What is an MPEG File?
An MPEG file refers to a digital video (and often associated audio) file encoded using standards developed by the Moving Picture Experts Group (MPEG). MPEG itself is not a single codec but a family of international standards for audio and video compression and transmission.
MPEG files typically use the file extensions .mpg or .mpeg, though they can also use other extensions depending on the specific MPEG standard used and the container format (e.g., .mp4 for MPEG-4 Part 14).
Key Facts About MPEG Files
Compression Focus: The core purpose of MPEG standards is to compress large amounts of audio and video data efficiently. This compression significantly reduces file sizes compared to uncompressed formats, making storage and transmission (like over the internet or broadcasting) feasible.
Lossy Compression: Most MPEG video formats use lossy compression. This means some original data is permanently discarded during encoding to achieve high compression ratios. The goal is to discard data least perceptible to the human eye/ear, aiming to maintain acceptable quality at much smaller sizes.
Common MPEG Standards
- MPEG-1: The first widely successful standard (early 90s). Crucial for Video CDs (VCD) and the basis for the original MP3 audio format (.mp3). Offers relatively basic quality by modern standards.
- MPEG-2: The dominant standard for DVD-Video, digital television broadcasting (ATSC, DVB, ISDB), and early HDTV. Provides significantly better quality than MPEG-1 and supports interlaced video. Often found in .mpg, .mpeg, .vob (DVD), and .ts (transport stream) files.
- MPEG-4: A much broader standard (late 90s onwards) focusing on multimedia delivery across diverse networks (including low-bandwidth internet). Encompasses advanced video codecs like:
- MPEG-4 Part 2 (DivX, Xvid compatible): Offered improved efficiency over MPEG-2 but largely superseded by H.264/AVC.
- MPEG-4 Part 10 / H.264 / AVC: Arguably the most influential video codec ever. Became the standard for HD video on Blu-ray, internet streaming (YouTube, Netflix, Vimeo), broadcasting, video conferencing, and more. Commonly wrapped in .mp4 or .mkv containers.
- MPEG-H Part 2 / H.265 / HEVC: The successor to H.264/AVC, providing roughly double the compression efficiency, enabling 4K UHD and HDR content at manageable file sizes and bitrates. Also often found in .mp4 or .mkv containers.
Core Functionality: Motion Compensation & Prediction
MPEG video compression relies heavily on:
- Inter-Frame Compression: Instead of storing every frame (image) completely, MPEG stores key frames (I-frames) fully. Subsequent frames (P-frames and B-frames) only store the differences from reference frames (I or P frames). This exploits temporal redundancy (areas that don't change between frames).
- Motion Vectors: For moving objects, the encoder calculates how blocks of pixels moved between frames and stores this motion information (vectors), rather than all the pixel data again.
Advantages & Disadvantages
- Advantages:
- Small File Sizes: Highly efficient compression.
- Universal Compatibility: Extremely wide support across devices, software, and platforms (especially MPEG-2, H.264).
- Proven Standards: Backed by international bodies (ISO/IEC).
- Disadvantages:
- Loss of Quality: Artifacts (blockiness, blurring) can occur, especially at high compression levels.
- Processing Overhead: Decoding complex MPEG streams (especially newer standards like HEVC) requires significant computing power.
- Patents/Licensing: Implementing some MPEG standards (like AVC, HEVC) may require licensing fees.
In essence, an MPEG file is a compressed video (and audio) file adhering to one of the MPEG family of standards. These standards revolutionized digital video by enabling practical storage and transmission, with MPEG-2 (DVD, Broadcast TV) and H.264/AVC (online video, Blu-ray) being particularly defining formats globally. Newer standards like HEVC continue this evolution for higher resolutions.
