Ordinary CDs use digital technology, but they simply digitize analog signals. In order to digitize the analog signal, the analog signal must first be sampled. According to the Nyquist sampling law, the sampling frequency is usually at least twice the highest frequency component in the signal. For high-quality audio signals, the frequency range is from 20Hz to 20KHz. So its sampling frequency must be above 40KHz. The 44.1KHz sampling frequency is used in the CD. After sampling the analog signal, it must also be stratified over its amplitude. In the CD, the layered amplitude signal is represented by a 16-bit binary signal, that is, the analog audio signal is divided into 65,536 layers in amplitude. In this way, its dynamic range can reach 96 dB = 20 Log 65536 (6 dB/bit). This direct analog-to-digital (A/D) conversion method is also referred to as PCM encoding. The biggest drawback of direct digitization is the very high bit rate. It reaches 44.1x16=705.6kbps, or 88.2kBbps. A high bit rate means that the required storage capacity is large. To record 1 minute of music, you need 5.292MB of storage. For two stereos, 10.584MB is needed. Recording hundreds of minutes of music requires hundreds of megabytes of storage capacity.
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