TLDR: Longer active HDMI cables usually have a micro USB or USB-C connector to supply power to the internal chipset used to amplify the signal. These ports are no longer needed if you purchase a cable and source device certified to support HDMI Cable Power.

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The HDMI Forum has just announced a new amendment to its HDMI 2.1a standard. It adds support HDMI power cableallowing active cables to be powered directly from the connector without the need for a separate power cable.

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Passive HDMI 2.0 cables can be over 10 meters long while maintaining maximum bandwidth. The newer HDMI 2.1 specification nearly tripled the maximum net data rate, but that comes at a price. Currently, the best passive cables that support a full 48 Gbps can be about 3 meters long before signal attenuation problems arise.

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If you need to go further than this, active cable system this is the answer. They have chips and retimers inside that work together to reduce signal jitter. Standard HDMI cables are rated up to 50mA, which is usually not enough to power their internal circuits.

HDMI Cable Power raises this limit to 300mA at 5V, eliminating the need for a separate USB connector for power. Essentially, HDMI Cable Power enabled active cables will connect in the same way as passive cables, except they will be unidirectional, with one end connected only to the source and the other end only to the display.

Finally, the source device must also support the HDMI Cable Power specification. HDMI Forum says consumers should check device specifications or packaging to determine if this feature is supported.