The world of high-definition displays relies heavily on HDMI for seamless image and audio transmission. But have you ever wondered about the technology behind the HDMI? Let's explore the key aspects, including signal transmission topology, security, and more.
Transition-Minimized Differential Signaling (TMDS)
TMDS is a technique for serially transmitting high-speed digital signals, it minimizes signal transitions to reduce electromagnetic interference (EMI) and maintain signal integrity over longer distances. In HDMI transmissions, there are three TMDS channels (Red, Green, Blue) that carry the signal information, along with a TMDS clock that regulates the speed of the signals to prevent timing or synchronization issues.
The "transition minimized" aspect is achieved through the use of the 8b/10b encoding algorithm in TMDS, this TMDS encoding was created by Silicon Image in 1999 and is not the same as the original 8b/10b encoding introduced by IBM in 1983.
While both TMDS and 8b10b are codes that expand 8 bits into 10 bits, their objectives are significantly different. The purpose of 8b10b is to generate enough transitions to recover the bit or symbol clock. In contrast, the goal of TMDS is to minimize transitions since it explicitly transmits a symbol clock. In Summary the data flow takes place in the following order.
The sending device encodes the signal, arranging the ones and zeros to minimize the risk of signal degradation.
Two copies of the signal are transmitted over separate conductors, with one being an "out-of-phase" version of the original signal.
The receiving device aligns the out-of-phase signal back in phase, compares the two versions, and disregards any discrepancies (noise) between them.
In TMDS encoding, consecutive data bits are either XORed or XNORed to create an output symbol bit, which is then either inverted or left unchanged based on cumulative DC balance. This means that for certain 8-bit input bytes, the 10-bit output symbols can have 8 consecutive bits without any change.
The following two images shows the TMDS signal from an oscilloscope capture and the FPGA simulation for HDMI Tx respectively.
TMDS can support up to 18 Gbps of bandwidth, which is sufficient for 4K at 60Hz but not higher resolutions or refresh rates. For higher bandwidth the HDMI 2.1 uses Fixed Rate Link (FRL) instead of TMDS. FRL supports 48 Gbps of bandwidth, enabling resolutions up to 10K and refresh rates up to 120Hz, as well as other advanced features like Dynamic HDR and Variable Refresh Rate (VRR).
Fixed Rate Link (FRL)
FRL is a major upgrade introduced in HDMI 2.1 to meet the higher bandwidth demands of modern high-resolution and high-refresh-rate video formats. Unlike the continuous data streams used by TMDS, FRL utilizes a packet-based transmission method. This involves dividing data into packets, each containing video, audio, and auxiliary information. FRL supports multiple fixed data rates, ranging from 3 Gbps to 12 Gbps per lane, and HDMI 2.1 can utilize up to four lanes, offering a maximum total bandwidth of 48 Gbps.
FRL is backward compatible with TMDS, using the same data and clock channels for high-speed communication. The FRL lanes correspond as follows: Lane 0 (TMDS Data 0), Lane 1 (TMDS Data 1), Lane 2 (TMDS Data 2), and Lane 3 (TMDS Clock). Unlike TMDS, which uses 8b/10b encoding, FRL employs 16b/10b encoding, resulting in a 9% increase in efficiency. For transmission, FRL can use both AC & DC coupling while TMDS uses only DC coupling.
I will write another article covering the TMDS and FRL in detail.
Display Data Channel (DDC)
DDC is also called as i2c channel, it is actually a bi-directional communication channel between the source and the sink. DDC allows your monitor and graphics card (or other video source) to talk to each other and exchange information for example:
Monitor identification
Display configuration
Supported resolutions and refresh rates
On the sink end, this information is stored in a non-volatile memory called Extended Display Identification Data (EDID). EDID allows the source device (like your computer) to identify the monitor and configure the video output accordingly. This ensures you get the best possible picture quality and resolution on your screen.
High-Bandwidth Digital Content Protection (HDCP)
This authentication protocol enables the sending and receiving devices to verify each other's credentials (stored on each device's EDID chip). If everything checks out, they generate a shared key used to encode and decode the data exchanged between them. This process, called a handshake, occurs almost instantaneously at the start of a session, ensuring that unauthorized devices cannot intercept the data during transmission.
Consumer Electronic Control (CEC)
CEC is a one-conductor, bi-directional serial bus that enables a person to control multiple CEC-enabled devices with a single remote control. Additionally, it allows a CEC-enabled device to control other CEC devices automatically, without any user intervention.
Hot Plug Detect (HPD)
HPD Is a feature that allows devices connected with an HDMI cable to detect when the other device is plugged in or unplugged while they are both powered on. This is called hot plugging, as opposed to cold plugging, which requires one or both devices to be turned off before connecting or disconnecting.
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