Before audio and video can be transmitted over a computer network, it must be digitzed and compressed.
Compression is important because uncompressed audio and video comsume a tremendoous amount of storage and bandwidth;
removing the inherent redundancies in digitized audio and video singals can reduce the amount of data that needs to be stored
and transmitted by orders of magnitude.
The fields of audio and video compression are vast. They have been active areas of research for more than fifty
years, and there are now literally hundreds of popular techniques and standards for both audio and video compression.
Audio Compression in the Internet
A continuoously varying analog audio signal is normally converted to a digital signal as follows:
- The analog audio signal is first sampled at some fixed rate. The value of each sample is an arbitrary real number.
- Each of the samples is then rounded to one of a finite number of values. This operation is referred to as quantization.
The number of finite values--called quantization values--is typically a power of two.
- Each of the quantization value is represented by a fixed number of bits. Each of the samples is coverted to its
bit representation. The bit representations of all the samples are concatenated together to form the digital representation
fo the signal.
Vidoe Compression in the Internet
A viedo is a sequence of images, typically being displayed at a constant rate. There are two types of redundancy
in video, both of which can be exploited for compression. Spatial redundancy is the redundancy within a given image.
Temporal redundancy reflects repetition from image to subsequent image.
The MPEG compression standards are among the most popular compression techniques. These include MPEG 1
for CD-ROM quality video, MPEG 2 for high-quality DVD video, and MPEG 4
for object-oriented video compression. The MPEG standard draws heavily on the JPEG standard for image compression by
exploited by JPEG. The H.261 video compression standards are also very poppular in the Internet.
Ther are nemerous proprietaary sechemes.
