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)

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

years ago

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-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 DA B C DB D C AB 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