What is HDMI's TMDS signal? Analysis of the relationship between HDMI bandwidth and TMDS

The transmission principle of HDMI is the same as that of DVI. It is a differential signal transmission technology invented by Silicon Image (TMDS). TMDS is a differential signal mechanism that uses a differential transmission method. This is not only the basis of DVI technology, but also the basic principle of HDMI technology.

1. TMDS Principle

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 magnitude of the voltage between the two pins. That is, two wires are used to transmit signals, one wire transmits the original signal, and the other wire transmits the opposite signal of the original signal. In this way, the receiving end can shield electromagnetic interference by subtracting the signal on one wire from the signal on the other wire, thereby obtaining the correct signal.

Each standard HDMI connection includes 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. In addition, there is a display data channel (DDC), which is a signal line for reading the extended display identification data (EDID) that indicates the display capabilities of the receiving end display, such as the clarity. The DDC line is also used for password key authentication between the sending and receiving devices equipped with HDCP (High-bandwidth Digital Content Protection, high-bandwidth digital content protection technology).

One HDMI includes three TMDS data channels and one TMDS clock channel.

Each data channel uses a coding algorithm (XOR, XOR, etc.) to convert 8-bit data into 10-bit data. The first 8 bits of data are obtained by calculating the original signal, the 9th bit indicates the calculation method, and the 10th bit is used to correspond to the DC balance. This algorithm makes data transmission and recovery more reliable.

1. Minimize transmission

Step 1: Send 8-bit parallel RED data to TMDS Tx. Step 2: Parallel/serial conversion. Step 3: Minimize transmission processing and add the 9th bit, which is the encoding process. The 9th bit of data is called the encoding bit.

2. DC-balanced

DC balance: During the encoding process, the DC offset of the channel is guaranteed to be zero. The method is to add 1 bit after the original 9th ​​bit of data, so that the number of "0" and "1" sent by TMDS remains basically the same. In this way, the transmitted data tends to be DC balanced, which reduces the electromagnetic interference of the signal on the transmission line and improves the reliability of signal transmission.

3. Differential signal

The signal is transmitted using the voltage difference between two pins.

2. HDMI and Resolution

PCLK: Pixel Clock
Taking 1920x1080p/60hz as an example: 1920*1080*60=124.4MHz
Taking 1280x720p/60hz as an example: 1280*720*60=55.3MHz

Bandwidth: the amount of data transmitted within 1 second (bit)

Bandwidth required for 4K frequency: Choose the most commonly used 3840x2160 resolution. The color depth is usually 8 bits, and the RGB three colors are 24 bits. Our goal is 60Hz refresh rate (60fps)

→→→ 3840*2160*24bit*60fps=11.94Gbps

HDMI1.4 pixel clock is up to 340MHz, that is, the maximum bandwidth is:
→→→ 340MHz*10bit (10bit encoding)*3 (3 data channels) = 10.2Gbps

However, since HDMI uses 8bit/10bit encoding, the actual efficiency is 80% of the theoretical value, so the maximum video bandwidth that 10.2Gbps can transmit is 10.2*0.8=8.1Gbps.

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 is this value obtained? What is its 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.

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, and three 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 the amount of data required to display 1.65G pixels (a complete pixel information consists of R/G/B primary color information) in one second. In terms of digital audio, HDMI flexibly supports 32kHz, 44.1kHz and 48kHz, 16-bit quantized stereo digital audio signals that comply with the IEC60985 L-PCM standard and IEC 61937 standard single-channel uncompressed PCM digital audio signals with a sampling rate of 192KHz and 24-bit quantization, 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.