Institute of Information Science - University of Miskolc

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===== Principles of MPEG Compression =====

**MPEG** compression offers various methods for both video and audio. In **MPEG**, it is crucial to keep audio and video synchronized over time, so audio and video data include timestamps.

**MPEG** (Moving Picture Experts Group) compression is a widely used standard for reducing the file size of video and audio content while maintaining an acceptable level of quality. It achieves high compression ratios by eliminating redundant information spatially within a frame and temporally between consecutive frames.

**Temporal Redundancy Removal**: Temporal redundancy exists because consecutive video frames often have many similarities. MPEG compression uses motion compensation to identify and encode only the differences between frames, instead of storing every frame in full detail. This is done by using the following frame types:

  * **I-frames** (Intra-coded frames): are complete images, compressed independently of other frames, much like a JPEG image.

  * **P-frames** (Predicted frames): store only the changes from the preceding I-frame or P-frame, thus reducing the required data.

  * **B-frames** (Bidirectional frames): use data from both preceding and succeeding frames, which allows for even greater compression by predicting changes from both directions.

**Spatial Redundancy Removal**: Spatial redundancy within a single frame is reduced using techniques similar to JPEG compression, including Discrete Cosine Transform (DCT) to convert spatial domain data into frequency domain data, allowing for more efficient quantization and compression.

**Audio Compression**: MPEG audio compression, such as MPEG-1 Layer 3 (commonly known as MP3), also takes advantage of the human auditory system's limitations. By discarding inaudible parts of the sound, MPEG audio can significantly reduce the bit rate without a perceivable loss in quality. Psychoacoustic models identify which parts of the audio can be removed or more coarsely quantized without affecting the listening experience.

**Applications**: MPEG compression is used in various multimedia applications, including digital television, video streaming, DVD, and Blu-ray discs. Its ability to maintain synchronization between audio and video streams makes it suitable for broadcasting and interactive media applications.

==== Container Types for MPEG Compression ====

**MPEG** compression is often used with different container formats that store video, audio, subtitles, and other metadata. Some common container formats include:

**MP4 (MPEG-4)**: MP4 is one of the most popular container formats for storing video, audio, subtitles, and images. It is highly compatible with various devices and platforms, making it ideal for streaming and sharing. MP4 supports various codecs, including MPEG-4 and H.264, and is commonly used for online video and mobile applications.

**AVI (Audio Video Interleave)**: AVI is a container format introduced by Microsoft. It is known for its simplicity and compatibility with many media players. However, AVI files tend to be larger than other formats due to limited compression options. While it can use various codecs, AVI is less efficient for streaming applications.

**FLV (Flash Video)**: FLV is a container format developed by Adobe and commonly used for delivering video content over the internet. It was widely used on platforms like YouTube in the early days. FLV supports streaming and progressive download, making it suitable for web-based video, but it has fallen out of favour in recent years due to the decline of Adobe Flash.

**MKV (Matroska Video)**: MKV is an open-source container format with unlimited video, audio, subtitle tracks, and other metadata. It is highly versatile and often used for HD video. MKV is known for its support of different codecs and its flexibility, making it a popular choice for storing Blu-ray rips and high-quality video files.

**MPEG-TS (Transport Stream)**: MPEG-TS is a container format designed for broadcasting and streaming applications. It is used for live video broadcasting, such as digital television and over-the-air broadcasts, as it can handle transmission errors and provide synchronization for continuous streams.

**MOV**: Developed by Apple, MOV is similar to MP4 and is often used with Apple's QuickTime framework. It can store multiple tracks, including video, audio, and text, making it suitable for editing and playback. MOV files are commonly used in professional video editing environments.

Each of these container formats has its own advantages and is chosen based on the specific requirements of the content delivery, such as compatibility, compression efficiency, and intended use. The container format determines how the compressed video and audio streams are packaged together, including information on storing the video frames, audio samples, and synchronization data. Video is typically stored as a series of compressed frames (using codecs like H.264 or H.265), while audio is stored as compressed streams (e.g., MP3, AAC). The container also includes metadata that helps synchronize audio and video during playback, ensuring that lip-sync and timing are maintained. This affects playback capabilities and compatibility with different devices and software.