Subject: comp.periphs.printers - FAQ - Part 2 of 11
Version: 3.09
Last-Modified: 1997/01/05
Summary: This posting contains a list of Frequently Asked
         Questions (and their answers) about printers and
         should be read by anyone wishing to post to the
         comp.periphs.printers newsgroup.
Posting-Frequency: monthly
Date: Sun, 5 Jan 1997 05:25:00 GMT
Archive-Name: comp-periphs-printers-faq

Subject: 02 Printer Classification

_______________

Subject: 02.01 Dot-Matrix Printers (Impact Printers)

A dot-matrix printer produces output by having a printhead with
a series of metal pins (typically 9, 24 or 48) which slam into
an ink-impregnated ribbon or a carbon ribbon as the printhead
is dragged back and forth across the paper to form images. The
pins are arranged in one or more vertical rows.

Another, less common, technology called "Comb Matrix" or
"Shuttle Matrix" uses a horizontal row of pins (optimally one
pin for each horizontal position) so that a single row of dots
may be printed very efficiently. In cases where there are not as
many pins as there are horizontal positions, the comb moves back
and forth slightly (hence "Shuttle") in order to print all the
necessary dots. With this technique the paper can move almost
continuously, and so it's heavily used in high speed matrix
line-printers since it is much faster than single character
printing.

Impact dot-matrix printers are very versatile in that the
matrix of pins can be manipulated to produce different character
fonts, and even graphics.

Since impact dot-matrix printers create an image through
hammering an inked ribbon, they can be used to print copies
together with the original. These printers are often used with
continuous paper, but most models can handle single sheets and
labels as well. 

Due to the printing method, dot-matrix printers are louder than
other printers (even in the so-called 'quiet mode').

The printing performance is normally given in cps (characters
per second), often for different fonts (Draft/LQ) and for
different cpi (characters per inch) values (where applicable).

_______________

Subject: 02.02 Daisywheel printers and Typewriters

A daisywheel printer produces output using a wheel which has
stamped images of each letter and symbol. As the printhead
moves back and forth across the paper, an electromagnetic
hammer is fired as the wheel is spun to position each letter
as it's needed.

Daisywheel printers were very popular because they were able to
produce much better output than other early printers such as 9-
pin dot-matrix printers. Their disadvantages include slow speed,
a character set limited to what's on the printwheel, and an
inability to do graphics.

Like daisywheel printers, typewriters were (and sometimes) are
popular since they were already established in the offices when
electronic data processing began. By adding an interface to the
typewriter you can connect it to your PC giving you a personal
printer as well as a typewriter for filling forms etc.

Daisywheel printers and typewriters are impact printers, so they
can print copies together with the original. Daisywheel printers
print on continuous paper as well as on single sheets or labels,
typewriters normally print on single sheets and labels.

Daisywheel printers and typewriters are as loud as dot-matrix
printers.

The printing performance is normally given in cps (characters
per second).

_______________

Subject: 02.03 InkJet Printers

An inkjet printer is very similar to a dot-matrix printer, in
that it produces output by putting a set of dots onto the
paper. Inkjets, however, spray drops of ink onto the page
directly. They can achieve much denser patterns of dots than
dot-matrix printers, allowing for much better output.

Newer inkjet printers, instead of projecting the ink
electrostatically or with very tiny pumps, vaporize the ink
in a heated capillary with the pressure of the expanding vapor
"squirting" the liquid ink onto the page. For our purposes, they
are otherwise just like original inkjets.

Inkjet printers are non-impact printers, so you have to print
copies separately. Inkjets print onto cut sheet paper, like a
photocopier. Some models need special quality paper for best output,
but for most printers standard paper quality should suffice.
Some printers may also handle continuous paper, and labels may
be printed if they accept ink well.

Inkjet printers are relatively quiet printers.

Inkjet printers tend to be faster than impact dot-matrix
printers. The printing performance is therefore given either in
cps (characters per second), sometimes for different font
qualities and/or cpi (characters per inch) values, or in
ppm (pages per minute).

