Transcript 计算机交叉学科 - Shenzhen University

MULTIMEDIA TECHNOLOGY
AND APPLICATIONS
CHAPTER 5. INTRODUCTION TO
VIDEO
5.1 Video Concept
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Video is an excellent tool for delivering multimedia.
Video places the highest performance demand on computer and
its memory and storage.
Digital video has replaced analog video as the method of choice for
making and delivering video for multimedia.
5.1 Video Concept
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Digital video device produces excellent finished products at a fraction
of the cost of analog.
Digital video eliminates the image-degrading analog-to-digital
conversion.
Many digital video sources exist, but getting the rights can be difficult,
time-consuming, and expensive.
5.2 Analogue Video
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Video information that is stored using television video signals,
film, videotape or other non-computer media
Each frame is represented by a fluctuating voltage signal known
as an analogue wave form or composite video.
5.2 Analogue Video
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Composite analogue video has all the video components:
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brightness, colour and synchronization
Then combined into one signal for delivery
Example : traditional television
Problems: colour blending, low clarity, high generation lost, difficult to
edit.
DIGITAL
ANALOGUE
Digitizing Video
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Digital video combines features of graphics and audio to create
dynamic content for multimedia products.
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Video is simply moving pictures.
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Digitized video can be edited more easily.
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Digitized video files can be extremely large.
Digitizing Video
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Digital video is often used to capture content from movies and television
to be used in multimedia.
A video source (video camera ,VCR, TV or videodisc) is connected to a
video capture card in a computer.
As the video source is played, the analog signal is sent to the video card
and converted into a digital file (including sound from the video).
VCR
Video Overlay Board /
Video Capture Card
PC
Analogue signal from VCR
Converted to DIGITAL
by VIDEO CAPTURE CARD
The converted
signal is
entered inside a
computer
Signal is processed
Video is edited
using video editing
software software
5.3 Digital Video
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Digital video is the digitisation of analogue video signals into numerical
format
It creates the illusion of full motion by displaying a rapid sequence of
changing images on a display device.
Conversion from analogue to digital format requires the use on an ADC
(Analogue to Digital Converter)
A Digital to Analogue Converter (DAC) can be used to output digital video
on analogue equipment
File Size and Formats
There is an important consideration:
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file size in digitized video which included
1.
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3.
frame rate
image size
color depth.
File Size and Formats
Frame Rate
1.
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animation is an illusion caused by the rapid display of still
images.
television and movies play at 30 fps but acceptable playback
can be achieved with 15 fps.
File Size and Formats
2.
Image Size
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A standard full screen resolution is 640x480 pixels but to safe
storing space a video with 320x240 for a computer display is still
acceptable.
New high-definition televisions (HDTV) are capable of resolutions
up to 1920×1080p60,
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1920 pixels per scan line by 1080 scan lines, progressive, at 60
frames per second.
File Size and Formats
Color Depth
3.
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The quality of video is dependent on the color quality (related
to the number of colors) for each bitmap in the frame sequence.
File Size and Formats
Color Depth
3.
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The color depth below 256 colors is poorer-quality image.
The frame rate to below 15 fps causes a noticeable and
distracting jerkiness that unacceptable.
Changing the image size and compressing the file therefore
become primary ways of reducing file size.
24 bit
16 bit
8 bit (256 colors)
Video Editing Terminology
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Linear
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It plays end to end in one direction, usually pertains
to videotape editing specifically the editing of linear
tape segments into one final master tape.
Video Editing Terminology
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Non-linear
Refers to the editing of disk-based digital video.
 The software provides an on screen map of what the final
video sequences should look like incorporating the edits,
splices, special effects, transitions and sound tracks.
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Special Effects
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Transitions
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Such as fading, wiping, splatters, scrolling, stipple and many
more are available by simply dragging and dropping that
transition between the two video clips.
5.4 Video Compression
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The video compression/decompression programs are used so that video can fit on a single
CD and the speed of transferring video from a CD to the computer can be increased.
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Let us say that a sequence of 25fps video is about 25MB.
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CD-ROM transfer rate is calculated as follows:
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1X= 150KB per second
10X=1.5 MB per second
100X= 15 MB per secondt
To overcome large video size, CODECS were developed.
