I am new to electronics, I'm trying to get my head around this concept.
As I understand it, on any processor, you will be able to switch an output on or off, at a rate no faster than the speed of that processor. So a 8Mhz chip could (ideally) switch an input on or off at a rate of 8 million times a second.
HDMI 2.0 specification calls for a transmission rate of up to 18 Gbit/s. How is this possible without an 18Gzh processor?
Such fast outputs are never handled directly by a processor. That would be called "bit banging" and is really only practical for bit rates that are a small fraction of the CPU instruction cycle rate (note that a chip with an 8MHz external clock might internally run at 2MHz or 200MHz instruction rate).
Instead there are peripherals that are dedicated arrangements of gates, flip-flops and registers that are used to serialize data, perhaps directly from memory (DMA = direct memory access). There may be an internal memory bus dedicated just to getting data out of a frame buffer at that rate. In the case of HDMI there would likely be a dedicated graphic processor (GPU) optimized for video processing.
In the case of the Broadcom BCM2835 SOC used in the Raspberry PI, the GPU is a VideoCore 4 ARM1176. It would be nice to see the internals of such a chip, but Broadcom holds the cards close to the chest, and an NDA and a substantiated intent to use a substantial volume of chips is the price of entry.
