ANALOG AND DIGITAL VIDEO - Columbia Universityhgs/teaching/ais/slides/2015 AIS video 0424.pdf · ANALOG AND DIGITAL VIDEO Henning Schulzrinne Columbia University COMS 6181 - Spring

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

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Example: DVI connector

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analog RGB + Hsync

DDC

DRed1

DGr

Dred 2

video only (no audio)

Example: HDMI connector

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

Analog TV: CRT

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howstuffworks.com

cathode

conductive coating

anode phosphor-coated screen

shadow mask

TFT LCD

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Analog TV

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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

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Mark Handley

Interlacing

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Basic Analog TV: Interlacing

Each interlaced frame has twofields, captured at different times.

Source: www.100fps.com

Interlacing artifacts

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www.100fps.com

motion

scene change

Composite video

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horizontal retrace vertical: 400-500 µs

black

white

howstuffworks.com Wikipedia

Color

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Color

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rods cones

Wikipedia TeXample.net

additive metamerism: tristimulus values

Subtractive color: printing

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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

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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”

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