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What cables should use between coaxial or twisted-pair wiring in HDMI 2.1

屏幕截图 2022-12-11 211902

Due to its features of simultaneously transmitting video and audio data, easy connectivity, and good compatibility, HDMI (High Definition Media Interface) has been widely used in consumer electronic products such as TVs, set-top boxes, and projectors.

The HDMI system can be divided into four categories: Source, Sink, Cable, and Repeater. In order to ensure good compatibility of these devices, the specifications have requirements for signal integrity of electrical signals. Currently, the mainstream HDMI standards on the market are 1.4b and 2.0, which support maximum resolutions/refresh rates of 4K@30Hz and 4K@60Hz, respectively.

With increasing demands for higher resolutions and refresh rates, the HDMI Association released the HDMI2.1 version in November 2017, which added the FRL (Fixed Rate Link) mode and supports 8K@60Hz, 4K@120Hz, and increased bandwidth to 48Gbps. The Microsoft Xbox Series and Sony PS5 are equipped with HDMI 2.1 interfaces, and the new ultra-fast HDMI cable ensures high-bandwidth independent features, including uncompressed 8K video with HDR support. The cable has low EMI (electromagnetic interference), which reduces interference with nearby wireless devices.


The cable is also backward compatible and can be used with existing HDMI devices. As HDMI 2.1 becomes more widely adopted, more audio and video devices such as computers, smart TVs, amplifiers, projectors, and players may support HDMI 2.1 in the future. However, the current biggest issue is whether HDMI 2.1 cables should use coaxial or twisted-pair wiring for production. This is a contentious issue in the industry, as the physical performance of the coaxial version, such as flexibility and outer diameter, is superior to that of the twisted-pair version, which has inherent defects of a larger outer diameter and insufficient flexibility. For more info HDMI2.1, Please check https://hdmi.org

High-frequency performance can be adjusted to minimize the differences between the two versions, but the production cost of coaxial cables is significantly higher than that of twisted-pair cables. The industry is watching to see the actions of major manufacturers. The most difficult parameter for HDMI 2.1 Cat 3 cable specifications is EMI, and any issues with TMDS signals are hard to solve. The impedance imbalance of the line causes incomplete signal transmission, and the quality and margin of the wire will affect the quality of EMI testing.

Coaxial cables are a composite of two conductors, with the center conductor used to transmit signals and the metal shield acting as a common ground for the signal to provide current and as a shield to suppress electromagnetic noise interference. The foam polypropylene insulation layer between the center conductor and the shield determines the cable’s transmission characteristics and effectively protects the middle conductor.

Nowadays, USB4, Thunderbolt 4, and HDMI2.1 use two coaxial lines for differential signal transmission, and ucox consists of two single lines to form a differential pair. The mode conversion is not easy to control, and the standard has been relaxed from -20dB to -17dB. There is no clear standard to judge which structure is better. In the short term, the production cost of twisted-pair cables is lower and is conducive to market promotion. In the long term, the performance of coaxial versions is easier to achieve and is more stable. With the improvement of manufacturing processes, the performance of coaxial versions will be better than that of price-oriented choices, otherwise, USB4 and Thunderbolt 4 will not endorse the coaxial structure!

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