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.
FaceBookShareMany of the topics presented online are from Music Notes by Stephen
Gislason.
Download a free copy of the work in progress. Your comments are welcome.
Persona Digital Studio is located on the Sunshine Coast, Sechelt,
British Columbia, Canada.
www.personadigitalstudio.com email
music@personadigitalstudio.com. Our Music catalogue includes recorded
performances by the P2500 Band, Em4U, and the Persona Classical Consort.
Music downloads are available from iTunes, Napster, AmazonMP3 and from our
companion website: Persona Digital
Online.
We have enjoyed association with Reverb Nation who provides an online service
for musicians to present and market their music.
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