Losing Control of Your Video Performance

Link training is not anything new in the world of High-Speed Digital Signaling. Whether it be Ethernet, Fiber, InfiniBand, Display Port, or SAS (Small Computer System Interface), link training has been utilized in some way to “Tune” a system’s cable transmissions for best overall integrity. Typically used for data rates between 10Gbs and 400Gbs, link training can improve system environment connectivity and reliability. Link Training’s only purpose is to provide the best optimal transmission integrity for moving high-speed data from a source to some form of receiving device (sink).

In the past HDMI transmissions have been using a transmission protocol designated as TMDS (Transmission Minimize Digital Signaling). Up until now this type of transmission has provided screen resolutions up to 4K and data rates up to 6Gbps per channel. However, moving into higher resolutions required higher data rates with a bit more efficiency. A more efficient protocol has now been introduced designated as FRL (Fixed Rate Link) that more efficiently supports data rates up to 12Gbps per channel.

As these data rates increase so must its cable performance used to transfer it. As with any transmission line, the higher the data rate the more difficult it is to transfer in a well-organized fashion providing the necessary integrity for the safe passage of super-high data rate Digital Video signals.

By incorporating Link Training into the specification certain decisions can be executed and generated within the system to provide the best signal transfer. Its basic operation is similar to other communication systems used in the world today. Here is a brief overview of how Link Training can work:

  • Determine the appropriate speed required to support the transmission from a compatibility list within the hardware.

  • Link speeds get negotiated between both Source and Sink.

  • The sources originate the Link Training process allowing the sink to negotiate the highest FRL data rate.

  • If sink is satisfied with the data rate the system will continue to function producing a pattern code LTS:P for pass.

  • Reaffirming Integrity continues infinitely by monitoring BER (Bit Errors) from the sink.

  • If the signal fails to meet minimums from start-up, the sink will re-negotiate for a new data rate until a satisfactory result occurs.

  • If the signal fails to meet minimums at any time during operation, the sink can request an updated correction during operation.

  • If signals continue to fail under FRL, Link Training will change the system’s protocol from FRL to TMDS with pattern code LTS:L for TMDS.

This process provides some assurance that the system will continue to function in a wide variety of operating levels. The one downside to Link Training is its effect on picture quality. When Link Training reduces the data rate due to signal integrity issues it will also reduce the systems picture performance as per the published specification.

Example: If a system’s electronics are designed will full 8K 48Gbps capabilities and the cable does not support this level of performance, Link Training may detect this where the display can request a reduced data rate from the source. The source compiles and reduces the data rate until the system is satisfied. The downside to this is if the viewer is expecting an 8K 60Hz frame rate resolution and Link Training reduces the data rate from 48Gbps to 32Gbps the viewer will now only get 8K at a 30Hz frame rate. Televisions will have a function in their remote-control GUI (Graphic User Interface) that can be used to display the video performance numbers on the screen.

With DPL Labs® True Image Testing process, cables advertised performance levels are subjected to a series of stress tests guaranteeing the advertised data rate. This process adds P-O-M-G (Peace of Mind Guaranteed) for each consumer so that their system’s Link Training continues to support the system’s advertised performance limits.