Introducing video codecs

A codec is quite simply an algorithm. An algorithm is a structured list of instructions that is capable of performing a task, and in the context of a codec, these algorithms define the method used to compress data into fewer bytes. A key feature of a codec though is that these instructions can be reversed to obtain the uncompressed file. By following these instructions, applications such as encoders and players can reliably compress and decompress data. Some codecs are ‘lossless,’ meaning that the data they encode can be reconstructed in their entirety, while others are ‘lossy,’ which means that the encoded data, when decoded, may not contain some of the information or ‘bits’ that it previously did. The MP3 audio codec, for example, eliminates sound in frequencies that are inaudible by humans, as part of its strategy to reduce audio file size. This makes it a lossy form of compression, as the original sounds cannot be reconstructed in their entirety.

The primary function of a multimedia codec is to reduce the size of a media file, enabling a higher level of portability and distribution across a network, such as the internet. Most codecs also allow content authors to determine the bit rate of a streaming file, which coupled with the duration of the media, determines the size of the compressed file. The quality of a media file is determined by many factors. Even though the effects of these factors change with different compression technologies, they apply uniformly across most codecs. One of these factors is the bit rate of the file, which is an important consideration when using codecs for specific tasks. For a given codec, a higher bit rate will result in higher quality, but the bit rate itself is determined by factors such as frequency and the number of channels in audio files, and factors such as image resolution, frame-rate and colour depth of the image in video files. The bit rate is in fact a function of the above-mentioned factors.

Next week we will catch a glimpse of the Windows Media 9 series of codecs that was adopted as an open standard in April this year as VC-1, and looks set to make a big impact in the world of multimedia. You too can join in the discussion by writing into technopage@gmail.com

Print this!

There are several major printer technologies available. These technologies can be broken down into two main categories with several types in each:

Impact printers

These printers have a mechanism that touches the paper in order to create an image. There are two main impact technologies:

*Dot matrix printers use a series of small pins to strike a ribbon coated with ink, causing the ink to transfer to the paper at the point of impact.

*Character printers are basically computerised typewriters. They have a ball or series of bars with actual characters (letters and numbers) embossed on the surface. The appropriate character is struck against the ink ribbon, transferring the character's image to the paper. Character printers are fast and sharp for basic text, but very limited for other use.

Non-impact printers

These printers do not touch the paper when creating an image. Inkjet printers and laser printers are well known technologies that are part of this group, which also includes:

* Inkjet printers use a series of nozzles to spray drops of ink directly on the paper.

* Laser printers use dry ink (toner), static electricity and heat to place and bond the ink onto the paper.

* Solid ink printers contain sticks of wax-like ink that are melted and applied to the paper. The ink then hardens in place.

* Dye-sublimation printers have a long roll of transparent film that resembles sheets of red, blue, yellow and grey-coloured cellophane stuck together end to end. Embedded in this film are solid dyes corresponding to the four basic colours used in printing: cyan, magenta, yellow and black (CMYK). The print head uses a heating element that varies in temperature, depending on the amount of a particular colour that needs to be applied. The dyes vaporise and permeate the glossy surface of the paper before they return to solid form. The printer does a complete pass over the paper for each of the basic colours, gradually building the image.

* Thermal wax printers are something of a hybrid of dye-sublimation and solid ink technologies. They use a ribbon with alternating CMYK colour bands. The ribbon passes in front of a print head that has a series of tiny heated pins. The pins cause the wax to melt and adhere to the paper, where it hardens in place.

* Thermal auto chrome printers have the colour in the paper instead of in the printer. There are three layers (cyan, magenta and yellow) in the paper, and each layer is activated by the application of a specific amount of heat. The print head has a heating element that can vary in temperature. The print head passes over the paper three times, providing the appropriate temperature for each colour layer as needed.

 
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