Although all inkjet inks that I've seen are theoretically water-
soluble, they vary greatly in their response to getting wet.
This may matter if the printout will be used for presentations,
business purposes, or in humid/wet environments (e.g., some inks
will come off when touched by a moist finger, even weeks after
printing).

Inks that I've been told smear easily and significantly when a
drop of water is dropped on them include: all Canon inkjet
inks, DEC 520ic (black), and the TI Micromarc inks. The HP
Deskjet ink (black) is as nearly water-resistant as laser
printing. The Epson inkjet inks (black and color) seem to be
mostly water resistant: characters diffuse but don't tend
to smear when wet.

Dick Miller points out: "Although you don't mention it, DEC 520
and TI Micromarc ink[jet]s are both from Olivetti in Italy. So
are many others, which also share the same Olivetti printer
mechanisms in most cases: DEC 100- and 500-series printers,
Citizen ProJet printers, Tandy/Radio Shack JP1000, etc. 
(Identification tip: these usually include an Olivetti 
"JP350.DRV" Windows printer driver.)

Color inkjet printouts are not as long-lasting as photographs.
Some can withstand light, but are undone by humidity.  Others fade
quickly in bright light. Laminating can extend the life of the
output. Generally speaking, modern color ink jet printers haven't
been around long enough to test the longevity of the newest inks.

_______________

Subject: 02.04 Laser and LED Printers

(See also the description at Environmental Laser's web site:
<http://www.toners.com/how.html>)
 
Laser and LED printers are non-impact printers, so you have to
print copies separately. Typically these printers use single
sheet paper. Labels may be printed as well if they withstand the
heating process during burn-in of the toner. Some exotic models
support continuous forms, especially highest-speed models with a
feed of 200 ppm and more. These printers tend to be used in
mainframe environments and for printing direct mail solicitations.

The following is a basic description of what happens in
a typical "laser" printer. There's some simplification 
involved because we don't want to lose everyone. I think
this gets the idea across, if you disagree, please let me
know and we'll take another pass at it.

The drum inside the print cartridge is first charged
electrostatically. Then portions of the drum are discharged
by means of a laser or an LED line. These discharged dots
accept the toner which has a charge similar to that of the
drum. The discharged dots are the mirror image of the positions
where "ink" is to appear on the final paper output.

The paper passes a corona wire (or roller) which gives the
paper an electrical charge opposite of the charge on the
dots on the drum. Now the paper passes the drum where the
toner is deposited by the difference in charges.  Finally,
the paper passes a heating element that fuses the toner onto
the paper.

Most laser printers these days don't have a corona wire -
they actually charge the paper with a roller.

Laser and LED printers produce very sharp, crisp output,
typically of 300 dpi (dots per inch) or more, with some
printers having as much as 1200 dpi. Some printers also use
techniques such as "Resolution Enhancement" to manipulate dot
placement for reducing aliasing (the jaggies).

Laser and LED printers are relatively quiet printers.

Some early models produce surprisingly high levels of ozone.
Ozone is an oxygen module that has three oxygen atoms instead
of the normal two. During the print process ozone is produced
as a by-product of the corona energizing process. Some people
react to ozone with eye, nose, throat and lung irritation or
with headaches. So it's essential to place the printer in a
well-ventilated space, and to change the ozone filter of the
printer regularly if the printer has one.

Since most modern lasers don't have a corona wire, they don't
produce ozone.

The performance of laser and LED printers is measured in ppm
(pages per minute) in copy mode (this means that the page image
is already in the printers memory and is only copied as fast as
possible to the paper). Unfortunately the ppm value is no
direct measurement for the printer's performance.

On the subject of reliability of lasers versus LEDs, Peter
Lorraine reports: "I asked the same question you have about
reliability. The answer from the salesperson was the same as
that from one of the computer support people at work. According
to them, the optics in a laser system can get out of alignment
over time. In a laser system, a single light source is scanned
across the image drum with a system of lenses and mirrors. In an
LED system, an array of LEDs in direct proximity to the drum is
used. The head in the LED printer comes with an extended
warranty beyond the other components."

