With HDMI, it’s about copy protection. But let’s first address that coax jack. The coaxial cable input on a TV’s back panel is provided to connect an antenna designed to pull in digital TV broadcasts. A tuner inside the TV then demodulates the signal, stripping audio/video from the radio frequency carrier. Next, the MPEG-compressed A/V stream is decoded so that your TV can display it.
With digital cable, the process is similar, except a different modulation method is used to convey signals over the cable TV system’s wired network to your cable box.
Once HD signals are decoded by your cable, satellite, or other-type receiver, the uncompressed video and audio gets routed to an HDMI output. Those signals are then encrypted using a DRM (Digital Rights Management) scheme called HDCP (High-bandwidth Digital Content Protection). For the source device to pass the signals, a handshake must occur with he receiving device. This process creates a secure digital connection that prevents any content from being copied.
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