Category: Audio and video

The world is in a slow but constant evolution: many times without realizing it, we are abandoning things that we did in the past to adapt to the new times. How to buy film roll boxes, in the corner bracket, to place in the camera. Whether on the camera phone or the digital camera itself, that no longer matters to us. But at that time, at the height of analog photography, it was common to know how to order one type of film or the other, saying for example “ASA 100 film” or “ASA 400 film”. There was another specification, very popular in Germany, DIN, with values ​​as similar as the Celsius and Fahrenheit temperature scales … Looking for an international standardization, the ISO (International Organization for Standardization),

The specifications of film labels were standardized, but the world presented new challenges: computers, increasingly present in our daily lives, brought digitization to the living room or the television: the old VCR gave way to the video player. DVD. Still images and then videos invaded the cell phone. However, to get here, new standards have become necessary over time.

One of them, older, concerned digital photographic images. The same ISO for film boxes (which also created dozens of other standards) joined in 1986 with the CCITT (Consultatif International Téléphonique et Télégraphique Committee) to create the Joint Group of Photographic Experts, whose work would result in 1994 in the JPEG compression standard. . for digital still images.

Compression is necessary in the digital world to reduce the high volume generated by numerical information resulting from audio and video digitization processes. In this process thousands of samples of each part of the image or sound are obtained, which are then converted into sequences of numbers that represent the brightness / color / sound values ​​of the samples. Then it is necessary to have a storage medium capable of storing all these numbers. Since these media typically do not contain as much information, the compression process comes into play.

And that’s not all: compression also has advantages in video transmission, allowing you to send a signal with better quality, compressed, occupying the same space as a lower quality signal, without compression.

In the JPEG standard, the image is divided into 8×8-pixel squares and by complex mathematical calculations an ‘average’ color is determined for these 64-pixel blocks. This results in loss of detail in compression. The processes in which this occurs are known as “lossy”. There are processes in which there is no loss (‘lossless’), used for example in the GIF standard (Graphic InterFace, developed by Compuserve) to compress photographic images. However, standards such as GIF are suitable for graphics and not for photographic images, because the set of colors treated by them (256 colors) is much smaller than that used in standards such as JPEG.

In January 1988, ISO formed a working group called the Motion Picture Experts Group together with the IEC (International Electrotechnical Commission), generating an acronym that would become well known: MPEG. The group, formed to create international video and audio compression standards, then created the MPEG-1, MPEG-2 and MPEG-4 algorithms that are used today.

The use of MPEG algorithms always involves two phases: the compression phase, where the image and / or audio are compressed, generating content with a smaller total size, ready to be recorded on an optical disc, for example, and the recovery phase of this content, also called expansion, where the original image and / or audio is reconstituted. Compression is done by software called a compressor and expansion by software called an expander. These names have better known synonyms, encoder and decoder respectively. And the set of 2 forms the word codec, short for encoder (encoder) + decoder.

The MPEG standards brought a new philosophy in coding systems, where the encoder process is much more laborious and complex than the reverse of the decoder. Since decoding is simple and fast, it facilitates the proliferation of electronic circuits that perform this process efficiently, making players cheaper, an ideal situation for content distribution. This fact also allows new encoders to emerge over time, using different approaches in compression processes. There are thus encoders that produce better images than others, in addition to being generally parameterized, that is, the compression quality can be adjusted.