David Luke, adds: "Theoretically this is true, but in reality,
the difference is negligible. Over 96% of all LaserJets ever
sold (clear back to the Classic from 1984) are still in use."

| From: David Byram-Wigfield, the author of 'PracticalPostScript'
| ISBN 0-9525308-0-5
|
| A great laser and photocopier mystery is the exact 
| composition of toner and how many different flavours there 
| are. Monotoners are pre-magnetised whilst duotoners rely 
| on a mixing with magnetised iron filings (or is it the other 
| way round?). In any event, if you refill with the wrong kind 
| you either get unmagnetised toner blown all over the interior 
| of the laser or pre-magetised toner forever clogging up the 
| iron filing core. Testing an unlabelled bottle of toner with a 
| magnet *may* spot the difference but I wouldn't rely on it.
|| Some toners have finer particles than others and Hewlett-
| Packard always refuse to reveal the recipe of their finest micro-
| toner brew; an HP refill is never quite as good as an Authorised 
| Version.
|
| If you are buying a new laser or photocopier, always rub the two 
| halves of a sample printed page together. On some models you 
| will be surprised how easily the toner rubs off onto the 
| opposite page. I suspect much depends on an adequate fuser 
| temperature.

_______________

Subject: 02.05 Color Printers

A color printer uses any of several methods to produce multi-
colored output. Regardless of the method used to produce the
color, there are first, three different methods of specifying
colors:

1) RGB: the standard method used with display devices (terminals
   and TVs) is RGB color space (RGB stands for red-green-blue).
   It's an additive process to mix colors, you get white by mixing
   all three colors with the same intensity.

2) CMYK: a method used especially in commercial printing (CMYK
   stands for cyan-magenta-yellow-black). It's a subtractive
   process to mix colors, you get black by either mixing all
   three colors with the same intensity, or having an additional
   black color added in for richer, darker blacks.

3) Dithering: A method whereby the colors are not really mixed,
   but by placing dots of different colors in different numbers
   in a confined place it's possible to make an image look like
   more it has more than four colors in it. This is somewhat
   comparable to the method of grayscaling on a pure b/w display
   by setting more or less pixels to black.

There are color dot-matrix printers using a colored ribbon (i.e.
one with colors on it besides black). Inkjet printers have color
ink cartridges, some produce black by mixing the colors, others
have a separate black ink cartridge.

Thermal transfer printers use a film with four colors of wax on
it. The printer moves the paper through four times, once for each
color. As each color is deposited on the paper, the full color
image comes through. Others use a wax, crayon-like stick which
is used to fill a tray in the printer.

Lately, there have been a couple of manufacturers introducing
color laser printers which have four separate bins of toner,
one for each of the basic colors.

Color laser printers generally use four different toner colors:
the three subtractive primaries (Cyan, Magenta and Yellow), plus
Black. Note that toners printed on a white piece of paper reduce
(or subtract colors from) the reflection of a white light shone
on the page, in order to form the resulting color. By contrast,
light emitting devices such as CRT's add emitted colors together
to form the resulting color; this is why CRT's work with the
additive primaries Red, Green and Blue (RGB), whereas printers
work with the subtractive primaries (CMY), and somewhere along
the way there is a conversion between the two.

Since the visible spectrum is continuous, you can only approximate
visible colors by mixing the subtractive primaries; and mixing Cyan,
Magenta and Yellow toner all together actually produces a sort
of off-looking gray, rather than a solid black. Adding the Black
toner pass helps to approximate more colors, and gives the nice
crisp looking black that laser printer users are used to.

All color laser printers generate a range of colors by "mixing"
their toners together to form the color on the paper, but they
do so in one of two fundamentally different ways:

1) Continuous tone printers can vary the amount of toner put down
   for each color, in each pixel. They are generally very
   expensive, and provide high quality photo-like reproductions.
   For a 32-bit color continuous tone printer, each pixel can
   have 4,294,967,296 different toner combinations, although not
   all of these necessarily make sense to print; for example, 
   solid Black mixed with any combination of C, M and Y will look
   the same, merely wasting the C, M and/or Y toner.
   Furthermore, the translation from 24 bit RGB to 32 bit CMYK is
   generally hidden from the user, so the user actually gets
   access to 16,777,216 of these colors. 

