Digital Media

Dr. Jim Rowan

ITEC 2110

Video

http://en.wikipedia.org/wiki/Interlacehttp://en.wikipedia.org/wiki/Progressive_scanhttp://en.wikipedia.org/wiki/Telecine

http://en.wikipedia.org/wiki/Digital_terrestrial_televisionhttp://en.wikipedia.org/wiki/ATSC_Standards

http://en.wikipedia.org/wiki/NTSChttp://en.wikipedia.org/wiki/PALhttp://en.wikipedia.org/wiki/SECAM

Lecture URLs

http://www.bealecorner.com/gl1/res/gl1resint.jpg Test Pattern

Next Several Lectures

Film & TV & Video & Animation

Issues that arise from conversion

Analog vs Digital

TV Broadcast…Digital replaces Analog

Why Digital Broadcast?reduced spectrum usegreater capacity

multiple programs on one freqbetter quality pictureHDTVcan use compression

allows multiple HD signals on one freq.allows user interaction

TV Broadcast…

Standards

Analog (old school)NTSC (north America)PAL (western europe)SECAM (eastern europe)

DigitalATSC (see map)

TV Broadcast…Difference with poor reception

Analog…as signal gets weakerimage gets less distinct“ghosts (white shadows) appear”gracefully degrades

Digital…with digital, you either have signalor you don’t have signal so…lose signal

everything goes blackaudio stopsungraceful degrading

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 & 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

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)– 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

http://en.wikipedia.org/wiki/Interlace

http://www.infocellar.com/television/files/interlace1.gif

Interlaced fieldsRaster scan

Interleaved scanhttp://www.geniusdv.com/weblog/archives/2007/08/14/interlaced_vs_progressive.gif

Interlaced problem:

• Rapid motion resulted in the “comb effect”

http://en.wikipedia.org/wiki/File:Interlaced_video_frame_%28car_wheel%29.jpg

Interlace problem:

the center-column

images are interlaced

the left-column

images are

progressive scan

the right-column

images use line doublers

bottom images are anti-aliased

Progressive scanhttp://www.geniusdv.com/weblog/archives/2007/08/14/interlaced_vs_progressive.gif

Progressive scanhttp://en.wikipedia.org/wiki/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

Video… it’s bigHow do you deal with it?

• Playback 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

• 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 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– It can be in the video card

• 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 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!!!

iMovie

video capture card

computer

miniDV

compression

compression

webCam

analog signal

digital signal

640 x 480 = 307,200307,200 can be represented by < 24 bits, call it 3 bytesRLE: 307,200 (3bytes) + RGB (3 bytes) ~ 6 bytes

Our video cameras compress using jpeg

the scene

640 x 480 = 307,200 bytesNoise makes each pixel a little differentRLE: 307,200 bytes x RGB (3bytes) = 921600 bytes

!!!NOISE!!!

• Converting TV to Video is problematic– Interlacing

• comb effect

Next:

• Converting Film to Video is problematic– Matching 24 frames to 30 frames

• Telecine problem

We’ve seen…

Film to Video

• Problematic– 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…

Film to interlaced video: http://en.wikipedia.org/wiki/Telecine