1. Digital Media
Dr. Jim Rowan
ITEC 2110
Video

2. Lecture URLs http://en.wikipedia.org/wiki/Interlace

3. In the next several lectures…
Film & TV & Video & Animation Issues that arise from conversion Analog vs Digital

4. Test Pattern

5. Digital replaces Analog
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

6. TV Broadcast…
Standards
Analog (old school)
NTSC (north America)
PAL (western europe)
SECAM (eastern europe)
Digital
ATSC (see map in wikiPedia)

7. Difference with poor reception
TV Broadcast…
Difference with poor reception
Analog…
as signal gets weaker
image gets less distinct
“ghosts (white shadows) appear”
gracefully degrades
Digital…
with digital, you either have signal
or you don’t have signal so…
lose signal
everything goes black
audio stops
ungraceful degrading

8. 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

9. 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

10. 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

11. 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

12. 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

13. Film Trivia 2 Watch the credits… Foley artist? 24 frame manager?
Telecine?

14. Video & TV Two versions Interlaced Progressive scan
Rising from a TV legacy
Progressive scan
Rising from a computer legacy

15. Interlaced Captured (and displayed) as “fields”
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

16. Interlaced fields Raster scan

17. Interleaved scan

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

19. 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

20. Progressive scan

21. 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

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

23. 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”

24. 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

25. 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

26. 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)

27. 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

28. 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

29. 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

30. 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

31. 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

32. Our video cameras compress using jpeg
analog signal
!!!NOISE!!!
computer
webCam
video capture card
compression
digital
signal
iMovie
miniDV
the scene
compression

33. Our video cameras compress using jpeg
analog signal
!!!NOISE!!!
computer
webCam
video capture card
compression
digital
signal
iMovie
miniDV
the scene
compression
640 x 480 = 307,200 bytes
Noise makes each pixel
a little different
RLE:
307,200 bytes x RGB (3bytes)
= bytes
640 x 480 = 307,200
307,200 can be represented
by < 24 bits, call it 3 bytes
RLE:
307,200 (3bytes) +
RGB (3 bytes) ~ 6 bytes

34. We’ve seen… Converting TV to Video is problematic Next:
Interlacing
comb effect
Next:
Converting Film to Video is problematic
Matching 24 frames to 30 frames
Telecine problem

35. 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…

36. Film to interlaced video: