Fiber optic hdmi cable to break audio ground loop?

markebenson

Senior Member
Location
fl
I have a new venue where bands and djs will connect on stage to the main pa . They often arrive with unbalanced equipment. No major issues yet except when a laptop with unbalanced audio is connected WITH it's hdmi video connected to the video system also, then we get noise. If there is no hdmi connected there is no noise. I know I can isolate the audio side with a direct box but somene suggested simply using a fiber optic hdmi cord. I was curious if this would work.


Thx
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
I have a new venue where bands and djs will connect on stage to the main pa . They often arrive with unbalanced equipment. No major issues yet except when a laptop with unbalanced audio is connected WITH it's hdmi video connected to the video system also, then we get noise. If there is no hdmi connected there is no noise. I know I can isolate the audio side with a direct box but somene suggested simply using a fiber optic hdmi cord. I was curious if this would work.


Thx
HDMI is not an optical format; there are no transmitting lasers or optical receivers in an HDMI connector. There is no simple cable solution, but there are wireless HDMI transmitter receiver pairs you might consider, or maybe there's a similar fiber optical solution as well.
 

markebenson

Senior Member
Location
fl
Let me clarify. There is no problem with the video, it is clear as day. When the hdmi is plugged into the video system a ground loop is created and we hear audio noise.
 

Todd0x1

Senior Member
Location
CA
HDMI is not an optical format; there are no transmitting lasers or optical receivers in an HDMI connector. There is no simple cable solution, but there are wireless HDMI transmitter receiver pairs you might consider, or maybe there's a similar fiber optical solution as well.

So actually there are fiber HDMI cables. Each HDMI connector has an optical transceiver built in, with some the fiber optic cable is fixed and on some its detachable.
 

Todd0x1

Senior Member
Location
CA
Let me clarify. There is no problem with the video, it is clear as day. When the hdmi is plugged into the video system a ground loop is created and we hear audio noise.

Do you have any issues when audio and the laptop charger are plugged in but no hdmi? I have had this happen but through the laptop's charger.

I think I would be inclined to isolate the audio before the video. Whirlwind PC-DI or something like that.

Out of curiosity have you measured potential difference between the plugged in laptop's HDMI shield and the shield of the unplugged HDMI cable which is connected to the video system? Might want to make sure its just an audio issue and you don't have anything more serious going on.
 

synchro

Senior Member
Location
Chicago, IL
Occupation
EE
I think I would be inclined to isolate the audio before the video. Whirlwind PC-DI or something like that.
I agree with that. Also, I'd be concerned about the fragility of the HDMI cables with fiber in a stage environment, and they are costly to replace.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
So actually there are fiber HDMI cables. Each HDMI connector has an optical transceiver built in, with some the fiber optic cable is fixed and on some its detachable.
Here is the HDMI spec: https://en.wikipedia.org/wiki/HDMI

There is no mention of optical transceivers, only that "specialist optical extender solutions exist to extend distances even further by sending the signal over fiber optic cable"

I think that those solutions involve encoder/decoder hardware pairs that terminate HDMI on each end and have fiber between them, similar to the wireless HDMI transmitter/receiver pair that I have that uses RF instead of fiber. Is that what you are talking about?

That being said, it's possible that a simpler solution exists. For example, audio signals sent over balanced lines only use the grounded shield as protection from RF intrusion while unbalanced transmission uses the shield as part of the circuit. Conversion from unbalanced to balanced is simply a matter of a line transformer, and you can break the shield in a balanced circuit at some point to isolate the grounding of components to prevent ground loops.
 
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I'd be concerned about the fragility of the HDMI cables with fiber in a stage environment, and they are costly to replace.
Fiber extenders would usually be used either with built-in fiber (e.g. booth to stage) or so-called "tactical fiber" cable which is made to be handled- that's what most a/v and production companies are using now. And for simple isolation very short cables are quite cheap and often less than a quality HDMI cable.

Anyway, use a good transformer-based DI box for the audio, but you should always do that.
 

Todd0x1

Senior Member
Location
CA
Here is the HDMI spec: https://en.wikipedia.org/wiki/HDMI

There is no mention of optical transceivers, only that "specialist optical extender solutions exist to extend distances even further by sending the signal over fiber optic cable"

I think that those solutions involve encoder/decoder hardware pairs that terminate HDMI on each end and have fiber between them, similar to the wireless HDMI transmitter/receiver pair that I have that uses RF instead of fiber. Is that what you are talking about?

