What are “black level”, “white level” and signal-to-noise ratio?

What are "black level", "white level" and signal-to-noise ratio?

What is “Black Level” and “White Level”

Black level: Define the corresponding signal level when the image data is 0. Adjusting the black level does not affect the amplification of the signal, but only translates the signal up and down. If you adjust the black level up, the image will be darker, if you adjust the black level down, the image will be brighter. When the black level of the camera is 0, the corresponding level below 0V is converted into image data 0, and the level above 0V is converted according to the magnification defined by the gain, and the maximum value is 255. Black level (also called absolute black level) setting, which is the lowest point of black. The so-called black lowest point is the electron beam energy emitted from the CRT picture tube. When the energy of the electron beam is lower than the basic energy that makes the phosphor (fluorescent substance) start to emit light, the black at the lowest position is displayed on the screen. The US NTSC color TV system positions the absolute black level at 7.5IRE, that is, signals below 7.5IRE will be displayed as black, while the Japanese TV system positions the absolute black level at the OIRE white level.

The white level corresponds to the black level, which defines the corresponding signal level when the image data is 255. The difference between it and the black level defines the gain from another angle. In quite a few applications the user cannot see the white level adjustment because the white level is fixed in the hardware circuit.

What is the signal to noise ratio

Signal-to-noise ratio (S/N, Signal/Noise) refers to the ratio between the signal strength of the maximum undistorted sound produced by the sound source and the noise strength at the same time, which is called the signal-to-noise ratio. That is, the ratio of useful signal power (Signal) to noise power (Noise) is referred to as signal-to-noise ratio (Signal/Noise), usually expressed in S/N, and the unit is decibel (dB). This calculation method is also applicable to image systems.

The ratio of the maximum fidelity output of a signal to unavoidable electronic noise in dB. The larger the value, the better. Below the index of 75dB, noise may be found in silence. In general, the signal-to-noise ratio of a sound card is often unsatisfactory due to too much high frequency interference in a computer.

The signal-to-noise ratio of the image captured by the camera and the sharpness of the image are both important indicators to measure the quality of the image. The image signal-to-noise ratio refers to the ratio of the size of the video signal to the size of the noise signal. The two are generated at the same time and cannot be separated. The noise signal is a useless signal, and its existence has an influence on the useful signal, but it cannot be separated from the video signal. Therefore, when choosing a camera, it is enough to select some useful signals that are relatively larger than the noise signals to a certain extent, so the ratio of the two is taken as the standard of measurement. If the signal-to-noise ratio of the image is large, the picture of the image will be clean, and there will be no noise interference (the main picture has snowflakes), and people will look very comfortable; if the signal-to-noise ratio of the image is small, the picture will be full of snowflakes, which will affect the normal viewing effect.