Understanding Digital Sound: From Analog to CD
Introduction to Digital Sound
The development of the compact disc revolutionized audio recording by digitizing sound, solving problems associated with the wear and tear of discs and tapes, such as warbling and tremolo. This innovation improved the signal-to-noise ratio and dynamic range, offering advantages inherent in digital systems, including precision storage, transmission, and processing.
Unlike analog signals, which can take any value and vary gradually, digital signals are characterized by taking only two values or states, each called a bit, corresponding to two voltage levels. To convert an analog signal to digital, an Analog-to-Digital (A/D) converter is used. This process involves sampling the analog signal at a specific frequency. The sampling frequency, in the case of CDs, is 44.1 kHz, meaning 44,100 samples are taken per second.
The values collected from each sample are discrete voltage values. Since a value must be chosen for a range of values, a process called quantization occurs. Each discrete value obtained is then replaced by a binary or digital code through encoding. The compilation of these codes from each sample generates digital information, which is recorded on the disc, replacing the analog signal.
CDs use a 16-bit code, allowing for 65,536 combinations, providing a high degree of precision in signal conversion. The speed of sampling also affects precision. For example, if a sample is taken every millisecond (1000 samples per second), the sampling frequency for reproduction would be 1000 Hz.
To reproduce the digital signal recorded on a CD, a Digital-to-Analog (D/A) converter is needed. This converter recovers the original analog signal by decoding the digital codes, obtaining discrete values, and then using a filter to eliminate abrupt changes in the signal.
Compact Disc Technology
The compact disc format was established in 1979 by Philips, defining the current system with a 12 cm diameter and a capacity equivalent to 74 minutes of audio, enough to accommodate Beethoven’s Ninth Symphony. It was launched in 1982.
The recording process uses Pulse Code Modulation (PCM) to convert the analog signal into digital form. This signal is then modulated, resulting in an Eight-to-Fourteen Modulation (EFM) signal, which produces variations in the length of the pits recorded on the disc. Binary information is recorded in the form of pits and lands. When a thin laser beam hits the disc, the lands reflect the entire signal, while the pits reflect a less intense signal, enabling the reading of ones and zeros.
The EFM signal converts an 8-bit code into a 14-bit code, providing more precise decoding. It uses an error protection system to ensure only valid codes, or words, that comply with the rule of having at least two zeros and a maximum of ten between each logic 1 are used. This allows the system to correct and reconstruct the original signal even if an error occurs during reading and a bit is lost.