HDMI: How Scrambling Enables Higher Data Rates

Scrambling for EMI/RFI Reduction

Scrambling is done in all the three data channels ­- TMDS channels 0, 1, and 2. EMI/RFI reduction in the TMDS clock channel is achieved by reducing clock frequency to one-fourth and reducing the clock amplitude. By default, the scrambling is enabled in HDMI 2.0 at TMDS bit rates above 3.4 Gbps to 6.0 Gbps. The Source enables scrambling for TMDS rates below 3.4 Gbps if both the Source and the 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, an 8-character period, is transmitted unscrambled, it is referred to as an Unscrambled Control Period (UCP) as indicated in the red portion of the figure below. The entire control period except UCP, is referred to as a Scrambler Synchronization Control Period (SSCP). One SSCP per field can be transmitted to maintain character synchronization.

The table below indicates how different periods are scrambled and encoded.

Linear-Feedback Shift Register (LFSR) is used for encoding each data channel on the Source side and for decoding each data channel on the Sink side. The Source simultaneously initializes the LFSRs with the appropriate seed values, 16’hFFFF for data channel 0, 16’hFFFE for data channel 1, and 16’hFFFD for data channel 2, when it transmits the 8-character sequence of Unscrambled Control Codes (UCC) in SSCP. The Sink initializes the LFSRs with the seed values when it receives an 8-character sequence of UCC in SSCP on the three data channels simultaneously. The seed values are used to scramble/descramble the first character on each channel following the UCC sequence.

The snapshot below shows the TMDS bit, character, and clock rate, along with LFSR data, 8-bit TMDS input data, 8-bit TMDS scrambled data, and 10-bit TMDS encoded data, which will get transmitted to the Sink for all three channels.

Use of UCC in Scrambling

UCC is used for character synchronization, inter-channel synchronization, and to reset the LFSRs. These codes are transmitted during SSCP. Source performs the following functionality during UCC:

• Transmit UCC on all three data channels simultaneously
• Transmit one SSCP with one set of eight UCC, once per field
• Limit the number of UCC to exactly eight
• Reinitialize the LFSRs on Source side when the UCC is transmitted