Video Data

Topic 4: Multimedia Technology

What is Video?

• A video is just a collection of bit-mapped images that when played quickly one after another give the illusion of a moving image

• It is a sequence of individual pictures or frames

• The standard rate at which these frames are taken is 25 frames per second (this gives realistic movement)

• Work in the same way as digital cameras

• They use the same light sensors, called CCDs

• They have a lower resolution than cameras

• They are designed to capture lots of images (frames) fast

Input (Capture):Digital Video Camera (Camcorders)

Input (Capture):Digital Video Camera (Camcorders)

• Consumer cameras use a RGB filter which is of poorer quality than professional ones.

• Professional cameras use 3 CCDs each one with a different colour filter: – Red, Green & Blue

25 - 30 years ago

20 - 25 years ago

10 – 20 years ago

More recently

Input (Capture):Digital Video Camera (Camcorders)

Input (Capture): Webcam

• Designed for the purpose of creating videos to be transmitted over the Internet

• They do not need to be high resolution, keeping down the bit rates and price.

• They use low resolution array CCDs and low quality lenses, keeping costs down

Input (Capture): Video Capture Card

• A video capture card is needed to capture videos from analogue sources– video tape players, television

broadcasts and analogue video recorders

• Some modern graphics cards include the ability to capture video

Input (Capture): Specialist Video Capture Card

• They often have faster ADCs and can capture videos at higher bit rates

• They usually capture sound as well. – Allows synchronisation

• Have a hardware codec – Allowing it to be processed and stored in a

compressed format as it is being captured.

Video Data:Storage of Video data

Topic 4: Multimedia Technology:

AVI (Audio Video Interleave)

• Type of RIFF file (container file)

• Uncompressed

• AVI ratio set during saving– The audio is embedded into the video at

different intervals.

• Lossy compression cuts out unnecessary parts of a video clip

• Saves each frame of video as a JPEG– These are called ‘i-frames’

• Data that stays the same in following frames is removed

• The next frames only store data on what has changed since last i-frame

MPEG

MPEG

• MPEG-1 – VHS video quality with 353 x 240 pixels and 30 fps frame rate

support• MPEG-2

– The standard for DVD-Video and Digital Television • MPEG-3

– Intended for HDTV but these revisions were incorporated into MPEG-2)

• MPEG-4 – Designed for low-bandwidth networks - e.g. video phones) (Part

used by DivX• MPEG-7

– Builds on the interactive and extra data capabilities of MPEG-4 and is a full multimedia description format

MPEG Standards

Video Quality And File Size

• Colour depth: – Increasing colour depth improves quality and file

sizes.

• Resolution:– Increasing resolution improves quality and increases

file sizes.

• Frame rate: – Measured in frames per second (FPS). – Increasing frame rate increases file size. Lower

frame rates reduce file size but make video clip ‘jerky’.

Video Quality And File Size

• Video time: – increasing or reducing the time of a video is

the obvious way to affect the file size. – Quality of the display of the clip is not

affected.

• Lossy compression: – Using MPEG4 compression reduces file sizes

without unduly affecting quality.

Calculations

File Size (Bytes) = Frame Size (Bytes) x Frame Rate (fps) x

Video Time (s)

Calculations

Uncompressed AVI

Frame Rate 24 fps

No of Pixels 320 x 240

Colour Depth 16 bits

Duration 9s

File Size =

Video Data:Video Editing

Topic 4: Multimedia Technology:

• Each frame is displayed as a thumbnail image.

• Each frame can be individually edited

• The audio would also be on an timeline

• Some packages provide multiple timelines

Timeline

• Simple plan of final product

• Usually freehand and rough

• Important process for all multimedia applications

• Produced at Analysis stage

Storyboards

• Basically, cutting or removing the parts you don’t want.

• You may want to remove a frame or a whole scene.

Crop

Sequencing

• Once you’ve got your video cropped, edited you have to put it in sequence.

• In other words, put it in the correct order

A B C D

B D C A

Transitions

• Used to ‘join up’ different scenes/frames

• Different types are available, for example:

Wipe

Transitions

• Used to ‘join up’ different scenes/frames

• Different types are available, for example:

Dissolve

Transitions

• Used to ‘join up’ different scenes/frames

• Different types are available, for example:

Box Out

Transitions

• Used to ‘join up’ different scenes/frames

• Different types are available, for example:

Fade

Video Data:Output Hardware

Topic 4: Multimedia Technology:

Output hardware• As mentioned in the topic on bitmap graphics, the

graphics card is responsible for the output of image data.

• Due to the complex codecs and high bit-rates needed to output video data,

• video output cards used to be needed in order for a computer to be able to display videos at full quality.

• Graphics cards have become much more powerful (driven mainly by the games market) and any modern graphics card should be able to comfortably decode and display full quality videos on a computer.

Digital to Analogue Converter (DAC)

• As most people have analogue displays (LCD displays with digital (DVI) inputs are available) graphics cards must be able to generate the analogue signals needed for the monitors.

• The DAC changes the digital video signal in the computer into an analogue video signal that the monitor can use to display the image.

Digital Signal Processor

• The Digital Signal Processor (or GPU) plays a key role in allowing computer to display full-quality videos.

• The GPU on the graphics card is responsible for decoding the video signal

• Takes the pressure off the CPU