Compresses
when saved
CODEC
Decompresses
when needed for
display
File format used
such as: Avi,
Mpeg, Mov
5.4 Video Compression
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Digital video compression schemes or codecs is the algorithm used to
compress (code) a video for delivery.
The codec then decodes the compressed video in real-time for fast
playback.
Streaming audio and video starts playback as soon as enough data has
transferred to the user’s computer to sustain this playback.
FULL
VIDEO
Taken from the CD
into memory buffer
MEMORY BUFFER
DISPLAY ON
SCREEN
5.4 Video Compression
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Flavors of file formats brand to choose:
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Microsoft’s AVI format
QuickTime
MPEG
Div-X
Wmv (Windows Media Video)
5.4 Video Compression
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Standards have been established for compression
programs, including JPEG (Joint Photographic
Experts Group) and MPEG (Motion Picture
Experts Group).
JPEG (Spatial)
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Often areas of an image (especially backgrounds) contain
similar information.
JPEG compression identifies these area and stores them as
blocks of pixels instead of pixel by pixel reducing the
amount of information needed to store the image.
These program reduce the file size of graphic images by
eliminating redundant information.
MPEG (Temporal)
The changes in the image from frame to frame.
 Key frames are identified every few frames the
changes that occur from key frame.
 Provide greater compression ratios than JPEG.
 Initially, it requires extra hardware for
multimedia.
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Video Hardware and Software
VCR
MULTIMEDIA PC
Video Overlay Board /
Video Capture Card
Video digital
Editing Software
Advantages of using Video
Captures interest
 Increase retention
 Clarifies complex physical actions and
relationships
 Can incorporate other media
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Disadvantages of using Video
Is expensive to produce
 Requires extensive memory and storage
 Requires special equipment
 Does not effectively illustrate abstract concepts
and static situations
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Summary
Digital video method is used for making and
delivering video for multimedia.
 Compression techniques help to reduce the file
sizes to more manageable levels
 Two types of compression lossless and lossy.
 Standards for compression program are JPEG
and MPEG.
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Analog Video
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Represented as a continuous (time varying) signal
Analog Video
From T1
NTSC (National Television System Committee)
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It uses the familiar 4:3 aspect ratio (i.e., the ratio of picture width to its height)
Uses 525 scan lines per frame at 30 frames per second (fps).
NTSC follows the interlaced scanning system, and each frame is divided into two
fields, with 262.5 lines/field.
Thus the horizontal sweep frequency is 525x 29.97 =15,734 lines/sec, so that each
line is swept out in 63.6 µ sec
(1/15.734 x
3
10 sec )
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63.6 µ sec = 10.9 µ sec for Horizontal retrace + 52.7 µ sec active line signal
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For the active line signal during which image data is displayed
NTSC (National Television System Committee) [3]
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NTSC video is an analog signal with no fixed horizontal resolution
Pixel clock is used to divide each horizontal line of video into
samples. Different video formats provide different numbers of
samples per line
Uses YIQ Color Model
PAL (Phase Alternating Line)
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Widely used in Western Europe, China, India, and many other parts of
the world.
Uses 625 scan lines per frame, at 25 frames/second, with a 4:3 aspect
ratio and interlaced fields
Uses the YUV color model
Uses an 8 MHz channel and allocates a bandwidth of 5.5 MHz to Y,
and 1.8 MHz each to U and V.
Digital Video
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Advantages over analog:
Direct random access --> good for nonlinear video editing
 No problem for repeated recording
 No need for blanking and sync pulse
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Almost all digital video uses component video
High Definition TV (HDTV)
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The main thrust of HDTV (High Definition TV) is not to increase
the definition in each unit area, but rather to increase the visual field
especially in its width.
The first generation of HDTV was based on an analog technology
developed by Sony and NHK in Japan in the late 1970s.
 Uncompressed HDTV will demand more than 20 MHz bandwidth,
which will not fit in the current 6 MHz or 8 MHz channels
 More than one channels even after compression.
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High Definition TV (HDTV)
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The salient difference between conventional TV and HDTV:
HDTV has a much wider aspect ratio of 16:9 instead of 4:3.
 HDTV moves toward progressive (non-interlaced) scan. The rationale
is that interlacing introduces serrated edges to moving objects and
flickers along horizontal edges.
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