Digital Media Lecture 9: Video, TV & Film Georgia Gwinnett College School of Science and Technology Dr. Jim Rowan

In the next several lectures… Film & TV & Video & Animation Issues that arise from conversion Analog 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 – 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 fields Raster scan

Interlaced scan

Interlaced problem: Rapid motion resulted in the “comb effect”

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-column images are interlaced the right-column images use line doublers bottom images are anti-aliased

Video… it’s big How do you deal with it? Playback degradation (graceful degradation) Compression

Video… it’s big Handling 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 conversion... 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 about Analog 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 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:

Next Lecture 10: Video & compression techniques