Digital Video

Chapter 6

OutlineVideo =

Motion Pictures

Analog Television Video

Digital Television Video

Analog vs. Digital Video

What is Video?

Take many pictures per second, sequence

them in order, and play them back at about

the same rate as they were taken.

Animation is different.

Video Recording

• Optical device that can capture light and store it.

Technology is pretty much the same as general

photography

•Negative film (analog)•Digital device•Magnetic film•Hard drive•Flash memory

How to store it

Challenge: Video requires taking and

storing many picture per second.

Motion (moving) Pictures

• Motion Pictures: the first widely-viewed form of real life captured video

• Captured on Film Stock (Negative Film) – Pioneered 1900-1920– Still used today

Television

• Real-time Analog Signal transmitted over a wire– Pioneered 1950

• At first, TV was only live.• Later, stored on Magnetic Tapes and re-broadcasted– Pioneered 1956

• Even Later, TV signals and Movies stored on Consumer VCRs and VHS cassette tapes – 1971– VCR – Video Cassette Recorder– VHS – Video Home System

Movie Cameras vs. TV Cameras

•Very Mechanical•Lots of moving parts•Surprisingly few electronics

Movie Cameras store

images on negative film.

•Signal is either•Broadcasted over wires•Saved on Cassette Tapes/Reals

TV Camerasdo NOT use

negative film

Modern TV Camera vs. Camcorder

•Higher quality, more expensive•Interfaces with lot of different devices for•Broadcast•Recording

TV Camera

•Lower quality all-around•Self contained•Recording device is usually part of the camera.

Camcorders

Digital Movies

• Most major motion pictures are still shot on negative film (analog technology)

• Film is high resolving medium (as good as the best digital capturing technology)– Academy camera US Widescreen:

• 21 × 11 mm 2970 × 1605– Anamorphic Panavision ("Scope"):

• 21 × 17.5 mm 2970 × 2485– Super-35:

• 24 x 10 mm 3390 × 1420

Digitizing Film

• Converting the Film (analog) to digital form.• Film Scanners are used• Prices range from $100 to $20,000• Google “Negative Film Scanners”

How Film Works

• Film has millions of light-sensitive silver halide crystals (silver + halogen) – held together by electrical

attraction. • When crystals are struck by light,

silver ions build up uncharged atoms which forms a latent image

• Developing chemicals create a visible image by building up the metallic silver where ever the uncharged atoms formed, i.e., the latent image.

Film Width

• Each image is stored sequentially on film role.• To achieve higher resolution, you can increase the width of

the film• Only part of the width can be used to capture images

35mm

21mm

Film Width & Orientation

• 35mm Film can achieve High Definite (HD) resolutions

• HD Wide Screen 1920+ × 1080+• 70mm can go way beyond

the HD seen on BluRay and HD DVD.

• IMAX used 70mm film and changes the orientation.

• 10000 X 7000 is possible.

Direct Digital Capture for Movies

• Digital Video Cameras are just starting to be widely used for filming movies– Speeds up the processing.– Eliminates a step: Film Digital Media Storage– Digital editing can immediately begin

• Interestingly, in Hollywood, there is great resistance to direct digital capture– Union resistance– Puts technicians out of work

OutlineVideo =

Motion Pictures

Analog Television Video

Digital Television Video

Analog vs. Digital Video

Analog TV Broadcast

• Is now ancient history, as of February 2009.• Or is it?

Analog Broadcast TV

3 standards for analog television

broadcast.

standards = technical details

Standards include

3 Standards

•North America, Japan, Taiwan, Caribbean and South America.

NTSC - National Television Systems

Committee

•Australia, New Zealand, Western Europe and Asia.

PAL - Phase Alternating Line

•France, the former Soviet Union, and Eastern Europe.

SECAM - Séquentiel Couleur avec Mémoire

Africa and parts of Asia are mostly influenced by their colonial histories.

Frame RatesVideo Type Frames Per Second (fps)

NTSC 29.97

PAL 25

SECAM 25

Motion Picture Film 24

NTSC was 30 fps for black-and-white TV, Frame rate was lowered to 29.97 fps to accommodate for color encoding.

Lines

• Old televisions (CRT technology) are made up of horizontal lines.

• Lines are drawn (refreshed) across the screen one line at a time. – NTSC frame: 525 lines (480 picture safe zone). – PAL and SECAM frames: 625 lines (576 picture

save zone).

Columns?

• The signal breakdown into lines (rows) is needed for CRT technology to properly display the picture

• In principle, the rows could have been continuous and the signal could be broken down into columns

No Pixels

• Analog video signals transmit each line as a continuous signal, i.e., no pixels at all.

– The only reason the signal is broken down into rows is for 2D display on a CRT.

– The laws of physics limit how fast the electrical signal can be sent, the resolution is only limited by the speed and density of the signal.

Progressive Scan vs. Interlacing

• Progressive Scan: Computer monitors (CRT) display lines from top to bottom in one pass.

• Interlacing: Television Standards (NTSC, PAL, and SECAM) display the picture in two passes: – first pass is odd-numbered lines (upper field)– second pass is even-numbered lines (lower field)

Interlacing – Progressive Scan

Interlacing

• The original video may have had all the lines (odd and even) for each frame

• But, the broadcast signal only includes half the lines for each frame

• Each frame alternates• Frame 1 odd lines• Frame 2 even lines• Frame 3 odd lines• Frame 4 event lines• …

Why Interlace?

