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Music Studio Technology
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 now, 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. The digital result is described as linear pulse-code modulation (PCM).
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 of audio files Digital audio PCM files are large and video files are larger. Compression audio information 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.
For audio recordings, Digital PCM audio can be compressed to MP3, Window's WMA files or Apple's AIFF-C.
MPEG-1, Audio Layer 3 audio compression, MP3, is most popular for internet music downloads and streaming. Layer 3 (MP3) requires more digital processing and delivers better quality sound at lower bit rates. Streaming audio on the internet requires a reduction in the bit rate or bandwidth of the signal. An MP3 audio file compressed to 192 bits/second will stream continuously through a high speed cable connection. MP3 files can be compressed to slower bit rates with the option of a constant or variable bit rate.
DVD movies are compressed with the MPEG-2 video codec. DOLBY AC-3 is used for 5.1 surround sound DVDs . The audio compression removes inter-channel redundancies and streams at 320 kbps with impressive audio quality.