Transcript Digital Video

Digital Video
Chapter 6
Outline
Video =
Motion Pictures
Analog
Television Video
Digital
Television Video
Analog vs.
Digital Video
What is Video?
Take many pictures per second, sequence
them in order, and play them back at
about the same rate as they were taken.
Animation is
different.
• Take/make pictures at a slower rate
• then play them back at a faster rate.
• Obviously, not live, not real-time.
Video Recording
Technology is pretty
much the same as
general photography
How to
store it
• Optical device that can
capture light and store it.
• Negative film (analog)
• Digital device
• Magnetic film
• Hard drive
• Flash memory
Challenge: Video requires taking and storing many picture per second.
Motion (moving) Pictures
• Motion Pictures: the first widely-viewed form
of real life captured video
• Captured on Film Stock (Negative Film) –
Pioneered 1900-1920
– Still used today
Television
• Real-time Analog Signal transmitted over a wire
– Pioneered 1950
• At first, TV was only live.
• Later, stored on Magnetic Tapes and rebroadcasted
– Pioneered 1956
• Even Later, TV signals and Movies stored on
Consumer VCRs and VHS cassette tapes – 1971
– VCR – Video Cassette Recorder
– VHS – Video Home System
Movie Cameras vs. TV Cameras
Movie
• Very Mechanical
Cameras store
• Lots of moving parts
images on
• Surprisingly few electronics
negative film.
TV Cameras
do NOT use
negative film
• Signal is either
• Broadcasted over wires
• Saved on Cassette Tapes/Reals
Modern TV Camera vs. Camcorder
TV Camera
Camcorders
• Higher quality, more expensive
• Interfaces with lot of different devices for
• Broadcast
• Recording
• Lower quality all-around
• Self contained
• Recording device is usually part of the
camera.
Digital Movies
• Most major motion pictures are still shot on
negative film (analog technology)
• Film is high resolving medium (as good as the
best digital capturing technology)
– Academy camera US Widescreen:
• 21 × 11 mm  2970 × 1605
– Anamorphic Panavision ("Scope"):
• 21 × 17.5 mm  2970 × 2485
– Super-35:
• 24 x 10 mm  3390 × 1420
Digitizing Film
•
•
•
•
Converting the Film (analog) to digital form.
Film Scanners are used
Prices range from $100 to $20,000
Google “Negative Film Scanners”
How Film Works
• Film has millions of light-sensitive
silver halide crystals (silver + halogen)
– held together by electrical
attraction.
• When crystals are struck by light,
silver ions build up uncharged atoms
which forms a latent image
• Developing chemicals create a visible
image by building up the metallic
silver where ever the uncharged
atoms formed, i.e., the latent image.
Film Width
• Each image is stored sequentially on film role.
• To achieve higher resolution, you can increase the width of
the film
• Only part of the width can be used to capture images
35mm
21mm
Film Width & Orientation
• 35mm Film can achieve High Definite (HD)
resolutions
• HD Wide Screen 1920+ × 1080+
• 70mm can go way beyond
the HD seen on
BluRay and HD DVD.
• IMAX used 70mm film and
changes the orientation.
• 10000 X 7000 is possible.
Direct Digital Capture for Movies
• Digital Video Cameras are just starting to be
widely used for filming movies
– Speeds up the processing.
– Eliminates a step: Film  Digital Media Storage
– Digital editing can immediately begin
• Interestingly, in Hollywood, there is great
resistance to direct digital capture
– Union resistance
– Puts technicians out of work
Outline
Video =
Motion Pictures
Analog
Television Video
Digital
Television Video
Analog vs.
Digital Video
Analog TV Broadcast
• Is now ancient history, as of February 2009.
• Or is it?
Analog Broadcast TV
3 standards for analog television broadcast.
• Encoding format
standards =
technical details • Transmission format.
Standards
include
• frame rate
• scan lines in each frame.
3 Standards
NTSC - National Television
Systems Committee
PAL - Phase Alternating Line
SECAM - Séquentiel
Couleur avec Mémoire
• North America, Japan, Taiwan,
Caribbean and South America.
• Australia, New Zealand,
Western Europe and Asia.
• France, the former Soviet
Union, and Eastern Europe.
Africa and parts of Asia are mostly influenced by
their colonial histories.
Frame Rates
Video Type
Frames Per Second (fps)
NTSC
29.97
PAL
25
SECAM
25
Motion Picture Film 24
NTSC was 30 fps for black-and-white TV, Frame rate was
lowered to 29.97 fps to accommodate for color
encoding.
Lines
• Old televisions (CRT technology) are made up
of horizontal lines.
• Lines are drawn (refreshed) across the screen
one line at a time.
– NTSC frame: 525 lines (480 picture safe zone).
– PAL and SECAM frames: 625 lines (576 picture
save zone).
Columns?
• The signal breakdown into lines (rows) is needed for
CRT technology to properly display the picture
• In principle, the rows could have been continuous
and the signal could be broken down into columns
No Pixels
• Analog video signals transmit each line as a
continuous signal, i.e., no pixels at all.
– The only reason the signal is broken down into
rows is for 2D display on a CRT.
– The laws of physics limit how fast the electrical
signal can be sent, the resolution is only limited by
the speed and density of the signal.
Progressive Scan vs. Interlacing
• Progressive Scan: Computer monitors (CRT)
display lines from top to bottom in one pass.
• Interlacing: Television Standards (NTSC, PAL,
and SECAM) display the picture in two passes:
– first pass is odd-numbered lines (upper field)
– second pass is even-numbered lines (lower field)
Interlacing – Progressive Scan
Interlacing
• The original video may have had all the lines
(odd and even) for each frame
• But, the broadcast signal only includes half the
lines for each frame
• Each frame alternates
•
•
•
•
•
Frame 1 odd lines
Frame 2 even lines
Frame 3 odd lines
Frame 4 event lines
…
Why Interlace?
