HDMI Corner: Be Proactive About Active Cables’ Power Demands

As True 4K Ultra HD products hit stores, you’re going to have to use ‘active’ cables with much higher power demands. Where will all that power come from?

Jeff Boccaccio

With True 4K Ultra HD products hitting stores, you are going to be forced into using transmission lines that are “active” in some way. The bandwidth is just too high for longer traditional passive copper products to support and we will see even higher data rates to come.

Active transmission lines will grow for both copper and fiber products and all need power to run. Where do they get it? 

If these products try to use the low power on the HDMI bus some tricky power management must be implemented; it was never intended for powering anything, and that hasn’t changed.

What has changed is the technology in active devices allowing for major power reductions to make this happen. The problem is, when product developers get the nod from their suppliers that their products can work with only the bus power they tend to believe the claims and want to market the hell out of “No External Power Needed.”

As we move into 2017 these power demands will have to increase, and you should be aware of what is really happening. Power is everything, which includes the current being delivered and the amount of filtering for super-low-noise applications.

The HDMI inter-face incorporates a relatively weak 5-volt output that originates from any source and eventually is consumed at the destination sink. One of its jobs when it arrives at the sink is to initiate the critical HotPlug trigger. Without the 5 volts, HotPlug can be jeopardized, killing the system before it starts. Rule 1, don’t mess with the 5 volts! It is weak and has its limits.

We mostly test long active products and when they go long (like over 10 meters) or very small in gauge, the voltage drops over distance due to higher current demand. This robs the lifeblood out of the bus.

One of our tests is to see how well the line handles this 5-volt source. We challenge the transmission by introducing only the exact power limits the specification calls for. If the product’s cur-rent consumption is not damaging the 5 volts, guaranteeing its delivery, it gets a passing grade. This generally is not a big issue with passive products. However, once you go active the rules change.

If an active transmission line does not use an external supply, your alert ears should go up. Current techniques used by most testing firms do not account for active devices. Typically, a test like this is made by inserting the rated voltage and current into the source end of the active transmission line and measuring the dropped voltage at the other end. That would work under normal passive products, but with active products it is way off the mark.

As with any amplifier, whether it be A/V, RF or data, the rules are the same. As you drive the amp into its load, the current pull will put more strain on the power supply. Think about it: if you use a 1,000-watt audio amplifier its power up sequence pulls a ton of current to charge up its filters and then relaxes to its “idle current.”This can be as low as 0.25 amps.

However, when you begin to drive the amplifier, its power supply has to deliver more current to keep the voltage stable and constant to provide enough juice during high demands. This is engineering 101, no matter if it is a beefy audio amp or a teeny TMDS digital signal. 

So at DPL we drive the device while we measure its supply. This requires a special test fixture that we custom made. When testing in “real world” mode most of the active devices failed. We mostly test long active products and when they go long (like over 10 meters) or very small in gauge, the voltage drops over distance due to higher current demand.

This robs the life-blood out of the bus, jeopardizing the HDMI’s operation and reducing the performance of the active device. This does not even take into consideration the data rate. As the data rate goes up the current demand typically follows its lead, so reaching out to 18Gbps sets new power limits too.

When using active products be sure you get the skinny on this. It is one of those failures that will have you bouncing off the wall at install or months later when you’re rolling trucks.