That being said, it's possible that a simpler solution exists. For example, audio signals sent over balanced lines only use the grounded shield as protection from RF intrusion while unbalanced transmission uses the shield as part of the circuit. Conversion from unbalanced to balanced is simply a matter of a line transformer, and you can break the shield in a balanced circuit at some point to isolate the grounding of components to prevent ground loops.

Here's a random example from google, of the cable I am talking about. As I previously mentioned these have optical transcievers built in to the HDMI connectors.

 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Here's a random example from google, of the cable I am talking about. As I previously mentioned these have optical transcievers built in to the HDMI connectors.

I didn't know they existed, but they are pricey. I would try running the audio though a balanced line with isolated shields first.

How long is the HDMI run?
 

markebenson

Senior Member
Location
fl
The hdmi cable itself is only 6ft, it connects to a cat 6 extender that coverts it back to hmdi at the distribution point in the it room. There is no audio noise unless the video hdmi cable
is also connected to the laptop.
 

retirede

Senior Member
Location
Illinois
The hdmi cable itself is only 6ft, it connects to a cat 6 extender that coverts it back to hmdi at the distribution point in the it room. There is no audio noise unless the video hdmi cable
is also connected to the laptop.

I’d consider using the fiber optic in place of the Cat6 portion.
 

Todd0x1

Senior Member
Location
CA
The hdmi cable itself is only 6ft, it connects to a cat 6 extender that coverts it back to hmdi at the distribution point in the it room. There is no audio noise unless the video hdmi cable
is also connected to the laptop.

Understood, but the problem exists in the audio so that's what should be isolated. The ground loop is being completed by the HDMI connection where the ground isn't part of the signal. Who knows what the unbalanced audio ground inside the laptop connects to, its possible it's not even connected to the same ground as the hdmi which is whats causing the noise. I would just use a transformer isolated DI and be done with it.
 
The hdmi cable itself is only 6ft, it connects to a cat 6 extender that coverts it back to hmdi at the distribution point
That highly suggests that the HDMI extender sender is a/the problem (and it would have been useful to know of them up front). To the best of my knowledge, HDbase-T extenders (I assume that's what you use) are electrically isolated by internal transformers (as is all twisted-pair ethernet).
 

Todd0x1

Senior Member
Location
CA
That highly suggests that the HDMI extender sender is a/the problem (and it would have been useful to know of them up front). To the best of my knowledge, HDbase-T extenders (I assume that's what you use) are electrically isolated by internal transformers (as is all twisted-pair ethernet).
There are alot of extenders out there which are not hdbase-t or hdmi over ip. These other types may not be isolated. OR is the cat6 shielded?
 

GeorgeB

ElectroHydraulics engineer (retired)
Location
Greenville SC
Occupation
Retired
Most of what is written here agrees with my experiences. What I'd add is that 2160x3840 (commonly known as 4k) makes some solutions more difficult. There is a (not competitive with mikeholt so I don't think the link will bother anyone) forum, AudioVideoScience, https://www.avsforum.com/forums/hdmi-q-a-the-one-connector-world.168/ with a HDMI specific thread with several posters having lots of similar discussions, usually in the interest of isolation for preventing electrical damage to multiple devices.

The solutions which I think would suit a venue such as the OP's are in the compressed wireless range. I've __NO__ experience beyond reading.
 

Todd0x1

Senior Member
Location
CA
Most of what is written here agrees with my experiences. What I'd add is that 2160x3840 (commonly known as 4k) makes some solutions more difficult. There is a (not competitive with mikeholt so I don't think the link will bother anyone) forum, AudioVideoScience, https://www.avsforum.com/forums/hdmi-q-a-the-one-connector-world.168/ with a HDMI specific thread with several posters having lots of similar discussions, usually in the interest of isolation for preventing electrical damage to multiple devices.

The solutions which I think would suit a venue such as the OP's are in the compressed wireless range. I've __NO__ experience beyond reading.

When using transmission mediums that use compression in a live environment you have to watch out for latency. IMHO this whole thing is solved with a $150 PC-DI
 
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