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