What is going on with super-expensive testing that seems to be allowing products to claim passing certification even without meeting the necessary power requirements — particularly active optical cables (AOC) we’ve had in for testing at DPL Labs? Maybe they are just buying time until the interface’s power specification changes over perhaps by 2023. Still, what does one do until then? For starters, take a moment to understand what the real deal is here with power.
The HDMI interface has onboard its multiple wire configuration one that is reserved for 5 volts. Over time, this voltage level has become a standard due to its huge exposure in the consumer electronics industry with USB being the leading driver. From the 5 volts, other lower voltage levels can be created via many fantastic supply solutions in the semiconductor world, we use them all the time.
That being said, however, the workable current needed for most applications is relatively high compared to what’s available on the HDMI power bus.
In most applications, power demands typically hover around 200ma to 500ma (0.2 to 0.5 amps). But the HDMI interface only has a total guaranteed current of 55ma. I say guaranteed because the specification only requires the source to have a minimum of 55ma. That does not mean source manufacturers couldn’t up that to 100ma or even 200ma on their own; it happens all the time.
The problem is that integrators don’t know this from product to product — set-top boxes, Blu-ray disc players, etc. are all over the map when it comes to current offerings. So, it is somewhat of a crapshoot. But with 48G 8K transmission slowly coming into view, the need for active cable transmissions will be common and all will need some form of supply voltage.
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Let’s crunch the numbers. The HDMI bus’ designated 5-volt wire is used to not only flag the sink (display) device it is attempting to communicate with by way of HotPlug but can also be employed for internal circuitry used to maintain the bus for the entire system whether the system is off or on.
The way it works, when the system is in the power off mode the current requirement is 55ma. When the system is in power on mode this current number can drop to only 5ma according to the spec. So, really, the installer has about 5ma at his or her disposal to use for whatever they may want or need when the system is on or off. But historically, getting anything done at even 50ma is near impossible. Should the system require the full 50ma for normal operation, it doesn’t leave much room to supply enough power to an active device especially when you consider current demand when the system is fully loaded and operating.
AOC hybrid products have active circuits in both the source and sink side of the cable. The good thing is that this technology has experienced some major improvements in lowering the amount of current consumption, but unfortunately it still doesn’t meet the power consumption limits. Even if you consider the power “on” state as being your most important function it still only provides 50ma. DPL Labs has seen AOC products come in as high as 300ma and now they are coming down to less than 60ma in some cases and as close as that is, it does not make the numbers.
As mentioned, these low current numbers on the bus will eventually increase as more power has been ruled in but probably will not surface for some time. For now, when AV installations must use legacy source hardware, pay extra attention. When in doubt, some external devices can be added, but be careful since they are not necessarily capable of delivering full 48G throughput for all four video channels.
Companies testing these AOC products have the option to use these external devices and are included with each DUT (Device Under Test). Not only are they used to supplement the power to achieve these low currents, but they are also tested for all high-speed video evaluation and analysis guaranteeing each product’s performance with and without an external device. It may be a trial-and-error procedure if you attempt to do this on your own.
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