Networking & Cables

Boccaccio: A Better Way to Power Active HDMI Cables

HDMI cables, power supplies and DC currents don't always play well together.

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5 Comments
Posted by David Meyer on July 3, 2018

Thank you Jeff! I totally agree about the absolute need for integrators to take command of an active transmission line by using external power wherever available. Some active cable vendors infer that a source not providing enough power to drive their product is somehow inferior. But in fact a source need only provide 55mA to be complaint, of which the sink needs to be able to draw 50mA. Providing anything more is purely optional. However, this is typically not specified by manufacturers, so how do you know? You don’t. Well, you do Jeff, but integrators don’t wink.

Further to this, HDMI 2.1 effectively outlaws the practice of in-line devices drawing anything more than 5mA through the HDMI port, and mandates the inclusion of external power for such active cables or extenders.

Relying on the unknown quantity of HDMI internal power is a recipe for a callback. I’ve long advised that if the active device has a power socket, use it.

Posted by Julie Jacobson on July 3, 2018

David, thanks so much for chiming in. Consider that I’m pretty ignorant on the video side of things, but is there a simple tool to test the power coming out of an hdmi port? The beauty about being a site moderator is that if this is deemed a stupid question, I can delete it. jj

Posted by David Meyer on July 3, 2018

Julie, that is actually a very good question, but sadly the answer is no. The reason is that power is never sent, only drawn. Voltage could be measured easily enough within the 4.7-5.3V range in question, but to measure a source’s current output capacity would entail a tester drawing current, starting low and ramping up to see where the fail point is. The problem is that it doesn’t really indicate safe or stable limits, and it’s also possible that damage could result to the source (rare, but possible nonetheless).

The other issue is that active cable/extender manufacturers don’t usually disclose their operating power requirements either. So we’re in the dark on both power needed and power available. But Jeff’s absolutely right about going to the only predictable external power source. USB 2.0 only defines 100mA per unit load, and 150mA for USB 3.0, but many products need >200mA. Many USB ports can handle multiple unit loads, but they’re not always labelled! A wall wart or USB power adapter main be a pain, but it’s the only way to be sure.

Posted by Targa100 on July 4, 2018

We have tested a bunch of these from all walks of life and most did not come up with the correct numbers. One big reason for this was in fact the USB cable used with it. So in order to actually do a physical test on the supply we had to tap into the 3 power pins on the USB connector directly so there would be no other form of resistance distorting the test. Even doing that we still could not get most of the devices to meet spec. So you can see that this issue can manifest themselves by using long USB cables and relatively small gauge.

We also ran some numbers on “average current demand” from a sampling of products that need external power where David is spot on. They came in anywhere from 160ma to as much as 300ma.  You must also take into account that these numbers must be measure with the device fully operational with all three TMDS channels operating at rail into a differential load of 100 ohms with the necessary pull ups, any additional features like LED’s and such, and all DDC Data operating at specified rail (5volts) and running at 100Khz.

Power supply’s have always been critical and will continue to be as we move into Rev 2.1.

Posted by Jeff Boccaccio on July 4, 2018

Sorry folks, my last comment from Targa 100 was with my old CEpro account.
My new one is boccaccio

5 Comments
Posted by Jeff Boccaccio on July 4, 2018

Sorry folks, my last comment from Targa 100 was with my old CEpro account.
My new one is boccaccio

Posted by Targa100 on July 4, 2018

We have tested a bunch of these from all walks of life and most did not come up with the correct numbers. One big reason for this was in fact the USB cable used with it. So in order to actually do a physical test on the supply we had to tap into the 3 power pins on the USB connector directly so there would be no other form of resistance distorting the test. Even doing that we still could not get most of the devices to meet spec. So you can see that this issue can manifest themselves by using long USB cables and relatively small gauge.

We also ran some numbers on “average current demand” from a sampling of products that need external power where David is spot on. They came in anywhere from 160ma to as much as 300ma.  You must also take into account that these numbers must be measure with the device fully operational with all three TMDS channels operating at rail into a differential load of 100 ohms with the necessary pull ups, any additional features like LED’s and such, and all DDC Data operating at specified rail (5volts) and running at 100Khz.

Power supply’s have always been critical and will continue to be as we move into Rev 2.1.

Posted by David Meyer on July 3, 2018

Julie, that is actually a very good question, but sadly the answer is no. The reason is that power is never sent, only drawn. Voltage could be measured easily enough within the 4.7-5.3V range in question, but to measure a source’s current output capacity would entail a tester drawing current, starting low and ramping up to see where the fail point is. The problem is that it doesn’t really indicate safe or stable limits, and it’s also possible that damage could result to the source (rare, but possible nonetheless).

The other issue is that active cable/extender manufacturers don’t usually disclose their operating power requirements either. So we’re in the dark on both power needed and power available. But Jeff’s absolutely right about going to the only predictable external power source. USB 2.0 only defines 100mA per unit load, and 150mA for USB 3.0, but many products need >200mA. Many USB ports can handle multiple unit loads, but they’re not always labelled! A wall wart or USB power adapter main be a pain, but it’s the only way to be sure.

Posted by Julie Jacobson on July 3, 2018

David, thanks so much for chiming in. Consider that I’m pretty ignorant on the video side of things, but is there a simple tool to test the power coming out of an hdmi port? The beauty about being a site moderator is that if this is deemed a stupid question, I can delete it. jj

Posted by David Meyer on July 3, 2018

Thank you Jeff! I totally agree about the absolute need for integrators to take command of an active transmission line by using external power wherever available. Some active cable vendors infer that a source not providing enough power to drive their product is somehow inferior. But in fact a source need only provide 55mA to be complaint, of which the sink needs to be able to draw 50mA. Providing anything more is purely optional. However, this is typically not specified by manufacturers, so how do you know? You don’t. Well, you do Jeff, but integrators don’t wink.

Further to this, HDMI 2.1 effectively outlaws the practice of in-line devices drawing anything more than 5mA through the HDMI port, and mandates the inclusion of external power for such active cables or extenders.

Relying on the unknown quantity of HDMI internal power is a recipe for a callback. I’ve long advised that if the active device has a power socket, use it.