Relatedly, why is PCIe x16, say, faster than PCIe x1? What functions are the extra pins performing if data cannot be sent multiple bits at a time?
In: Engineering
PCIe is, at its core, made up of a transmit pair and a receive pair (*”Pair” is used to denote a “Differential Pair”, a type of signalling method frequently used in high speed electronics that uses two wires for a single data line–using two wires and a ‘differential’ signalling method improves signal integrity and allows longer cables to be used*). These transmit and receive pairs can be thought of as two one way roads, one allows travel from place A to place B, and another from place B to place A.
Now, when you have 1 transmit line and 1 receive line, this is called PCIe x1, where the number after the “x” indicates how many lanes the ‘road’ has. Think of it as two separate one-lane roads. A PCIe x1 link requires 4 pins: “TX+, TX-, RX+, RX-” (A positive and negative transmit pair, and a positive and negative receive pair).
A one-lane road can only allow 1 car to drive down it at a time. We’ll call this car “Packet 1”. Packet 1 can travel from A to B pretty quickly, but before the next car (let’s call it “Packet 2”) can use the road, Packet 1 needs to reach its destination. And subsequent cars need to wait for Packet 2 to complete, etc etc.
Now, if you ***add*** another lane to the road, now ***two*** cars can drive down it at the same time, allowing more people to travel from place A to place B at the same time. A two lane road would be PCIe x2. Now Packet 1 and Packet 2 can drive from place A to place B at the same time, and Packet 3 and Packet 4 will have to wait for the roads to become free. A PCIe x2 link uses 8 pins.
You can keep adding lanes, resulting in PCIe x4 (16 pins), x8 (32 pins) and up to x16 (64 pins). When you have a 16-lane road, you can now have 16 cars driving side by side, meaning you have more room for more traffic, and you can get a larger number of cars moving from one place to another.
So while PCIe is considered a type of serial communication at its most basic (PCIe x1), it can use multiple lanes in a parallel-***like*** method to dramatically increase data throughput.
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In the case of HDMI, the large number of pins is because there are actually 3 separate serial channels, along with a clock channel. Like PCIe, HDMI uses differential pairs for data transmission (a positive and a negative ‘pair’ of wires per signal); ***unlike*** PCIe, HDMI only ever goes one-way (meaning each channel only needs 2 wires/pins).
Using the road analogy, HDMI is effectively a 4-lane, one-way road travelling from place A to place B. Lane 1 (red channel) only allows red cars, lane 2 (green channel) only allows green cars, lane 3 (blue channel) only allows blue cars, and lane 4 (clock channel) is reserved for police cars that keep all the other cars at the speed limit.
As displayed images are made up of the colors red, green and blue, you need to combine the data of the red, green and blue channels while using the clock channel to synchronize the data received by the three serial color channels. TVs and Monitors have special circuits that can compose images based on the data received from these channels, and quite literally fills in the image in a similar way we read text on a page (it fills it in pixel by pixel across a line, then the next line, and the next, until the full image is ready and it then gets displayed).
HDMI also has ***other*** pins that are not specifically used for sending image data from a PC to a monitor–these include a special ‘power’ pin, a CEC pin, EDID Clock, EDID Data, a Detect pin (to detect when something is connected), though these are specialized pins that are not necessarily required for sending serial image data from a device to a screen.
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