Digital recording
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In digital recording, the analog signal of a sound is converted into a stream of discrete numbers, representing the changes in air pressure through time; thus making an abstract template for the original sound.
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History
- In 1937, the Brit Alec Reeves files the first patent describing Pulse-code modulation.
- In 1957, Max Mathews of Bell Telephone Laboratories developed the process to digitally record sound via computer.
- In the 1970s, Thomas Stockham makes the first digital audio recordings using standard computer equipment.
- The first digital compact discs marketed in 1982.
- In 1990, digital radio begins in Canada, using the L-Band.
- DVD players begin selling in Japan in 1996.
- On January 5, 2004, the first HD car radio was sold in Cedar Rapids, Iowa.
Process
Recording
- The analog signal is transmitted from the input device to an analog to digital converter (ADC).
- The ADC converts this signal to a series of binary numbers. The count of the numbers produced per second is called the sample rate.
- A bundle of wires transmits these numbers into storage. (Such as a hard drive or compact disc burner).
Playback
- The sequence of numbers is transmitted from storage into a digital to analog converter (DAC), which converts the numbers back to sound.
- Sound is transmitted to the loudspeaker.
Getting the bits recorded
Even after getting the signal converted to bits, its still difficult to record. The hardest part is finding a scheme that can record the bits fast enough to keep up with the signal. Say you want to record a song at 44.1 khz sample rate, and use a 16 bit word size. This means your software has to handle 1,411,200 bits per second.
Techniques to record to commercial media
For digital cassettes, the read/write head moves as well as the tape in order to maintain a high enough speed to keep the bits at a manageable size.
For CDs, a laser is used to burn microscopic pits into the plastic. A weaker laser is used to read these signals. This works because the plastic is reflective, and the pits disrupt that reflection, giving the data.
Concerns with digital recording
Word Size
The number of bits used to represent a single audio wave (the word size) directly affects the distortion of a signal. Since each bit used doubles the number of values possible for the wave, the more accurately the wave's shape can be sculpted from them.
Sample Rate
The sample rate is even more important a consideration than the word size. If the sample rate is lower than the sound's frequency, entire waves could be missed, causing the output wave's shape to be severely altered. This problem is called aliasing.
Also if the sample rate is exactly the same as the sound's frequency, it would take its number from the same point on every wave every time, causing the output wave to be shaped in a perfectly straight line. Sample rates of exactly twice the frequency have this same problem, just skipping a wave in the process. To prevent this problem, the Shannon-Nyquist sampling theorem was developed (or, more simply, Nyquist's rate, which is double the sound's frequency as the lowest possible sample rate.)
Error Correction
Since these bits are physically extremely small, some are bound to be lost and destroyed during the process of recording or using them. This is why error correction schemes are necessary to find the lost information and to fix it. Here are some techniques used to recover lost data:
External links
A debate on the rules of digital recording (http://www.usatoday.com/tech/news/2002/04/16/copy-usat.htm)