Detailed explanation of HDMI transmission principle

Just like the top-level engine is the guarantee for F1 racing, the reason why the HDMI standard has powerful data transmission capacity is inseparable from its transmission principle. Let's take a look at the transmission principle of the HDMI standard.
The HDMI standard continues to use the same TMDS (Time Minimized Differential Signal) minimized transmission differential signal transmission technology invented by Silicon Image as DVI. TMDS is a differential signal mechanism that uses differential transmission. This is not only the basis of DVI technology, but also the basic principle of HDMI technology.

TMDS differential transmission technology is a technology that uses the voltage difference between two pins to transmit signals. The value of the transmitted data ("0" or "1") is determined by the positive and negative polarity and size of the voltage between the two pins.

Each standard HDMI connection contains three TMDS transmission channels for data transmission and one independent TMDS clock channel to ensure the uniform timing required for transmission. In one clock cycle, each TMDS channel can transmit a 10-bit data stream. And this 10-bit data can be composed of several different encoding formats.

TMDS encodes the three primary colors of RGB in each pixel as 8 bits respectively.

TMDS data transmission timing structure
Generally speaking, the encoding format of HDMI transmission should include video data (each pixel uses 24 bits before HDMI1.3 version), control data and data packets (data packets contain audio data and additional information data, such as error correction codes, etc.). Each TMDS channel should contain a 2-bit control data, 8-bit video data or a 4-bit data packet during transmission. In the HDMI information transmission process, it can be divided into three stages: video data transmission cycle, control transmission cycle and data island transmission cycle, which correspond to the above three data types respectively.

The relationship between HDMI bandwidth and TMDS
The bandwidth specified in the HDMI standard is set at a maximum of 4.96Gbps in version 1.0. So how did this value come from? What is the relationship with TMDS? Let's look at the following formula:

This is a formula applicable to the bandwidth calculation of all serial transmission interfaces. In all digital circuits, there is a component responsible for providing the basic frequency - the crystal oscillator. It is like an accurate alarm clock, and all components in the circuit act in unison according to its rhythm. For example, the crystal oscillator frequency of a certain operation circuit is 100Hz, which means that this circuit can perform 100 operations in one second. It can be seen that the higher the operating frequency of the crystal oscillator, the more operations can be processed per second, and the more powerful the data processing capability will be. This principle also applies to the HDMI standard.

HDMI encoding example: 3 TMDS transmits RGB data of pixels respectively.
The clock frequency in the HDMI circuit ranged from 25MHz to 165MHz when it was first formulated, which means that a TMDS channel can transmit up to 165MHz×10bit=1.65Gbit of data per second. 3 TMDS channels can transmit 1.65×3=4.95Gbit of data per second. Adding control data, the standard method represents a bandwidth of 4.96Gbps. If expressed in pixels, it means that the amount of data required to display 1.65G pixels (a complete pixel information consists of R/G/B primary color information) can be transmitted in one second.

In terms of digital audio, HDMI flexibly supports 32kHz, 44.1kHz and 48kHz, 16bit quantized stereo digital audio signals that comply with the IEC60985 L-PCM standard and IEC 61937 standard sampling rate of 192KHz, 24bit quantized single-channel uncompressed PCM digital audio signals, or 8-channel 96kHz sound data streams. In addition, Dolby Digital 5.1 and DTS digital audio formats commonly used in home theaters can also be transmitted directly via HDMI.