The basis of HDMI consists of TMDS (at least up to and including HDMI 2.0), which stands for Transition-Minimized Differential Signaling. TMDS was developed by Silicon Image as a method of transmitting high-speed digital data. The process uses an intelligent algorithm that reduces electromagnetic interference (EMI) and enables synchronization at amazing distances of up to 30 m. The method also works with a high signal skew in the cable. This is quite difficult as the thin strands are not actually capable of transmitting decent video images over long distances. With HDMI 2.0, the bandwidths are at least 600 MHz.
TMDS is similar to analog RGB transmission. It uses 4 channels: Red, Green, Blue and Sync. These are designed as a twisted pair and each take up 3 lines in the HDMI cable (see title picture). The actual audio and video signals are transmitted via HDMI via these 4 channels. Audio is embedded in the color channels. This is the reason why you cannot easily transmit audio signals separately from HDMI. You need an HDMI audio extractor that filters out the audio signal.
The TMDS method achieves a maximum bandwidth of 3 x 600 MHz, which means that data can be transmitted at up to 18 Gbit/s. This is sufficient for video resolutions up to 4K 60Hz. This is no longer enough for higher resolutions such as 4K 120Hz or even 8K. Therefore, the data rate has not only been increased with HDMI 2.1, but the entire transmission process has been redesigned. For higher resolutions, HDMI 2.1 uses the FRL (Fixed Rate Link) method. 4 data channels are used without a clock signal. Advantage: Simply by eliminating the clock signal, the possible data rate increases by 33%. In order to achieve the highest resolutions and color depths, signal compression was also agreed. This allows 48 Gbit/s data to be transmitted. More about HDMI 2.1 here.
EDID data is exchanged via the Digital Display Data Channel (DDC). You can find more information on this in our separate Post about EDID .
The 5V power feeds HDMI chips in connected but powered off devices. In this way, even inactive devices can provide EDID information and receive CEC commands. You can also use this power supply to remotely power low-power HDMI repeaters or HDMI splitters. According to the specification, an HDMI device must be able to supply at least 55 mA. At least 200 mA are common nowadays, some devices can also manage 1000 mA. Unfortunately, this is rarely documented. The performance of the plug-in power supply provides a first indication - if available. For example, if the power consumption of the main device is 8 W, but the power supply can deliver 12 V and 1 A (i.e. 12 W), there is at least a theoretical reserve for the HDMI power supply.
CEC is a single-ended, bi-directional serial bus. The HDMI devices communicate with the television via CEC (see Post via CEC ).
Pin 19 has several functions: Hot-plug detection registers a device that has been added during ongoing transmission. Since HDMI 1.4, the HDMI over Ethernet function has made it possible for an HDMI device to be connected to a network (LAN / Internet) and for the other HDMI devices to share this connection. That would be great if it were implemented. Unfortunately, this feature does not use a consumer device. The audio return channel uses the same line and is now widely used. For more information see Post about the ARC . Also the successor eARC uses these pins and does not necessarily require new cables or connectors.