Page 188 - DCAP303_MULTIMEDIA_SYSTEMS

Multimedia Systems

notes a codec is a computer program that both shrinks large movie files, and makes them playable on
your computer. Codec programs are required for your media player to play your downloaded
music and movies.
The codec is a short name for coder-decoder, the software that takes a raw data file and turns it
into a compressed file. Since compressed files only contain some of the data found in the original
file, the codec is the necessary “translator” that decides what data makes it in to the compressed
version and what data gets discarded.
Different codecs translate in different ways, so a video file compressed using the Intel Indeo
codec will be different from a file compressed using the Cinepak codec. For example, sometimes
the difference is noticeable, sometimes not, but it is good to be aware of what codecs are best for
what you are trying to do in order to maintain the best ratio of file size to quality.

11.2 Compression Quality

• Lossy codecs: Many of the more popular codecs in the software world are lossy, meaning
that they reduce quality by some amount in order to achieve compression. Often, this type
of compression is virtually indistinguishable from the original uncompressed sound or
images, depending on the codec and the settings used. Smaller data sets ease the strain on
relatively expensive storage sub-systems such as non-volatile memory and hard disk, as
well as write-once-read-many formats such as CD-ROM, DVD and Blu-ray Disc. Lower
data rates also reduce cost and improve performance when the data is transmitted.
• Lossless codecs: There are also many lossless codecs which are typically used for archiving
data in a compressed form while retaining all of the information present in the original
stream. If preserving the original quality of the stream is more important than eliminating
the correspondingly larger data sizes, lossless codecs are preferred. This is especially
true if the data is to undergo further processing (for example editing) in which case the
repeated application of processing (encoding and decoding) on lossy codecs will degrade
the quality of the resulting data such that it is no longer identifiable (visually, audibly or
both). Using more than one codec or encoding scheme successively can also degrade quality
significantly.

The decreasing cost of storage capacity and network bandwidth has a tendency
to reduce the need for lossy codecs for some media.

11.3 types of Compression

11.3.1 Lossless Compression algorithms
There will be no data loss in this type of compression as it is defined by the name. Both original
data and the compressed data are the same in this compression. The algorithms for the compression
and decompression are exact inverse of each other in the Lossless Compression. The main
mechanism in this compression is removing the redundant data in the compression and adding
them in the decompression.
Advantage: The original format of the data remains even it is compressed.
Disadvantage: Reduction of the size of the data is a small. Sometimes the size can increase instead
of decrease.
run-length encoding
Run-length encoding can be named as the simplest method of encoding. Any combination of
symbols can be compressed by this encoding technique. The concept of this algorithm is removing
continuous sequence of symbols using another shorten symbol which represent it. Practically
what is does is to replace the number of occurrences with the numeric value and the symbol itself.
For example, a text like ‘AAAACBBBBB’ is replace by ‘A04C01B05’. This method is very much
efficient when there are only two symbols.

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