Digital recording
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Template:CleanupDate In digital recording, the analog signal of a motion-picture/sound is converted into a stream of discrete numbers, representing the changes in air pressure (chroma and luminance values in case of video) through time; thus making an abstract template for the original sound or moving image.
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History
- In 1937, the Brit Alec Reeves files the first patent describing Pulse-code modulation.
- In 1943, Bell Telephone Laboratories developed the first digital scrambled speech transmission system, SIGSALY.
- In 1957, Max Mathews of Bell developed the process to digitally record sound via computer.
- In the 1970s, Thomas Stockham makes the first digital audio recordings using standard computer equipment, as well as developing a digital audio recorder of his own design, the first of its kind to be offered commercially (through Stockham's Soundstream company).
- 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, it is still difficult to record: the hardest part is finding a scheme that can record the bits fast enough to keep up with the signal. For example, to record a song at 44.1 kHz sample rate with a 16 bit word size, the recording 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 CD-Rs, a laser is used to burn microscopic holes into the dye layer of the medium. A weaker laser is used to read these signals. This works because the metallic substrate of the disc is reflective, and the unburned dye prevents reflection while the holes in the dye permit it, allowing digital data to be represented.
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. Increasing a sample's word length by one bit doubles its possible values, likewise increasing the potential accuracy of each sample and the fidelity of the recording to the original. 24-bit recording is generally considered a current practical limit as this word length allows a signal-to-noise ratio exceeding that of most analog circuitry, which by necessity must be used in at least two points in the recording/playback chain.
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
(main articles: digital and Error correction and detection)
One of the great advantages of digital recording over analog recording is its resistance to errors. Since these bits are physically extremely small, some are bound to be damaged during the process of recording or using them. With analog recording techniques, any amount of damage is irreversable. As you use it, the increasing damage causes the noise to get worse and worse.
With digital recording techniques, small amounts of damage are completely irrelevant. When a crisp bump meant to represent a "1" gets a small notch knocked off or becomes worn, it's still very easy to distinguish it from a "0". Even when one particualar "1" bump is so well worn that it becomes indistinguishable from a "0", there are error correction schemes that can detect the lost information and to fix it. Here are some techniques used to avoid losing any data even when particular bits are completely destroyed.
error correction involves correcting not just one '0' but thousands per second. It replaces the data by approximating what data was there in the first place. It does this by using the millions of lines of code (PCM) that have entered the multiplexor. PLEASE read up on the digital process.
- Compact discs use Reed-Solomon error correction
- Many bits are stored on [redundant array of independent disks|RAID storage systems].