The impedance in coaxial cables is generally divided into 50 ohms and 75 ohms, because the 75 ohm cable is used for a relatively single purpose, in the entire coaxial cable, 50 ohms dominate. Although there are fewer types of 75 ohm coaxial cables, they have a lot of functions, and we are still worth taking them out for a separate analysis.
75 ohms are commonly used in major applications including bracket wiring of equipment, closed circuit television (CCTV), shared antenna systems (MATV) and the transmission of color or monochrome RF monitors. These applications do not require the selection of precision video coaxial cables with particularly tight electrical tolerances.
The characteristic resistance of the video coaxial cable is 75 ohms, and this value is not arbitrary. Physics proves that the optimal attenuation characteristics of the video signal occur at 77 ohms. In low-power applications, the material and design determine the optimal impedance of the cable at 75 ohms.
In the application of 75 ohm coaxial, SDI cable is one of the mainstream, and it also has more complex knowledge points so far. After a rough introduction to the 75 ohm coaxial cable, let's take a look at what is special about the SDI cable.
所谓SDI，英文名字为Serial Digital Interface，数字视频SDI接口：Digital video SDI interface: SDI接口是数字分量串行接口(serial digital interface)的首字母缩写。
This serial interface is an interface that sequentially transmits the individual bits of the data word and the corresponding data through a single channel. Due to the high data rate of serial digital signals, they must be processed before transmission, and they belong to digital baseband signals. It is actually a type of interface, which is a digital serial interface.
SDI interfaces can be roughly divided into SD-SDI, HD-SDI, 3G-SDI, 6G-SDI, 12G-SDI, etc., from this division standard, it can be seen that speed is an important indicator.
HD-SDI is a high-definition digital component serial interface. HD-SDI is a real-time uncompressed high-definition broadcast camera, which is another technological advancement in the field of security monitoring, providing high-definition image sources for the monitoring center.
It has been widely used in the radio and television industry, with the continuous development of the security industry, its high-speed uncompressed digital advantages have been gradually tapped, and a large number of 3G-SDI series products have been launched on the market, including SDI conversion equipment, SDI digital switching matrix equipment and SDI distributors.
In 2015, the SMPTE organization officially released 6G-SDI and 12G-SDI for serial data interfaces. 6G-SDI is designed for 2160p30 applications and corresponds to 30 frames per second refresh. 12G-SDI for 2160p60 (commonly known as true 4K) applications, corresponding to 60 frames per second refresh.
For a comparison of 3G-SDI, 6G-SDI and 12G-SDI, we can check out the table below:
Since SDI cables can also transmit video, what is the difference between them and HDMI cables? We will compare in four areas.
HDMI: Consumer Electronics: Jiayin Cinema, Game Consoles, Computers, etc.
SDI: Broadcast, Film and Professional Video Production.
HDMI: Generally within 15 meters.
SDI: Can reach hundreds of meters or more.
HDMI: The interface is usually small and suitable for all kinds of electronic devices.
SDI: The interface is usually larger, usually a BNC connector.
HDMI: Support refresh rate above 4K/60Hz, HDR, etc.
SDI: Support for high resolution video, multiple SDI connections can support 8K.