AVEM Classroom 03 - Signal compression

Compression allows us to reduce the size of a data stream or file. Simple Compressin process take advantage of repeated data patterns in a signal or file to reduce the number of times the pattern is stored or transmitted. The orginal signal can later be mathematically reconstructed to its original size and pattern without  any loss of accuracy. This is known as lossless compression and can work well in compressing text, music, and video streams, although the amount of compression can be relati-vely low.

 ZIP, FLAC, MPEG-2, PNG, and JPEG2000 are some examples of efficient lossless compression methods. Only lossless compression techniques are considered to be acceptable  for critically accurate applications such as text transmission and financial transactions.

Advance compression processes are based on the human brain's inability to perceive all of the available detail in an image or a sound, and they use a var-iety of signal processing techniques to reduce the amount of unnoticeable detail transmitted or stored. This is known as lossy compression and can b almost undetectable at moderate compression levels, although the lack of fidelity becomes quite noticeable at high compresion levels. MP3, JPEG, HEVC, and H.264 (aka AVC) are examples of high-efficiency lossy compression methods.

The critical steps in the compression process are analyzing and encoding the digital data to reduce its size, and later decoding the compressed data into its original format. The device and software processes that perform these functions are known as codecs.

Digital Media Formats

A digital media file contains two elements.

• Container  The container (such as AVI, which stands for Audio Video Interleaved) is the structure of the file where the data is stored. The container holds the metadata (data about the data) describing how the data is arranged and which codecs are used.
• Codec  The codec provides the method for encoding (compressing) and decoding (decompressing) the file. Many video and audio codecs must be installed in the operating system to play the file.

Some sophisticated replay applications such as the multiplatform, open source VLC media player can decode a wide variety of media file formats. Many applications and online services are available for converting media file between formats.

Formats can be confusing because the term codec is used interchangeably to describe the container and the codes used within the container. In addtion, some codec names describe both a codec and a container. An AVI container, for instance, could contain data encoded with an MPEG codec.