How video projectors work!

Subject:	How video projectors work!
Date:	Sat, 5 Dec 2009 08:54:51 -0800 (PST)

Welcome, class, to Projector Anatomy 201. To get the most out of this
short course, you should be familiar with the basic facts about
projectors=96=96what they do and where you can use them. If you=92re not,
please attend the introductory Projectors 101 class down the hallway.

You=92re going to learn here today how a projector works. We=92ll be
covering only the digital genus. The analog =93CRT=94 variety,
distinguished by its typically larger size and darker disposition,
will be discussed in a later course.

In order to explain the topic adequately, we will be splitting an
archetypal digital projector from stem to sternum (so to speak). We=92ll
talk about each of the major =93organ=94 systems in detail, as we delve
into its inner structure. Those of you who actually own projectors may
find this process a little disturbing. That=92s entirely natural=96=96you=
=92ve
come to think of your projector as a reliable friend, bringing you
laughs, tears, and thrills in equal measure. But it exists only as a
conduit for the moving picture=96=96what you should ultimately care most
about.

You have no doubt noticed that digital projectors come in many shapes
and sizes. New variations appear every year, and on occasion, an
entirely new species is discovered. Most are small and compact, but
some challenge CRT projectors in massiveness. Despite these obvious
outward differences, they all share the same fundamental structure and
work in the same basic way. As we explore the anatomy of this
creature, we will go into more detail, where significant differences
do in fact exist.

KNEE BONE'S CONNECTED...

A projector has only one purpose in life=96=96to expel light focused into
an image onto a diffusely reflective external surface (the screen). In
order to do that, it needs to be fed, at a minimum, a video signal (or
signals) and power. Most of its internal structure is devoted to
processing the video signals and making use of the power to,
primarily, produce light. It takes many =93organs=94 or systems to do
this, and they must be adequately supported and interconnected.

We can see this structure by removing the projector=92s =93skins=94 or
covers. In some species, these are made of metal, typically aluminum
or, where weight is very important, a magnesium alloy. Many species
use plastic, often coated on the inside surface with a copper spray to
suppress radiation of radio frequencies (required by the FCC). The
styling of projector skins has evolved to make them desirable to human
beings, a classic example of selective adaptation. But the covers are
more than cosmetic=96=96they ensure that the cooling system directs air
where it belongs; more about that later. Inside the skins, there is a
metal frame that mechanically supports the major components like a
skeleton. Between the components, a multitude of harnesses connect
them together, spreading like arteries and veins in a biological
organism.

Digital projectors typically contain very little empty space; they are
no larger than needed. While this can make them look very complicated,
we can make sense of their structure by examining each of the major
systems in turn. Let=92s begin where the video signals enter the
projector.

A projector has only one purpose in life=96=96to expel light focused into
an image onto a diffusely reflective external surface (the screen). In
order to do that, it needs to be fed, at a minimum, a video signal (or
signals) and power. Most of its internal structure is devoted to
processing the video signals and making use of the power to,
primarily, produce light. It takes many =93organs=94 or systems to do
this, and they must be adequately supported and interconnected.

We can see this structure by removing the projector=92s =93skins=94 or
covers. In some species, these are made of metal, typically aluminum
or, where weight is very important, a magnesium alloy. Many species
use plastic, often coated on the inside surface with a copper spray to
suppress radiation of radio frequencies (required by the FCC). The
styling of projector skins has evolved to make them desirable to human
beings, a classic example of selective adaptation. But the covers are
more than cosmetic=96=96they ensure that the cooling system directs air
where it belongs; more about that later. Inside the skins, there is a
metal frame that mechanically supports the major components like a
skeleton. Between the components, a multitude of harnesses connect
them together, spreading like arteries and veins in a biological
organism.

Digital projectors typically contain very little empty space; they are
no larger than needed. While this can make them look very complicated,
we can make sense of their structure by examining each of the major
systems in turn. Let=92s begin where the video signals enter the
projector.

