HDMI: How scrambling enables higher data rates

HDMI 2.1/2.0 offers significant improvements over previous versions in terms of speed, data integrity, and data transfer modes.

Scrambling is one of the most important features introduced in HDMI 2.0. It is used to reduce electromagnetic interference (EMI) and radio frequency interference (RFI). In this blog, we will introduce the scrambling feature introduced in HDMI 2.0.

Compared with previous versions, HDMI 2.0 increases the transition-minimized differential signaling (TMDS) character rate from 340 megacharacters per second (Mcsc) to 600 megacharacters per second (MCSC), and also adds EMI/RFI scrambling functions at different rates. The relationship between TMDS clock rate, bit rate, and character rate below 340 Mcsc or above 340 Mcsc is shown in the following table. The source will not transmit at a TMDS character rate higher than the maximum rate supported by the sink device.

Improve EMI/RFID reduction

Scrambling is done in all three data channels - TMDS channels 0, 1 and 2. EMI/RFI reduction in the TMDS clock channel is achieved by reducing the clock frequency by one-fourth and reducing the clock amplitude. Scrambling is enabled by default in HDMI 2.0 at TMDS bit rates above 3.4 Gbps to 6.0 Gbps. The source allows scrambling for TMDS rates below 3.4 Gbps if both the source and sink support scrambling at that TMDS character rate. Scrambling is applied to active video, data islands, guard bands, and most control periods. If a portion of the control period (a period of 8 characters) is transmitted unscrambled, it is called the Unscrambled Control Period (UCP), as shown in red in the figure below. The entire control period excluding the UCP is called the Scrambler Synchronization Control Period (SSCP). One SSCP can be transmitted per field to maintain character synchronization.

The following table shows how the different periods are scrambled and encoded.

A Linear Feedback Shift Register (LFSR) is used to encode each data channel on the source side and decode each data channel on the receiver side. When the source transmits a 16-character Unscrambled Control Code (UCC) sequence in SSCP, the LFSRs are initialized simultaneously with the appropriate seed values, 16'hFFFF for Data Channel 0, 16'hFFFE for Data Channel 1, and 8'hFFFD for Data Channel 2. When the receiver receives an 8-character UCC sequence in SSCP on three data channels simultaneously, the LFSRs are initialized with the seed value. The seed value is used to scramble/descramble the first character on each channel after the UCC sequence.

The snapshot below shows the TMDS bit, character, and clock rates, along with the LFSR data, 8-bit TMDS input data, 8-bit TMDS scrambled data, and 10-bit TMDS encoded data that will be transmitted to the receiver for all three channels.

Application of UCC in Scrambling

UCC is used for character synchronization, inter-channel synchronization, and resetting the LFSR. These codes are transmitted during SSCP. The source performs the following functions during UCC:

UCC transmission on all three data channels simultaneously

Transmit an SSCP using a set of eight UCCs, once for each field

Limit the number of UCCs to exactly 8

Reinitialize the source LFSR when transmitting UCC