Digital audio
General term for technologies that convert audio signals into digital codes, process, transmit, and record them. Also called PCM audio (PCM = pulse code modulation).
Sound is a phenomenon in which the minute vibrations of air particles and the accompanying minute fluctuations in atmospheric pressure propagate through the air, and both particle vibrations and pressure fluctuations often change continuously with time. Even if such sound is received by a microphone and converted into an electrical signal, the waveform is similar to the waveform of the original sound, and the property that the phenomenon changes continuously with time does not change. This method of transmitting or recording an electrical signal is called analog audio in the sense that it handles an electrical signal similar to a sound signal and is used in many fields to this day. It has a good track record. For example, there are AM broadcasting (AM = broadcasting using the amplitude modulation method), FM broadcasting (FM = broadcasting using the frequency modulation method), disk recordings, tape recordings and the like.
On the other hand, it is digital audio in which a continuous electrical signal is converted into a sequence of discontinuous codes, processed, transmitted and recorded. The foundation of digital audio is PCM technology. PCM converts a continuous signal into a discontinuous code sequence through three processes: sampling, quantization, and encoding.
Sampling is an operation in which a signal waveform whose amplitude changes continuously with time is observed at regular intervals as shown in the figure, and the original signal waveform is replaced with a sequence of observed values. . Jumping observations are called sample values, and the number of observations per second is called the sample rate. Since the signal waveform is not faithfully tracked, but only discretely observed, information appears to be missing in such operation, but the sampling frequency is more than twice the upper limit of the frequency component contained in the sign. , it is theoretically proven that the sequence of sample values can maintain the information contained in the signal without omission, and the original signal can be completely restored from now on (called the sampling theorem). In the case of a compact disc (CD), which is a typical digital audio system, the upper limit of the frequency of the signal to be recorded is set to 20 kHz and the sampling frequency is set to 44.1 kHz, which is more than double, with some concession. Even if you are nervous, observe 44,100 times per second in detail.
Quantization is an operation in which a fixed interval step is provided in the range from the minimum value to the maximum value of the signal amplitude, and the signal amplitude that changes continuously from moment to moment is replaced with the value in the closest step. . The picture is like replacing the slope with a ladder as shown in the figure. Since the smooth waveform becomes an irregular step, it is believed that signal deterioration will occur, but if the number of steps is large enough and the width of the step (the height of one step of the stairs) it becomes small enough. are as close to a slope as possible. The number of increments is expressed in bits (the number of digits when the number is expressed in binary) and is called the number of quantization bits. For a CD, the number of quantization bits is 16 bits, which is expressed in decimal as 65,536. Even if the slope is replaced by stairs, the number of steps is more than 60,000 and you can see that it is as close to the slope as possible.
Two operations, sampling and quantizing, result in a continuous signal waveform that is transformed into a discrete, discontinuous (discrete) sequence of signals, both temporal and oscillating. It should be noted that sampling or quantification can be done first, and actually both are often done at the same time.
Encoding is the operation of converting a quantized sample value to a numeric code based on a predetermined promise. From the binary code that corresponds to the voltage (or current) “yes” and “no” to the symbols “1” and “0”, the appropriate code conversion is carried out for each medium such as recording and broadcasting. The CD, which was put into practical use in 1982 (Showa 57), was encoded without any compression operation to reduce the data size, but the MiniDisc (MD)
