ANALOG AND DIGITAL VIDEO Henning Schulzrinne Columbia University COMS 6181 - Spring 2015

with material from Mark Handley

Video & image sources • VGA video (computer video output)

•  component video •  R, G, B + H (horizontal sync), V (vertical sync)

• Analog video from cameras •  NTSC or PAL coded color •  composite or component video

• Digital images •  scanners, copiers and fax machines •  digital cameras

•  Images = still pictures • Video = motion pictures

2

Example: DVI connector

3

analog RGB + Hsync

DDC

DRed1

DGr

Dred 2

video only (no audio)

Example: HDMI connector

4

•  fully digital signal •  audio •  video (similar to DVI) •  remote control signals •  no closed captioning

Analog TV: CRT

5

howstuffworks.com

cathode

conductive coating

anode phosphor-coated screen

shadow mask

TFT LCD

6

Analog TV

7

vertical retrace

scan lines

How many frames? • Movies use 24 fps

•  Not scanned – just photographs •  Flicker, with undersampling (stroboscopic) problems •  dark environment à less sensitivity to flicker

•  TV •  scanned •  fairly bright environment •  à use interlacing to reduce flicker

8

Mark Handley

Interlacing

9

4

Basic Analog TV: Interlacing

Each interlaced frame has twofields, captured at different times.

Source: www.100fps.com

Interlacing artifacts

10

www.100fps.com

motion

scene change

Composite video

11

horizontal retrace vertical: 400-500 µs

black

white

howstuffworks.com Wikipedia

Color

12

Color

13

rods cones

Wikipedia TeXample.net

additive metamerism: tristimulus values

Subtractive color: printing

14

magenta (complement of green) cyan

(complement of red)

yellow (complement of blue)

Color representation •  Luminance (Y) = brightness • Chrominance • RGB (red, green, blue)

•  basic analog components

• YPbPr (Y, B-Y, R-Y) •  color space for component video •  YUV •  YCbCr

•  digital representation

15

Color mapping •  True color

•  RGB in 8 bits each (common) à 16.7 million colors •  RGB in 16 bits each à 281 trillion •  Eye: 10 million colors

•  Indexed color •  color table •  8 bit index à RGB 24 bit •  approximate using dithering •  see “web safe colors”

16