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Re-railing HDMI Signal Power

If not for re-railing, most of the HDMI systems installed would be in the toilet. Most components are designed to boost HDMI voltage output, or re-rail it, by 1 volt.

Jeff Boccaccio · November 8, 2013

Many integrators are totally unaware what a repeater really does, whether under Rev 2.0 or anything else. The term “repeater” in the HDMI world describes a device that contains the necessary electronics to support both an HDMI receiver and HDMI transmitter. These are typically found in Blu-ray players, cable boxes, AVRs, extenders and distribution devices.

Since HDMI is a serial interface, an HDMI receiver positioned at the input brings in the serial data, de-serializes the data and converts it to standard digital video signal. It’s the same type of video we have used over the years for scalers, format converters, and televisions.

This is where manufacturers insert goodies such as OSD (On Screen Display), format conversion, color space modifications, and of course resolution either up or down. After they are done playing around with the video, it gets routed to an HDMI transmitter that re-serializes the standard digital video and converts it all back to an HDMI format as an output.

Needless to say, this process could not be possible unless these devices have some kind of power to supply the chipsets that do all this work. Remember, we are talking digital here: When data enters the component, it gets massaged by a ton of electronics that only know what a 0 and 1 is. The 0 has no bearing, it’s the 1 that makes this story so interesting. You see, to make a 1, the chipset must “PULL UP” the incoming data using its power supply to bring the data up to what we call a “rail.” With HDMI it is usually 1 volt.

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This is a walk in the park for these chips since they are fed as much as 5, 3.3 or even 1.8 volts DC. The chip will always output the data at the specified rail voltage it was designed for. In HDMI’s case it will PULL IT UP to 1 volt. So you see as long as there is enough dynamic range between the 0 and 1 that enters the component the data can be identified and re-railed.

Just like that, we have a refreshed HDMI signal coming out. If it were not for this re-railing process all those HDMI systems you installed would be in the toilet right from the start. Now don’t get overjoyed about this because it can also sting you. Ever notice that sometimes you can install a system and it works perfectly fine? Then you install the same exact system with one small addition like another repeater such as a distribution device and the system goes dark. It does not matter whether the repeater is a distribution device or AVR; the point here is that there was a change.

Most components are designed to boost HDMI voltage output, or “re-rail” it, by 1 volt.

Just because it went through another repeater does not mean that all is well. Sure, the chipsets in most cases do their job and supply the necessary rail needed for its output. However that’s only part of the story. Now each manufacturer of the device must get the HDMI signal to the outside world. In many cases this could make or break a system depending on the integrity of the build. We have seen products actually produce output signals worse than the ones going in!

The reason is due to the incredible speed being transported from one point to another. It requires a skilled designer that understands RF signaling to carry the HDMI data to the outside world. This can go the other way also. Using high rail devices that are inserted to a system can correct for low output systems. The key here is where to place them. Remember the re-railing process starts all over again when joined up with another repeater. If the added repeater is inserted between a source and an AVR does not mean it will correct anything between the AVR and the display. This phenomenon is just as susceptible with HDCP, Voltage, Hotplug and Timing, which make up the entire HDMI performance signature.

DPL is currently setting up a method to show the integrity of these signatures so that you have the ability to put the pieces of the puzzle together and yield one hell of a system.

Watch for “Voice of the Integrator” on the DPL site.

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  About the Author

Jeff Boccaccio is president of DPL Labs. Jeff can be reached at info@dpllabs.com. Have a suggestion or a topic you want to read more about? Email Jeff at jeff@dpllabs.com

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