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Optional
Audio Introduction
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Fields
and Frames When you get right down to it,
both motion pictures and TV are based solidly on an illusion. Strictly
speaking, there is no "motion" in TV or motion picture images. Interestingly, the
foundation for motion pictures was established in 1877 with a $25,000 bet.
For decades an argument had raged over whether a race horse ever had all four
hooves off the ground at the same time. (Some people must have a lot of time
on their hands to sit and debate things like this!) In an effort to
settle the debate once and for all, an experiment was set up in which a rapid
sequence of photos was taken of a running horse. And, yes, as discussed But this experiment
established something even more important. It was discovered that if this
sequence of still pictures was presented at a rate of about 16 or more per
second, these individual pictures would blend together, giving the impression
of a continuous, uninterrupted image. In this case, of course, the individual
pictures varied slightly to reflect changes over time, and the illusion of
motion was created when the pictures were presented in an uninterrupted
sequence. In the illustration
on the right you can more clearly see how a sequence of still images can
create an illusion of movement. A more primitive
version of this can be seen in the "moving" lights of a theater
marquee or "moving" arrow of a neon sign suggesting that you come
in and buy something. Although early silent films used a
basic frame
(or picture) rate of 16 and 18 per-second, when sound was introduced,
this rate was increased to 24 per-second. This was primarily necessary to
meet the quality needs of the sound track. (Actually, to reduce flicker
today's motion picture projectors use a two-bladed shutter that projects each
frame twice, giving an effective rate of 48 frames per-second.) Unlike broadcast
television that has frame rates of 25 and 30 per second depending on the
country, film has for decades maintained a worldwide, 24-frame per-second
sound standard. The NTSC (National Television System
Committee) system of television used in the United States, Canada, Japan,
Mexico, and a few other countries, reproduces pictures (frames) at a rate of
approximately 30 per-second. Of course, this
presents a bit of a problem in converting film to TV (mathematically,
24 doesn't go into 30 very well), but we'll worry about that later. A motion picture camera records a
sequence of completely formed pictures on each frame of film, just like the
still pictures on a roll of film in your 35mm camera. The motion picture
camera just takes the individual pictures at a rate of 24 per-second. Things are different
in TV. In a video camera each frame is comprised of hundreds of horizontal
lines. Along each of these lines there are thousands of points of brightness
and color information. This information is electronically discerned by in the
TV camera (and then later reproduced on a TV display) in a left-to-right,
top-to-bottom, scanning sequence. This sequence is similar to the movement of
your eyes as you read a section of this page. To reduce flicker and brightness
variations during the scanning process, as well as to solve some technical
limitations, it was decided to divide the scanning process into two halves.
The odd-numbered lines are scanned first and then the even-numbered
lines are interleaved in between to create a complete picture. Not
surprisingly, this process is referred to as interleaved or interlaced scanning. Note the extreme
closeup of a section of a TV image shown on the left below. In the
illustration on the right we've colored the odd lines green and the even
lines yellow so you can see how they combine to create a full video picture.
(A color TV picture, which is a bit more complex, will be described later.)