• It removes the flicker effect, without the need
to increase the frame rate.
• Progressive Scan at 15 frames per second
flickers, i.e., a noticeable blinking, like a strobe
light.
• To remover the flicker, you’d have to increase
the frame rate, which would require sending
more data
– 24+ frames per second instead of 15.
Refreshing
• 60 Hertz
– each pixel is refreshed 60 times per second.
• With Interlacing, you only have to refresh a pixel 30 times a
second with no noticeable difference.
Time
Refreshing
• Interlacing + latency
Time
Interlacing Artifacts
Upper Field & Lower Field Demo
• http://media.pearsoncmg.com/ph/esm/esm_
wong_dmp_1/chapter06/ylwong-videointerlace-fields.html
Artifact Example
• http://media.pearsoncmg.com/ph/esm/esm_
wong_dmp_1/chapter06/ylwong-dv-interlacedemo.mov
Overscan
• CRTs distort the signal near the edges the picture
tube.
– Caused by noise in the signal
• By design, the plastic
frame of the TV covers
the distorted area.
• The size of the covered
area depends on the
quality of both the
CRT and the signal
Overscan & Save Zones
• For NTSC The signal sent is larger (525 lines) than what
you see (480-520 lines).
• The area you do NOT
see is called the
overscan
• TV Broadcast
(especially News),
has to be careful
not to display important
information in
the overscan area.
• 480 lines is the save zone
TV Color Format
• Remember how Digital Images were encoded
using RGB or CYMK.
• Analog TV actually breaks the color signal into
only two parts
1. Luminance (Brightness)
2. Chrominance (Hue/Color)
– When there were only B&W TV’s the signal was only a
luminance signal.
– Chrominance was added later and it lengthened the
signal, which decreased the Frame rate.
Outline
Video =
Motion Pictures
Analog
Television Video
Digital
Television Video
Analog vs.
Digital Video
Interlacing & Digital HDTV
An interlaced signal is not meant for
LCD and Plasma display technology
LCD and Plasma TVs
display an entire
frame in one burst
(called a refresh).
CRT technology
naturally displays lines
drawn from top to
bottom, rapidly.
Interlacing & Digital HDTV
• While LCD technology can achieve refresh
rates of over 200 Hertz (200 frames per
second)…
• The problem is that an analog TV signal is
interlaced (sends half the lines 30 times a
second).
• Also, until recently TV signals were analog.
Analog  Digital Problem
• With an analog signal, LCD or Plasma TVs have to
1. Wait for two frames (odd lines then even) and
put them together, or
2. Interpolate (approximate) the missing lines
Both require added hardware
One of the reasons these TVs are more expensive
One of the reason they should be cheaper in the future,
i.e., no need to worry about analog signals anymore
Which do you think is better?
1. Wait for two frames (odd lines then even)
and put them together, or
2. Interpolate (approximate) the missing lines
Interpolation
HDTVs and Analog Signals
• The video quality of HDTVs (high resolution LCD
or Plasma) appears poor when given an analog
signal.
• Because the
– Interlaced signal need to be interpolated to a
progressive signal (now a minor issue)
– Signal can only be up-converted to a NTSC safe area
frame 640 X 480
• Consider that a 20” LCD ($150) has 1280 X 1024 pixels. Over
4 times as many pixels as the received signal.
• Buying an HTDV without a digital video source is a waste of
money.
Standard-Definition Digital TV Signals
640 x 480 progressive
• 24 fps
• 30 fps
• 60 fps
640 X 480 interlaced
• 60 fps
704 X 480 progressive
• 24 fps
• 30 fps
• 60 fps
704 X 480 interlaced
• 60 fps
High-Definition Digital TV Signals
1280 × 720
progressive (720p)
• 24 fps (720/24p)
• 30 fps (720/30p)
• 60 fps (720/60p)
1920 X 1080
interlaced (1080i)
• 60 fps (1080/60i)
1920 X 1080
progressive (1080p)
• 24fps (1080/24p)
• 30 fps (1080/30p)
What Signal will be transmitted when
Broadcast TV goes Digital?
• It can be any one of the Standard or High Definition
formats I just listed.
– 720/24p will probably be the standard definition default
– 1080/24p will likely be the HD default
– 1080i was supposed to be used because it works nicer with
CRTs, but CRTs will be phased out of existence soon.
• Depends on the TV network’s system or your signal
provider.
– TV Network = ABC, NBC, Local Affiliates, etc.
– Signal provider = DirectTV, TimeWarner, etc.
Outline
Video =
Motion Pictures
Analog
Television Video
Digital
Television Video
Analog vs.
Digital Video
Analog Video Signal
• The details here are not 100% correct, but the
principle is…
– Like the Bohr Model of the Atom
• Think of an analog TV signal as a wave
• The amplitude (height) indicates the brightness
• The frequency (pattern) indicates the color
• A segment of the signal generates a line on the
CRT
Digital TV Signal
• This is closer to being 100% correct
• Think of a digital signal as a wave
• Where wave/no wave indicate 1 or 0
• 1 0 0 0 0 0 1 1 0 1 0 0 1 0 0 1 1 1 0 0
• Each frame is encoded as bitmap/raster image
just as we previously studied.
• Color is RBG 24-bit
• Obviously, compression is used
Digital Video Fundamentals
• Sampling rate is…
– Capture resolution or each frame
i.e., 640 X 480 vs. say 1920 X 1080
– Capture rate
i.e., 30 frames per second vs. say 60
• Quantization Level is still
– The color depth
i.e., 8 bit color vs. say 24-bit