   Even then, some of those may map onto the same CMYK value
   depending on the color space mapping, which is related to the
   linearity of the printing process, toners, etc., etc. Also, not
   all the colors that can be represented on screen in RGB can be
   generated by the mix of four printer toners; some colors will
   lie outside the printer's "gamut" and can only be approximated
   by substituting a printable color which is perceptually close.
   Similarly, the printer will be able to print colors which you
   can never reproduce on your RGB screen.
 
2) Bi-level printers cannot vary the amount of toner put down for
   each color within each pixel. They are considerably cheaper,
   and their output is a good deal lower quality given equivalent
   resolutions, particularly for reproducing real-world images.
   Each of the four colors are either on or off in each pixel, so
   each pixel can have only 16 different toner combinations. 
   Furthermore, black mixed with anything will look black, so 
   eight of those combinations would look the same, meaning that
   each pixel can only really take on nine different colors. 

   Colors which cannot be represented directly are simulated via
   some form of dithering, so that the eye perceives the 
   intermediate colors in a similar manner to halftone 
   photographs in newspapers. Note that these printers cannot 
   print anywhere near the 16,777,216 colors that can be 
   represented by 24-bit RGB; they can print only nine different
   color values, and rely on dithering to represent the 24 bits
   of color information. Dithering effectively trades off
   resolution to represent these intermediate colors, compared
   to continuous tone. Bi-level printers can still have 24-bit
   inputs; this doesn't imply that they can mix varying amounts
   of each toner to produce a smooth range of colors for each
   pixel.

As Tapani Tarvainen points out, there are some new printers which
make the preceding kind of misleading:

  The HP DeskJet 850C and 855C could actually be called "quad-
  level" or something, in that they can squirt three different
  sizes of inkdrops on each pixel: nowhere near continuous tone 
  but much more than bi-level: with three colors each pixel can 
  be one of 64 colors, and up to 256 when black is mixed in (as 
  it can be, however most of the 256 combinations aren't really 
  useful; according to a HPer the number of useful ones varies 
  between 64 and 120 or so).

  So the distinction is no longer as clear-cut as it used to be: 
  color printers must now be measured by both resolution and 
  color depth: in addition to dots-per-inch we need something 
  like colors per dot.

_______________

Subject: 02.06 Other Printer Types

Another printer technology is that of line printers, mostly
seen in mainframe environments. They print by means of a
rotating cylinder or chain containing all printable characters.

In the case of the rotating cylinder, all columns contain a
complete character set, the cylinder rotates in the paper
transport direction and the paper is slammed against the ribbon
and the cylinder when the character to print is at the right
position. One revolution of the cylinder prints the complete
line.

In the case of the rotation chain, the chain contains the
character set repeated several times. The printer designer
chooses the number of times each character is repeated based
on how frequently it turns up in the text. For American English,
this is the old ETAON... sequence. The idea is if the more
frequent characters show up more often, the overall printing
will be faster. The chain rotates perpendicularly to the paper
transport direction and the paper is slammed against the ribbon
and the chain when the character to print is at the right position.

A variant of the chain printer is the "Belt Printer". The quality
is roughly equivalent to chain printers at lower cost, lower
power consumption and less noise. The standard belt or train
printer uses high-quality type set in a heavy chain (it IS heavy)
and support mechanism. In the IBM 3203 and 1403 printers each "slug"
was three characters in one position on the train. The use of the
word train is deliberate, as the characters are three to a
"carriage" which in turn follow after each other on a track.

In a belt printer the characters are mounted individually on
"fingers" (not all characters are repeated the same number of times. In
difference to cylinder and chain printers the belt is slammed
against the ribbon against the paper.

The printing performance is normally given in lps (lines per
second) or even in pps (pages per second). Printer types
ordered in increasing speed: belt printers, chain printers,
rotating cylinder printers (up to 70 lps).

Also there are thermal matrix printers printing characters and
graphics (e.g. barcodes) by heating the paper positions (dots)
that should become black.

There are a few other "exotic" technologies such as the thermal
aluminum coated paper used by Sinclair and also Spark discharge,
but they are VERY rare and largely obsolete.

_______________

=====
End of Part 2 of 11