THE DIGESTIVE TRACT

Projectors can eat a wide variety of video signals, which are input on
a variety of connectors, including RCA jacks, BNCs, 4-pin mini-DIN (S-
video), VGA, and DVI (or, increasingly, HDMI). These are typically
supported on a small circuit board. Some projectors allow plug-in
cards for signals they don=92t handle normally. Harnesses route the
signals to a main circuit board, which is where all the real action
occurs.

The main circuit board, usually the largest in the projector, contains
all the circuitry required to =93digest=94 or process the video signals.
The majority of this processing converts the incoming image=96=96whatever
its original format=96=96into an RGB (red, green, blue) image of the
specific size and bit depth (the number of bits in the digital word
for each pixel) required by the projector=92s spatial light modulators,
otherwise known as panels. (Some species use only a single panel; more
about this later.)

First, however, the projector has to choose from which of its many
video inputs it will accept signals. The projector will route the
selected signals to different circuits, depending on the formats it
detects, or knows it can accept, on those inputs. Since this is a
digital projector, analog signals in particular need to be routed
first to specialized circuits, to convert them to streams of digital
information before they can be further processed. To this end,
composite video and S-video signals are routed to a Video Decoder
circuit, while analog RGB or component (YPbPr) signals are routed to
ADCs (Analog-to-Digital Converters).

My use of the word =93circuit=94 does not necessarily imply separate
electronic components. As technology improves, more and more functions
are being included on fewer and fewer integrated circuits (ICs). Many
projectors are able to use one large IC for most of the so-called
=93front-end=94 processing, with a handful of smaller ICs surrounding it.
This helps to keep the overall cost and size of the projector down.

Two of the most important video processing functions, which are also
the most difficult to do well, are de-interlacing and resizing. De-
interlacing is required for standard- definition video images and for
1080- line high-definition images (1080i). For these formats, odd
lines and even lines are transmitted in successive fields: the
projector has to stitch them together for display on its panel(s). The
process is complicated by whether there is motion between every field
or whether some field pairs come from the same image, the latter being
true of most transfers from film. Faroudja pioneered some of the best
de-interlacing algorithms. These, or similar, are now built into ICs
from several different manufacturers. Hence, many projectors today,
particularly those created from the start for home theatre, can do
this function fairly well.

Resizing is also ubiquitous in video processing ICs, but high-quality
algorithms tend not to be so widespread. The process fits the incoming
X-pixel by Y-line image into an M-pixel by N-line matrix, where M and
N match the specification of the projector=92s panel(s). In most cases,
the goal is to preserve the source image=92s intended aspect ratio
(usually 16:9 or 4:3) while showing all of its pixels, but this can be
modified by user preferences. Without resizing, we would see only part
of the image or it would occupy only a small portion of the panel(s).
I referred earlier to =93front-end=94 video processing, so named for the
processing=92s position in the projector=92s =93digestive tract.=94 =93Back=
-end=94
processing also occurs, whereby the front-end processed digital image
data is transferred to the projector=92s panels. Sometimes the circuitry
for this is located on the main board, sometimes on a separate board
generically known as a =93panel driver.=94 The specifics of back-end
processing depend on the panel technology in use. T
here are three common types today: Liquid-Crystal Display (LCD),
Liquid-Crystal on Silicon (LCoS), and Digital Micromirror Device
(DMD). The first two require that the digital RGB data for each pixel
be converted to analog voltages, in order to modulate the liquid-
crystal material in the panels, a requirement that might seem ironic
for a =93digital=94 display.

DMDs, being true digital devices, don=92t require this. However, for
that same reason the back-end processing for them is much more
complicated. A DMD pixel can only be off (fully-black) or on (fully-
white). Intermediate grey levels are achieved by varying the ratio of
on-to-off time over the video frame. This requires that the image data
for a frame be sent to the DMD in multiple sequences, at a rate of
several thousand updates per pixel per second!

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