Chapter 21: Audio/Visual Devices^563
information is combined to make a full color image. The drawbacks of this
method are that in addition to the time it takes to make three passes, the image
quality can suffer from tiny inaccuracies in the alignment of the three sets of
data that were combined to create the composite image. A single-pass scanner
collects all of the color data in one pass. The result is usually a faster scan with
less potential for image distortion than a multipass scan.Interface
Scanners, like most external peripheral devices, attach to the PC through one of its avail-
able ports. The most commonly used connector is the parallel port, but several newer ver-
sions are now available with a USB interface as well. Higher-end scanners connect to the
PC through the SCSI interface.
SCSI SCSI (Small Computer System Interface) scanners work with either
a standard SCSI interface or with their own proprietary adapter card. SCSI
scanners are often faster than their parallel counterparts but can cost more
when the price of the SCSI adapter is included. Because a SCSI adapter is
required, installing a SCSI scanner can be more difficult than other types.
Parallel These scanners connect to the PC’s parallel port with a standard
DB-25 cable. Most parallel scanners include a pass-through connector to allow
a printer to share the same port. An advantage of parallel scanners is that they
do not involve the additional expense and trouble of a SCSI adapter, but there
is usually a trade-off in speed. In addition, some printers and other parallel
devices like Zip drives can have problems with a scanner’s pass-through port.
USB and IEEE 1394 (FireWire) These Plug-and-Play scanners eliminate most of
the problems of the SCSI and parallel port scanners. Their speeds are comparable
to SCSI scanners, and they have a lower price and a simpler installation. USB and
IEEE 1394 scanners will only work on systems with operating systems that support
these interfaces. All versions of Windows 98/2000 support USB and IEEE 1394, but
some USB scanners will not work with Windows 95, no matter what is tried.Delivery Method
A scanner uses a variety of delivery methods, which means the way it captures the image
of a document, that range from partial page, single-sheet, or automatic sheet feeders that
can scan a multi-page document. The delivery methods used are:
Drum scanners PMT scanners in which the original document to be scanned
must be mounted to a transparent cylindrical drum to capture its image.
Handheld scanners Popularized by Logitech in the early 1990s, these must
be moved across the surface of the original by hand. Because they are often
narrower than a typical page, more than one scan is usually needed to capture
a full-page image. Usually some image manipulation is required to stitch the
images together using software.