CS533 Information Retrieval Systems - Technical Presentation

BILKENT UNIVERSITY CS533 Information Retrieval Systems Technical Presentation Web Site

Barýþ UZ

Index of this page

Introduction

Push Technologies

Channels

CGI

ASP

Java

Streaming Media

Overview

Audio files

Video files

Links

A video or movie file is a digital representation of sounds and graphics. Each movie consists of a sequence of still images and an audio track. The frame rate, or number of frames captured per second, is the primary measure of reproduction quality.

Since video files need to store a lot of data, they can become quite huge. To demonstrate how big they can get, consider a 24-bit, full screen, 30 frames/sec, video file. It would consume:

3 (bytes) x 640x480 (pixels) x 30 (frames) x 60 (seconds) = 1,658,880,000 bytes/min

or 1.6 gigabytes of storage space for a minute of video, and that doesn't include the audio track, which would add another 5 to 10 megabytes.

Of course, there are several ways in which file size can be reduced:

Reduce the size of the playback window. For example, video files played over the Internet use a standard 160 x 120 pixels. However, the smaller the window, the harder to see detailed movements.

Decrease the number of colours, from 16 million to 256 or even 16 colours. Of course, visual quality is also reduced.

Reduce the frame rate from 30 down to 15 or less frames per second. The lower the frame rate however, the more "jerky" the motion becomes.

Compress the file. The best video compression techniques today can shrink file size in the order of 100:1.

Common Video File Formats

MPG/MPEG: has become probably the most popular format, and was developed by the Moving Pictures Expert Group, a group of over 70 multinational organisations. MPEG files use a powerful lossy compression routine, similar to JPEG image files. Additionally, it utilises a technique known as interframe compression, which takes advantage of the fact that most of the time, there is very little change from one frame to the next. In simple terms, a frame is compared to those immediately preceding and following it, and only the differences (i.e. the pixels that have changed) are stored. This enables a substantial reduction in the size of the subsequent file.

MOV: The Quicktime format was created by Apple for use in Macintosh computers, although Quicktime viewers are available for other platforms. These files can contain data of various formats, since Quicktime does not define a format at all, but a set of graphics display routines that are used to interact with the Macintosh Operating System. These routines can optimise playback on particular platforms.

AVI: The MS-Windows standard video format, and is becoming more popular on the Internet. Like Quicktime, it is an interface into a proprietry set of Windows graphics display routines that a video format as such, and so can be used to encapsulate data using the MPEG routines.

RealVideo: This format is the video equivalent of RealAudio sound files. It is a streaming format that allows a standard video file, such as an MPEG, to be viewed as it is received across the internet. This removes the need to download the entire file before it is viewed, meaning that the download can be stopped if the file being watched is not what was required, and lets program length video data be transmitted across the net. RealVideo streaming technology is lately finding new applications in areas such as corporate video conferencing.

PREVIOUS TOPIC	UP (First page of this topic)		FOR INTRODUCTION, CLICK HERE.
	PREVIOUS PAGE (of this topic)	NEXT PAGE (of this topic)