• It removes the flicker effect, without the need to increase the frame rate.

• Progressive Scan at 15 frames per second flickers, i.e., a noticeable blinking, like a strobe light.

• To remover the flicker, you’d have to increase the frame rate, which would require sending more data– 24+ frames per second instead of 15.

Refreshing

• 60 Hertz – each pixel is refreshed 60 times per second.

• With Interlacing, you only have to refresh a pixel 30 times a second with no noticeable difference.

Time

Refreshing

• Interlacing + latency

Time

Interlacing Artifacts

Upper Field & Lower Field Demo• http://media.pearsoncmg.com/ph/esm/esm_

wong_dmp_1/chapter06/ylwong-video-interlace-fields.html

Artifact Example• http://media.pearsoncmg.com/ph/esm/esm_

wong_dmp_1/chapter06/ylwong-dv-interlace-demo.mov

Overscan

• CRTs distort the signal near the edges the picture tube.– Caused by noise in the signal

• By design, the plastic frame of the TV coversthe distorted area.

• The size of the covered area depends on the quality of both the CRT and the signal

Overscan & Save Zones

• For NTSC The signal sent is larger (525 lines) than what you see (480-520 lines).

• The area you do NOTsee is called the overscan

• TV Broadcast(especially News), has to be carefulnot to display importantinformation inthe overscan area.

• 480 lines is the save zone

TV Color Format

• Remember how Digital Images were encoded using RGB or CYMK.

• Analog TV actually breaks the color signal into only two parts

1. Luminance (Brightness)2. Chrominance (Hue/Color)– When there were only B&W TV’s the signal was only a

luminance signal.– Chrominance was added later and it lengthened the

signal, which decreased the Frame rate.

OutlineVideo =

Motion Pictures

Analog Television Video

Digital Television Video

Analog vs. Digital Video

Interlacing & Digital HDTV

An interlaced signal is not meant for LCD and

Plasma display technologyLCD and Plasma TVs display

an entire frame in one burst (called a refresh).

CRT technology naturally displays lines drawn from

top to bottom, rapidly.

Interlacing & Digital HDTV

• While LCD technology can achieve refresh rates of over 200 Hertz (200 frames per second)…

• The problem is that an analog TV signal is interlaced (sends half the lines 30 times a second).

• Also, until recently TV signals were analog.

Analog Digital Problem

• With an analog signal, LCD or Plasma TVs have to1. Wait for two frames (odd lines then even) and

put them together, or2. Interpolate (approximate) the missing lines

Both require added hardwareOne of the reasons these TVs are more expensiveOne of the reason they should be cheaper in the future,

i.e., no need to worry about analog signals anymore

Which do you think is better?

1. Wait for two frames (odd lines then even) and put them together, or

2. Interpolate (approximate) the missing lines

Interpolation

HDTVs and Analog Signals

• The video quality of HDTVs (high resolution LCD or Plasma) appears poor when given an analog signal.

• Because the– Interlaced signal need to be interpolated to a

progressive signal (now a minor issue)– Signal can only be up-converted to a NTSC safe area

frame 640 X 480• Consider that a 20” LCD ($150) has 1280 X 1024 pixels. Over 4

times as many pixels as the received signal.• Buying an HTDV without a digital video source is a waste of

money.

Standard-Definition Digital TV Signals

•24 fps•30 fps•60 fps

640 x 480 progressive

•60 fps640 X 480 interlaced

•24 fps•30 fps•60 fps

704 X 480 progressive

•60 fps704 X 480 interlaced

High-Definition Digital TV Signals

• 24 fps (720/24p) • 30 fps (720/30p)• 60 fps (720/60p)

1280 × 720progressive (720p)

• 60 fps (1080/60i)1920 X 1080interlaced (1080i)

• 24fps (1080/24p)• 30 fps (1080/30p)

1920 X 1080progressive (1080p)

What Signal will be transmitted when Broadcast TV goes Digital?

• It can be any one of the Standard or High Definition formats I just listed.– 720/24p will probably be the standard definition default– 1080/24p will likely be the HD default– 1080i was supposed to be used because it works nicer

with CRTs, but CRTs will be phased out of existence soon.• Depends on the TV network’s system or your signal

provider.– TV Network = ABC, NBC, Local Affiliates, etc.– Signal provider = DirectTV, TimeWarner, etc.

OutlineVideo =

Motion Pictures

Analog Television Video

Digital Television Video

Analog vs. Digital Video

Analog Video Signal

• The details here are not 100% correct, but the principle is…– Like the Bohr Model of the Atom

• Think of an analog TV signal as a wave

• The amplitude (height) indicates the brightness• The frequency (pattern) indicates the color• A segment of the signal generates a line on the CRT

Digital TV Signal

• This is closer to being 100% correct• Think of a digital signal as a wave

• Where wave/no wave indicate 1 or 0• 1 0 0 0 0 0 1 1 0 1 0 0 1 0 0 1 1 1 0 0• Each frame is encoded as bitmap/raster image just

as we previously studied.• Color is RBG 24-bit• Obviously, compression is used

Digital Video Fundamentals

• Sampling rate is…– Capture resolution or each frame

i.e., 640 X 480 vs. say 1920 X 1080– Capture rate

i.e., 30 frames per second vs. say 60• Quantization Level is still– The color depth

i.e., 8 bit color vs. say 24-bit