Digital Media
Lecture 9: Video, TV & Film
Georgia Gwinnett CollegeSchool of Science and Technology
Dr. Jim Rowan
In the next several lectures…
Film & TV & Video & AnimationIssues that arise from conversionAnalog vs Digital
Test Pattern
TV Broadcast…Digital replaces Analog
Why Digital Broadcast? reduced spectrum use greater capacity multiple programs on one freq better quality picture HDTV can use compression allows multiple HD signals on one
freq. allows user interaction
TV Broadcast…Difference with poor reception
Analog vs Digital
Analog… as signal gets weaker image gets less distinct “ghosts (white shadows) appear” gracefully degrades
TV Broadcast…Difference with poor reception
Analog vs Digital
Digital… with digital, you either have signal or you don’t have signal so… lose signal everything goes black audio stops ungraceful degrading
Two ways to make Moving Pictures:Video & Animation
In this class:– Video
• shot with a camera • captures images from the world• then play them back
– Animation • create frames individually• using inkscape and blender• play them back
Two ways to make Moving Pictures:Video & Animation
In this class:– Video
• shot with a camera • captures images from the world• then play them back
– Animation • create frames individually• using inkscape and blender• play them back
Video (and Film)
Works because of persistence of vision– human perception causes still images
played in rapid succession to fuse into motion
– http://en.wikipedia.org/wiki/Frame_rate Fusion frequency
– ~ 40 frames per second– depends on the brightness of the image
relative to the viewing environment Less than that
– first flickering– then individual images appear losing the
illusion of motion
Film: How it works
Plays at 24 frames per second– Show the image– Block the light to make it dark– Move to the new image– Allow the light through to show the
new image– Without “blacking out” the change
from one image to the next the image would be blurred
Film Trivia 1
Films are longer than one reel long How does the person who runs the
cameras know when to change? There are two projectors, one running,
one waiting A black hole in the film appears ~5 sec
before the switch is made Another black hole in the film appears
and the projector operator switches
Film Trivia 2
Watch the credits… Foley artist? 24 frame manager? Telecine?
Video & TV
Two versions– Interlaced
• Rising from a TV legacy– Progressive scan
• Rising from a computer legacy
Interlaced
– Captured (and displayed) as “fields”– First the odd numbered lines are
captured (or displayed)– Then the even numbered lines are
captured (or displayed)– …
Interlaced
– …– This reduced the bandwidth needed
to transmit images that moved for early TV• The glowing phosphor of the CRT stayed
glowing for a while after the electron beam was turned off
• Allowing the other field to be drawn and complete the TV image
Interlaced fieldsRaster scan
Interlaced scan
Interlaced problem:
Rapid motion resulted in the “comb effect”
Progressive scan
Progressive scan
• Each line on the screen is painted one after the other from top to bottom• Electronics are faster now so interlacing is not
required• If captured progressively, then the playback is
straight forward• If captured as interlaced fields, playing them
back progressively is problematic• disadvantage of progressive scan is that it
requires higher bandwidth than interlaced video that has the same frame size
Interlace problem:
the left-column images are progressive scan
the center-columnimages are interlaced
the right-columnimages use line doublers
bottom images are anti-aliased
http://en.wikipedia.org/wiki/File:Indian_Head_interlace.gif
Video… it’s bigHow do you deal with it?
Playback degradation (graceful degradation)
Compression
Video… it’s bigHandling with Playback
Transport or playback not fast enough to keep up with the story?– something’s got to give– there’s too much data to either transport or
display Some players just freeze the image and
halt the audio– this kills the ability to tell the story
Some players (like quicktime) make attempts to “degrade gracefully”
Video: Graceful degradation
Graceful degrading allows the story to continue
Some players drop frames– first showing as a “slide show” while continuing to
play the audio– then holding the last image while continuing to play
the audio stream– this effectively loses the illusion of motion but
continues the “story” as an audio stream …
Video: Graceful degradation
… Some play lower resolution images
while remaining synched to the audio stream– this continues the illusion of motion (at a lower
resolution) and continues the “story” with the audio stream
Video: The “progress bar”Quicktime example
Click to play a quicktime video Quicktime window opens It is in “play” mode (the pause icon is
showing Doesn’t begin to play, instead a gray
colored bar starts filling the progress bar
At some point it begins to play Why? It’s predicting how long the
download will take
The progress bar
Video is big so: reduce its size using compression
On the capture side– Digitization & compression can be
carried out by hardware to be fast– Can be done in the camera
(hardware)– Can be done in the computer
(software)
hardware vs software compression
Hardware compression... user has no control over it... it is hardwired– It can be in the camera
Software compression... is computationally expensive... it’s a slow process– Provides for the most flexibility since it can
be changed– Can use different software coder-decoders
(codec), picking and choosing what fits your needs better
Compression in the camera:hardware compression
Our cameras?– Mini DV format– Compress each captured image into a jpeg
image• This is called intra-frame compression
– Present a digital stream of bits to the computer over a firewire connection
With compression you get artifacts
with software compression…
Analog is presented to the computer through a video capture card
Compression is done (usually) in the video capture card
Allows for a really small camera because the work (the compression and the analog to digital conversion) is done elsewhere
More aboutAnalog vs Digital
An analog signal to the computer is susceptible to noise corruption
Digital signal is not What’s the big deal? Consider compressing a video of a wall
painted a solid color– Analog noise will cause small fluctuations
from pixel to pixel– RLE can’t compress it because each pixel is
a bit different
Comparing cameras iSight to MiniDV
iSight (or a webcam) is built into the Macs in this room– Presents an analog signal to the computer– Subject to analog noise
The cameras we can check out from the library are Mini DV format and record on tape– Presents a digital signal to the computer
iMovie
video capture card
computer
miniDV
compression
compression
webCam
analog signal
digital signal
Our video cameras compress using jpeg
the scene
!!!NOISE!!!
We’ve seen…
Converting TV to Video is problematic– Interlacing
• comb effect
Next: Converting Film to Video is
problematic– Matching 24 frames to 30 frames
• Telecine problem
Film to Video
Problematic (interleaved)– video is 30 frames per second– film is 24 frames per second
How do you make 30 frames from 24?
One way: The 3-2 pull down… AKA Telecine
Film to interlaced video:
Refer to Supplemental text:
Moving Images: Film Moving Images: TV Moving Images: Video
Next
Lecture 10:Video & compression techniques