Analogue to Digital Conversion

Interfaces between computers and the real world have had a great impact on the human experience. In the beginning, Analogue to Digital Conversion required expensive equipment. By 2007 AD converters are inexpensive micro chips that are found everywhere.  You can still pay thousands of dollars for professional converters, but consumer level digital players produce surprisingly good quality at low cost.

Recording A/D conversion changes an analog voltage signal into a string of samples, expressed as digital numbers. In other words, real-world analog signals such as sound, and measurements of temperature, pressure are converted to digital values that computer electronics can store and process.

Playback The conversion  can be reversed, changing digital values back to voltages that drive analogue devices such as audio speakers. Compact discs have replace vinyl albums and tapes. The digital information on the CD is converted to audio in a great variety of CD players. DVDs have the same storage strategy but larger capacity. Video and audio are converted together as digital files. Most players have video converters that restore analogue signals that most TVs understand. Increasingly, video cameras and TVs are designed to use only digital information.

CD quality sound conversion creates 44,100 samples per second at a 16 bit depth. DVD quality sound conversion creates 48,000 samples per second at 24 bit depth. Digital sound is often stored as a wave file  or as Apple's AIFF format

Compression  Digital audio files are large and video files are larger. Compression involves  hardware and/or software that reduces the digital file size.  Compression requires compromises between storage space requirements, quality of reproduction and the cost of the hardware/software required to decompress in a reasonable time. The software/hardware used to compress is called a codec.  Digital audio can be compressed to MP3, Window's WMA files or Apple's AIFF-C.

The MPEG-1 codec is used for Video CDs and online video. This is most universal codec for video players.
DVD movies are compressed with the MPEG-2 codec.