotation (28 revolutions total). Each jet travels
horizontally the distance of 117 pixels to lay down its 28 pixel columns within
that 117 pixel-wide field. The 117 pixel-wide field of each jet overlaps the
117 pixel-wide field of 3 other jets; this is where interleaving occurs. To lay
down its 28 pixel columns, each jet follows this sequence:
1. Print 1st column, step four columns to the right, print 2nd column,
step four columns to the right. Repeat until 7 columns are laid down.
2. Step 2 additional columns to the right.
3.
Print the 8th column, step 4 columns to the right, print 9th column,
step 4 columns to the right. Repeat until 7 columns are laid down
(total of 14 so far).
4. Step 1 additional column to the right.
5.
Print the 15th column, step 4 columns to the right, print the 16th
column, step 4 columns to the right. Repeat until another 7 columns
are laid down (21 total so far).
6. Step 2 additional columns to the right.
7.
Print the 22nd column, step 4 columns to the right, print the 23rd
column, step 4 columns to the right. Repeat until the last 7 columns
are laid down for a total of 28 pixel columns.
The intermediate 2-step, 1-step and 2-step movement between the 7th and 8th
pixel columns, the 14th and 15th columns, and the 21st and 22nd columns,
respectively, allow the 28 pixels columns of each jet to properly interlace with
the 28 pixel columns of the other jets it is combined with. A total of
2,432 columns, each 3,134 dots tall, are laid down (for a full-image, A-size print).
The advantage of this print method is that variability between jets is “averaged
out” by being interlaced with three other jets. As shown in the illustration, of the
28 pixel columns printed by any single jet, only two of its pixel columns are
actually ever adjacent. Usually they are separated by three other pixel columns
produced by other jets, hence the name four-jet interlace. Because of the 28 pixel
spacing between jets and the 112 pixel-wide field that each jet travels, the pixel
columns of any one jet actually interlaces with the pixel columns of six other jets,
although no more than four at any one time.