Introduction to MPEG, Media Synchronization – from IP Television Magazine.

MPEG Series

Media Synchronization
by: Lawrence Harte

Media synchronization is the process of adjusting the relative timing of media information (such as time aligning audio and video media). Media synchronization typically involves sending timing references in each media stream that can be used to align and adjust the relative timing of multiple media signals.

Media synchronization is especially important for packet based systems that can have variable amounts of delay between media sources. The variable packet transmission time may result in media components being recreated (rendered) at different times.

To provide media synchronization in MPEG systems, a program clock reference (PCR) is used. A PCR is a reference source of timing information that is used as a reference to all the media streams associated with that program. The PCR is a 42-bit field that is transmitted at least every 100 msec.

This article is Part 8 of a 8 Part Series
MPEG Series List	Month

Introduction to MPEG	Apr 06
Audio Compression	May 06
Video Compression	Jun 06
Media Streaming	Jul 06
MPEG Transmission	Aug 06
Program Tables	Sept 06
*Media Synchronization*	*Oct 06*
MPEG Profiles	Nov 06

Figure 1.28 shows how MPEG can be used to time synchronize multiple media channels with each other. This diagram shows that MPEG channels include program clock reference (PCR) time stamps to allow all of the elementary streams to remain time synchronized with each other.

Figure 1.28., MPEG Media Synchronization

18 Oct 2006 Definitions FREE at www.IPTVDictionary.com

Display Formatting

Display formatting is the positioning and timing of graphic elements on a display area (such as on a television or computer display). Display formatting may combine mixed types media such as video, animation, graphics and interactive controls on a video or television monitor.

MPEG has several protocols that can be used to position and sequence the presentation of media. These protocols include synchronized multimedia integration language (SMIL), binary format for scenes (BIFS) and active format description (AFD).

SMIL is a protocol that is used to control the user interface with multimedia sessions. SMIL is used to setup and control the operation of media files along with the placement and operation windows on the user's display. Binary format for scenes is part of the MPEG-4 standard that deals with synchronizing video and audio. AFD is a set of commands that are sent on a video bit stream that describes key areas of interest in a video or image sequence. The use of AFD allows the receiver or set top boxes to adjust or optimize the display for a viewer.

Figure 1.29 shows how MPEG can use BIFS to position and coordinate the timing of media to different areas of a display. This example shows a television that has is displaying different types of media. In the top left, a video is streaming in the window area. To the top left, an image of a new game that has been released is shown. On the bottom, text is display with interactive buttons on the right of each text line

Digital Video Quality (DVQ)

Digital video quality is the ability of a display or video transfer system to recreate the key characteristics of an original digital video signal. Digital video and transmission system impairments include tiling, error blocks, smearing, jerkiness, artifacts (edge busyness) and object retention.

Video quality is subjective and cannot always be directly related to signal quality levels and the distortion that occurs on analog video that is different than the distortion that occurs on digital signals. Some of the quality impairments that can occur in MPEG systems include tiling, error blocks, jerkiness, artifacts and object retention.

Figure 1.29., MPEG Display Formatting

Tiling is the changing of a digital video image into square tiles that are located in positions other than their original positions on the screen. Error blocks are groups of image bits (a block of pixels) in a digital video signal that do not represent error signals rather than the original image bits that were supposed to be in that image block. Jerkiness is holding or skipping of video image frames or fields in a digital video. Visual artifacts are the unintended, unwanted visual aberrations in an image (such as blocks on a video image or speckles on a picture image around sharp edges). Object retention is the keeping of a portion of a frame or field on a digital video display when the image has changed. Figure 1.30 shows some of the causes and effects of video distortion that may occur in IP Television systems. This example shows	that video digitization and compression converts video into packets that can be sent through data networks (such as the Internet). Packet loss and packet corruption results in distorted video signals. This example shows that some types of distortion include tiling, error blocks and retained images.
		Series Source: Introduction to MPEG $19.99 printed $16.99 eBook

Figure 1.30., Digital Video Quality