MPEG stands for the Moving Pictures Experts Group, an organization cooperating with International Standards Organization (ISO) for developing new standards for digital audio and video. Its first standard MPEG-1 was released in 5 parts during the period from 1993 to 1999. This standard led to all the modern digital audio/video compression standards adopted by the ISO. MPEG-2 and MPEG-4 are two major releases of the MPEG standards.
MPEG-2
MPEG-2 was developed to overcome the shortcomings of the MPEG-1 standard. MPEG-1 had audio compression system limited to two channels (stereo) and, for interlaced video, had standardized support with poor compression. Also, it had only one standardized “profile” (Constrained Parameters Bitstream), which was unsuitable for videos with higher resolution. MPEG-1 could support 4k video, but encoding video for higher resolutions was difficult. There were discrepancies in identifying the hardware that supported such encoding. Also, the colors were limited only to 4:2:0 color space.
MPEG-1 evolved into MPEG-2 by sorting the above issues. The eleven parts of the standard were released from 1996 to 2004, and still the standards are updated. Part 8 was abandoned due to lack of interest in the industry. The video compression standard is H.263 and specified in Part 2 while audio advancements are specified in Part 3 and Part 7. Part 3 defines multichannel specification and Part 7 defines the Advance Audio Encoding. The parts of the specification that define different aspects are shown below;
• Part 1-Systems: describe the synchronization and multiplexing of digital audio and video.
• Part 2-Video: compression coder-decoder (codec) for interlaced and non-interlaced video media signals
• Part 3-Audio: compression coder-decoder (codec) for perceptual coding of audio media signals. This enables multichannel extension and bit rates and sample rates for MPEG-1 Audio Layer I, II and III of MPEG-1 audio are also extended.
• Part 4: Methodology for testing compliance.
• Part 5: Describes systems for Software simulation.
• Part 6: Describes extensions for Digital Storage Media Command and Control (DSM- CC).
• Part 7: Advanced Audio Coding (AAC).
• Part 9: Extension for real time interfaces.
• Part 10: Conformance extensions for Digital Storage Media Command and Control (DSM-CC).
• Part 11: Intellectual property management (IPMP)
MPEG-2 standard is used in DVD`s and Digital television broadcasting methods (ISDB, DVB, ATSC). It is the base standard for MOD and TOD video formats. XDCAM is also based on MPEG-2.
MPEG-4
MPEG-4 is the latest standard defined by MPEG. It incorporates the features of MPEG-1 and MPEG-2 with the newer industry technologies and features such as Virtual Reality Modeling Language (VRML), 3D rendering, object-oriented composite files and facilitates the structure for externally specified Digital Rights Management. It was initiated as a standard for low bit-rate video communications, but later morphed into a comprehensive multimedia coding standard. MPEG is still a developing standard.
MPEG-4 Part 2 describes the visual aspects and forms the basis of the Advanced Simple Profile used by codecs integrated into software such as DivX, Xvid, Nero Digital, and 3ivx and by QuickTime 6. MPEG-4 Part 10 describes the video aspects of the standard. MPEG-4 AVC/H.264 or Advanced Video Coding used in the x264 encoder, Nero Digital AVC, and HD video media like Blu-ray Disc are based on this. Following is a summary of the Parts included in the specification of the standards.
• Part 1: Systems
• Part 2: Visual
• Part 3: Audio
• Part 4: Conformance testing
• Part 5: Reference software
• Part 6: Delivery Multimedia Integration Framework (DMIF)
• Part 7: Optimized reference software for coding of audio-visual objects
• Part 8: Carriage of ISO/IEC 14496 contents over IP networks
• Part 9: Reference hardware description
• Part 10: Advanced Video Coding (AVC)
• Part 11: Scene description and application engine
• Part 12: ISO base media file format
• Part 13: Intellectual Property Management and Protection (IPMP) Extensions
• Part 14: MP4 file format
• Part 15: Advanced Video Coding (AVC) file format
• Part 16: Animation Framework eXtension (AFX)
• Part 17: Streaming text format
• Part 18: Font compression and streaming
• Part 19: Synthesized texture stream
• Part 20: Lightweight Application Scene Representation (LASeR) and Simple Aggregation Format (SAF)
• Part 21: MPEG-J Graphics Framework eXtensions (GFX)
• Part 22: Open Font Format
• Part 23: Symbolic Music Representation (SMR)
• Part 24: Audio and systems interaction
• Part 25: 3D Graphics Compression Model
• Part 26: Audio Conformance
• Part 27: 3D Graphics conformance
• Part 28: Composite font representation
• Part 29: Web video coding
• Part 30: Timed text and other visual overlays in ISO base media file format
Parts 29 and 30 are currently under development.
MPEG-4 provides DVD quality video, but consumes a lower bit rate; therefore, it’s feasible to transfer digital video streams over computer networks.
MPEG2 vs MPEG4
• MPEG-2 and MPEG-4 are two releases of the ISO standards for digital audio and video. MPEG-4 is the latest standard.
• Both MPEG-2 and MPEG-4 use lossy compression in the encoding. The MPEG-2 uses H.262 encoding while MPEG-4 uses H.264.
• Compression of the MPEG-4 is relatively complex compared to MPEG-2. Discrete cosine transform, vector quantization, and wavelet compression methods are used to reduce the source signal, resulting in a relatively lower file size.
• MPEG-2 files are larger compared to MPEG-4; therefore, MPEG-4 standards are used in online/network related media transfers.
• MPEG-2 is the DVD quality; the quality of MPEG-2 is superior to MPEG-4, but not suitable for internet or network applications.
• MPEG-2 can have a bit rate in the range from 5 to 80 Mbits/sec. MPEG-4 bit rate is low relative to MPEG-2.