What are the sample rate, the number of quantization bits, and the clock?
There is some format jargon that you really need to know about CDs.
It is the “sample rate” and the “quantization bit number”. In connection with that, you will deepen your understanding if you also learn about the “clock” from the CD. Next time you learn “How to Read Specifications / Optical Discs”, it will go into your head.
■ What is the sampling frequency and the number of bits?
Digital audio recorded on a CD has a 44.1 kHz sample rate and a 16-bit quantization bit rate, right? Yes, that is correct. It has appeared several times so far, but it is the first time that we have explained it in detail from the basics.
First, let’s start with the image. Just the esoteric feeling of sampling and quantizing, and the “vertical slice” and “horizontal slice” of the signals first. Think of it like cutting a radish. First of all, I’ll cut it vertically with a kitchen knife. You can make a lot of cuts, but they were originally continuous. The solid curve is the analog voice, and the first thing to do when digitizing it is the “vertical slice” = “sample” image.
Next is the quantification work. Even if the cut is a cut, it is quantified to “cross” the kitchen knife on its side. Then the radish will be divided into small squares. Did you imagine that the finer the square, the closer it is to the original analog signal?
The CD format is the rule of how fine the radish is cut (analog signal). “The sampling frequency is 44.1 kHz and the number of quantization bits is 16 bits” means that the first sampling is done at a rate of 44,100 times per second, and then the level is read with an accuracy of 16 bits (2 to power step 16). . Sampling is also called sampling, but in the first place, sampling is the norm, and without sampling, the work of quantification cannot be done.
■ PCM conversion flow
Let’s summarize how analog music signals are digitized in PCM and burned to CD. PCM is an abbreviation for pulse code modulation. In Japanese, it translates to pulse code modulation method.
The music signal is originally a continuous analog signal. A continuous waveform that ripples like a wave won’t fit in the hole of a CD as is, so test it out first. What part of the rippling wave should be used as a sample? Of course, it is necessary to have regular intervals, and in the case of CD, it is decided to sample at 44.1 kHz. kHz is a unit of frequency and is the number of repetitions per second. We’re going to sample at a tremendous rate of 44,100 times per second. The job of sampling is sampling, and it does not mean that the waves are crushed separately.
After sampling in the direction of the time axis in this way, the next step is how to read the discrete data (points) with what precision. This is the quantification. It’s not used often, but in English it’s called quantizing. Since the vertical axis of the graph is the signal level, that is, the magnitude, the precision point is how many steps to read to the highest point of the wave. The unit is the number of bits.