Each of these
half-frame passes (either all of the odd or even-numbered lines, or the green
or the yellow lines in the illustration) is called a field. The completed (two-field) picture
is called a frame,
as we've previously noted. Once a complete
picture (frame) is scanned, the whole process starts over again. The slight
changes between successive pictures are fused together by human perception,
giving the illusion of continuous, uninterrupted motion. Today, rather than using an
interlaced approach to scanning, some video systems (including computer
monitors and some of the new digital television standards) use a progressive or non-interlaced scanning approach, where the
fields (odd and even lines) are combined and reproduced at the same time. Progressive scanning
has a number of advantages, including greater clarity and the ability to more
easily interface with computer-based video equipment. At the same time, it
also adds greater technical demands on the TV system. The interleaved
approach, although necessary before recent advances in technology, results in
some minor "picture artifacts," or distortions in the picture,
including variations in color. Most of today's TV receivers still rely on the
interleaved approach. As we will see in the
next module, the specifications for digital and high-definition television
(DTV/HDTV) allow for both progressive and interlaced scanning. The Camera's Imaging Device
The lens of the television camera
forms an image on a light sensitive target inside the camera in the same
way a motion picture camera forms an image on film. But, instead of film,
television cameras commonly use solid-state, light-sensitive receptors called
CCDs (charged-coupled devices, or
"chips") that are able to detect brightness differences at
different points The target area of
the CCD (the small rectangular area near the center of this photo) contains
from hundreds of thousands to millions of pixel (picture element) points, each
of which can electrically respond to the amount of light focused on its
surface. A very small section
of a CCD is represented below—enlarged several thousand times. The individual
pixels are shown in blue. Electronics within
the camera scanning system regularly check each pixel to determine the amount
of light falling on its surface. This sequential information is directed to
an output amplifier along the path shown by the red arrows. The sequential
readout of information is continually repeated, creating a constant sequence
of changing field and frame information. (This process, especially as it
relates to color information, will be covered in more detail in Module 15.) In a sense, this whole process is
reversed in the TV receiver. The pixel-point voltages generated in a camera
are then changed back into light, which we see as an image on our TV screens.
Electronic signals as they
originate in microphones and cameras are analog (also spelled analogue) in form.
This means that the equipment detects signals in As illustrated on the
left, in professional facilities these signals are then changed into digital
data (computer 0s and 1s) before progressing through subsequent electronic
equipment.
In order to change an
analog signal to digital, the wave pattern is sampled at a high rate of speed and the
amplitude at each of those sampled moments is converted into a number
equivalent. It's as if each of
the blue columns on the left (which represents a corresponding point on the
analog signal above) is instantly assigned a numerical value before it is
sent out to represent the original signal. Since we are dealing with
numerical quantities, this conversion process is appropriately called quantizing. The faster all this
is done, the better the audio and video quality will be, of course—but also
the more "space" (bandwidth) that's required to record or transmit
the signal. Thus, we are frequently dealing with the difference between
high-quality equipment that can handle ultra high-speed data rates, and
lower-level (less expensive) consumer equipment that relies on a lower
sampling rate. This answers the question as to why some video recorders cost
$500 and others cost nearly $100,000. Actually, our ears and eyes don't
need every bit of the information in a continuous analog wave to get a true
impression of the original signal. If the sampling rate is fast enough, we
won't notice the "holes" (the spaces between the blue lines above)
in the data stream. Thus, original analog
audio and video signals are always "compressed" to some degree in
the analog-to-digital conversion process. The issue of
"quality" rests on how much. We'll revisit this issue a bit
later when we focus on the issue of compression. Once the information
is converted into numbers, we can do some very interesting things (generally,
special effects) by adding, subtracting, multiplying and dividing the
numbers. Compared to the digital signal, an analog signal would
seem to be the most accurate and ideal representation of the original signal.
While this may initially be true, the problem arises in the need for constant
amplification and re-amplification of the signal throughout every stage of
the audio and video process. Whenever a signal is reproduced
and amplified, noise
is inevitably By converting the
original analog signal into digital form, this noise buildup can be virtually
eliminated, even though it's amplified or "copied" dozens of times.
Because digital signals are limited to the form of zeros and ones (0s and 1s,
or binary computer code), no "in between" information can creep in
to degrade the signal. When we cover digital audio,
we'll delve more deeply into some of these issues. Today's digital audio and video
equipment has borrowed heavily from developments in computer technology—so
heavily, in fact, that the two areas seem to be merging. Today, satellite
services such as DISH and Direct-TV make use of digital receivers that are,
in effect, specialized computers. Progressive radio and TV stations have
already switched over to digital signal processing. And, very possibly,
you regularly listen to music recorded on a shirt pocket-sized device that is
capable of storing several hours of digitized music. Some of the
advantages of digital electronics in video production are
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