Compact Disc Technology: Digital Audio Recording
The Compact Disc: Digital Sound
Introduction
The development of the compact disc, with audio recording in digital form, has solved problems such as wear on disks and tapes, *wow* and *flutter*. It has improved the signal-to-noise ratio and offers the advantages of a digital system, such as precision in storage, transmission, and any other information processing that is characteristically digital.
An analog signal can take any value and varies progressively and continuously. To record digitally on a CD, it is necessary to convert the analog signal into a digital one. This is accomplished with an Analog-to-Digital Converter (ADC) for digital recording on CDs. A digital signal is characterized by taking only two values (0 and 1), each of which is called a *bit* and corresponds to two voltage levels (0 and +5 V).
The A/D converter has to sample the analog signal with a given frequency. The number of samples per unit of time is called the *sampling frequency*, and in the case of CDs, it is 44.1 kHz. The values included in each sample are discrete values. The voltage should be chosen as a value for an interval of values; this is a kind of rounding, known as *quantization*. Each discrete value obtained should be replaced by a binary code in an encoding process. The collection of codes from each sample, one by one, generates the digital information that is recorded on the disc and replaces the analog signal.
The CD works with a 16-bit code, which enables 65,536 combinations. The precision of the conversion also depends on the sampling speed, for example, one sample every millisecond. To copy the digital signal recorded on a CD, it is necessary to convert the digital signal back to analog. The Digital-to-Analog Converter (DAC) recovers the original analog signal, for which it will have to decode the digital code.
The Compact Disc (CD)
The format was introduced in 1979 by Philips, which defined the current system with a diameter of 12cm and a capacity of 74 minutes, denoting the duration of Beethoven’s Ninth Symphony. The CD was released in 1982.
The CD is manufactured on a transparent plastic substrate 1.2 mm thick, with a reflective coating that has a thickness between 0.05 and 0.1 microns to enable scanning. Then, a varnish of about 30 microns is applied to protect the face where the disc titles are silkscreened.
The recording is done as follows:
- Digital data (music, song number, etc.)
- Sync signal (disc rotation speed)
- Code for errors
- Reading errors produced by the signal
EFM (Eight-to-Fourteen Modulation) converts an 8-bit code into a 14-bit one, plus perfect for encoding. This means that after a logic 1, there must be at least 2 zeros and a maximum of 10. This way, if an error occurs during reading and any bit is lost, the system can correct and reconstruct the original signal. This digital code provides an adequate sync signal that can control the speed of the disk.
Main Advantages
- The precision of the disk speed is perfect; there is no *wow* or *flutter*.
- The digital signal improves the signal-to-noise ratio to over 90dB.
- Distortion is less than 0.03%.
The linear speed of the disc must be very precise, so to start reading in the center of the disk, the angular speed is 500 rpm. As the beam moves towards the outside of the disc, the shaft speed decreases to 200 rpm so that the linear speed remains constant.