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Chapter 29 Multimedia (Detailed Notes)

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Chapter 29: Multimedia (Detailed Notes)

Overview

Recent advances in technology have significantly transformed how we use audio and video. Previously, we accessed audio via radio and video via television, with interactive communication through telephones. Today, people use the Internet not only for text and images but also for audio and video services.

Categories of Audio/Video Services

There are three broad categories of Internet-based audio/video services:

  1. Streaming stored audio/video: On-demand content where the user can start playback before downloading completes. Examples include music files, movies, and TV shows.

  2. Streaming live audio/video: Broadcasted in real-time through the Internet, such as Internet radio or potential future Internet TV.

  3. Interactive audio/video: Used for live communication between individuals, such as Internet telephony and video conferencing【7:0†source】【7:1†source】.

Digitizing Audio and Video

Audio and video must be digitized before being transmitted over the Internet.

  • Digitizing audio: An analog audio signal is converted into digital form using methods like sampling, with common rates being 8000 samples/second for voice and 44,100 samples/second for music. These generate 64 kbps for voice and up to 1.411 Mbps for stereo music.

  • Digitizing video: Videos are composed of frames (sequences of images). A common frame rate is 25 frames per second. For color video, each frame can consist of millions of pixels, leading to very high data rates (e.g., 944 Mbps). Compression is essential for reducing the data size to a manageable level【7:2†source】【7:11†source】【7:14†source】.

Compression Techniques

  1. Audio Compression

    • Predictive encoding: Used for speech, encoding differences between successive samples to save bandwidth.

    • Perceptual encoding (MP3): Takes advantage of psychoacoustics (how humans perceive sound) to compress music by masking certain frequencies and reducing temporal redundancy【7:12†source】.

  2. Video Compression

    • JPEG: Compresses images by dividing them into blocks of pixels and reducing redundant information.

    • MPEG: Combines spatial compression (compressing individual frames) and temporal compression (removing redundant frames across time). There are three types of frames:

      • I-frames: Self-contained, independent frames.

      • P-frames: Predicted frames, relying on previous frames.

      • B-frames: Bidirectional frames, relying on both preceding and following frames【7:12†source】【7:17†source】.

Streaming Stored Audio/Video

Streaming stored audio/video involves downloading content to a client for playback. There are several methods for this:

  1. Using a web server: The simplest method where the entire file is downloaded and then played, though the delay can be significant for large files.

  2. Using a metafile: A more efficient method where a small metafile directs the media player to stream the content.

  3. Using a media server: A dedicated server that streams content using protocols like UDP, avoiding the delays caused by TCP retransmissions【7:15†source】【7:19†source】.

Streaming Live Audio/Video

This method mirrors traditional broadcasting but through the Internet. It involves multicasting (sending to multiple users) and is sensitive to delay. Currently, live streaming often uses TCP and unicast, though multicasting via UDP and RTP (Real-Time Protocol) would be more efficient【7:19†source】.

Real-Time Interactive Audio/Video

Real-time interactive services, such as voice over IP (VoIP) and video conferencing, allow people to communicate live. This requires specialized handling in networks to maintain the timing and quality of the transmissions. Protocols like RTP are crucial for managing the real-time flow of data【7:16†source】.

Key Protocols for Multimedia

  1. RTP (Real-Time Protocol): Supports sequencing, time-stamping, and mixing of audio/video streams.

  2. RTCP (Real-Time Control Protocol): Provides control features like flow control and feedback to optimize quality.

  3. VoIP (Voice over IP): A real-time interactive audio service.

  4. SIP (Session Initiation Protocol): Establishes, manages, and terminates multimedia sessions.

  5. H.323: An ITU standard that connects telephone networks with the Internet for voice communication【7:0†source】【7:19†source】.

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