Audio / Video

Historically audio and video technologies were completely analog-based. An analog representation of a signal is usually electrical; for example, a voltage level represents the air pressure waveform of a sound. Today audio or video signals are generally converted to digital signals. Analog signals are continuous in time. To convert these signals into a flow of digital values it is necessary to take samples at a defined rate - the sampling rate or frequency. An analog-to-digital converter (ADC, A/D or A to D) is an electronic circuit that converts continuous signals to discrete digital numbers. The reverse operation is performed by a digital-to-analog converter (DAC).

There are lossless and lossy types of data compression. Lossless data compression enables the data to be reconstructed almost exactly as the original. Lossy compression may alter the data but is generally close enough to the original to be useful. Lossy compression is used frequently in current technologies using streaming media and telephony applications. These methods are typically referred to as codecs.

A codec is a device or program which encodes a data stream or signal for transmission, storage or encryption and decode it for viewing or editing. There are many types of codecs. Below is a small sampling of commonly used video codecs:

1. MPEG-1: Used for Video CDs, and online video. Comparable to VHS.
2. MPEG-2: Used on DVD, SVCD and in most digital video broadcasting and cable distribution systems.
3. H.263: Primarily for videoconferencing, videotelephony, and internet video.
4. MPEG-4: For internet, broadcast, and on storage media.
5. Sorenson 3: Used by Apple's QuickTime.
6. WMV: Microsoft format which can be used for low resolution video to HDTV.
7. RealVideo: Developed by RealNetworks, now losing popularity.

There is a difference between file formats and codecs. Although most audio file formats support only one audio codec, some file formats - such as AVI - support multiple codecs. There are three major groups of audio file formats:

1. Uncompressed formats, such as WAV, AIFF and AU. Professional users or audio experts may use the WAV format for maximum audio quality. Less popular for streaming media on the internet because the file size can be large.
2. Lossless compression formats, such as FLAC, Monkey's Audio WavPack, Shorten, TTA, Apple Lossless and lossless Windows Media Audio (WMA). Often used in professional applications.
3. Lossy compression formats, such as MP3, Vorbis, lossy Windows Media Audio (WMA) and AAC. Lossy file formats are based on psychoacoustic models that leave out sounds that are not in the human range of hearing. •