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yet another laptop noise problem - hdmi

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outkast View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote outkast Quote  Post ReplyReply Direct Link To This Post Posted: 04 February 2019 at 3:37pm
May or may not be relevant, but I had a noise problem with an Apple Macbook 2012 which had a slightly different power supply to my older 2006 Macbook. The noise went away when I used the older power supply on the newer laptop, so the issue was definitely with the power supply.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote concept-10 Quote  Post ReplyReply Direct Link To This Post Posted: 04 February 2019 at 4:08pm
Originally posted by cravings cravings wrote:

i'm sure that answers your question.

Fully sorted Wink
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Post Options Post Options   Thanks (0) Thanks(0)   Quote MattStolton Quote  Post ReplyReply Direct Link To This Post Posted: 04 February 2019 at 5:02pm
Are you taking audio out of the HDMI, or out of the headphone socket?

If out of headphone socket, or even a USB audio breakout, we use these pieces of nastiness, to remove hum, clicks, whirrs etc, that arise when PSU is plugged into mains and laptop...

http://www.avsl.com/product/201.086UK-ground-loop-isolator-rca-plugs-rca-sockets

Hate them but they work, every time...I'm sure if you measure one, the transformers saturate and don't transfer much below 50Hz, but that is less objectionable than the fizz of laptop PSUs.

If you want something posher, DI's with an earth lift switch tend to work, and give a balanced O/P.

Before I get flamed, I know that most PSUs are double insulated, so CPC is spurios and redundant, and the Laptop itself is SELV, and its "0v" is floating around in the ether or to "N", tied to only your mixers audio earth; but these cheap bodgit boxes just work. Easy to take apart, and hardwire cable of choice to the 2x 1:1 transformers PCB, and then tape or heat shrink the enclosure back together. And cheap.

Horrible things, but needs must.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote MattStolton Quote  Post ReplyReply Direct Link To This Post Posted: 04 February 2019 at 5:05pm
Sorry, just saw your issue is with HDMI embedded audio. So much for noise free digital audio quality!

Perhaps extract audio from HDMI, into analogue, and then ping it into Hi-fi. Then if noise still present, cheap bodgit boxes?
Matt Stolton - Technical Director (!!!) - Wilding Sound Ltd
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Post Options Post Options   Thanks (0) Thanks(0)   Quote MattStolton Quote  Post ReplyReply Direct Link To This Post Posted: 04 February 2019 at 5:13pm
HDMI over Cat5/6/7?

A lot of the things to extend HDMI past the notional 10M limit, and transfer by CatX cable, may well offr some form of galvanic isolation of the audio?

HDMI "RGB" is sent digitally, balanced (i.e. +1/0 and -1/0), but has a 0V core too. So there is three 0V. Audio is also digital data, but with a 0V reference, so another 0V ref, then the +5V has a 0V ref, and the overall screen!


Just thinking all that bodged over some form of CatX Tx/Rx set may get you down to just one 0V ref, which may help?


Whole range of them here:

https://cpc.farnell.com/c/computer-office/computing/extenders-splitters/cat5-extenders/hdmi-cat5-extenders/hdmi-extender-over-cat5
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Post Options Post Options   Thanks (0) Thanks(0)   Quote mini-mad Quote  Post ReplyReply Direct Link To This Post Posted: 04 February 2019 at 5:20pm
Have you tried a USB to HDMI adaptor? 
Maybe the noise won't be present on the USB sockets...


If it sounds like a gorilla is trying to escape, turn it down.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote MattStolton Quote  Post ReplyReply Direct Link To This Post Posted: 04 February 2019 at 5:24pm
Forget the TMDS clock has a screen too, so another source of 0V...And the DDC/EDID has a screen...but it still is 5 screens, and overall screen...
Matt Stolton - Technical Director (!!!) - Wilding Sound Ltd
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Post Options Post Options   Thanks (0) Thanks(0)   Quote MattStolton Quote  Post ReplyReply Direct Link To This Post Posted: 04 February 2019 at 5:26pm
Originally posted by mini-mad mini-mad wrote:

Have you tried a USB to HDMI adaptor? 
Maybe the noise won't be present on the USB sockets...




IMHO, if Video 0V is noisey, Audio 0V is noisey, the USB bnus 0V will be even noisier!

Worth a try though, if you have one kicking about, strangest things can cure this noise.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote mini-mad Quote  Post ReplyReply Direct Link To This Post Posted: 04 February 2019 at 5:33pm
Originally posted by MattStolton MattStolton wrote:

Originally posted by mini-mad mini-mad wrote:

Have you tried a USB to HDMI adaptor? 
Maybe the noise won't be present on the USB sockets...




IMHO, if Video 0V is noisey, Audio 0V is noisey, the USB bnus 0V will be even noisier!

Worth a try though, if you have one kicking about, strangest things can cure this noise.

The thought was the combined 0V rail MAY have an impact on the noise (in a good way) as they will all be steming from the same place on the USB output.

As you say... stranger things!

...to add, maybe a USB powered hub would isolate the laptops rails and then plug in a USB to HDMI adaptor... I know we are getting into the realms of silliness at this point but it saves removing the earth bond of the PSU or anything daft



Edited by mini-mad - 04 February 2019 at 5:36pm
If it sounds like a gorilla is trying to escape, turn it down.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote godathunder Quote  Post ReplyReply Direct Link To This Post Posted: 04 February 2019 at 6:43pm
Originally posted by kedwardsleisure kedwardsleisure wrote:

that's going to be a hell of a linear power supply! It'll need to use a switching regulator to get that sort of current without a heatsink the size of a potato waffle

really? it seemed fairly sensibly sized to me - 

Ive probably got a big enough 230-20ish v tx and rectifier etc somewhere
would I be able to parallel a pair of these to regulate it?

mounted on

watcha think?

edit: apparently paralleling regulators is not quite as easy as hoped




Edited by godathunder - 04 February 2019 at 7:58pm
LOUDER THAN LOUD
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Post Options Post Options   Thanks (0) Thanks(0)   Quote studio45 Quote  Post ReplyReply Direct Link To This Post Posted: 05 February 2019 at 5:08pm
You can use a series pass transistor to handle the current, controlled by the regulator - you're still going to end up with a right heavy brick-like unit, though. 
Conceptually, there's switching noise present on the audio because somehow, the path to ground via the HDMI has a lower RF impedance than the neutral connection of the power supply. Why that should be I couldn't say, but that will be the mechanics of what's happening. It all ought to be decoupled in the supply circuitry, but perhaps some caps are either missing or defunct? 
Can you add a real earth connection - put on a 3-pin plug, add a green/yellow wire - and tie it to the power supply output's 0v side? Unsure of the safety implications of that, but it should ensure that the lowest impedance path for noise is not through the HDMI grounds any more!
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Danielr Quote  Post ReplyReply Direct Link To This Post Posted: 05 February 2019 at 5:40pm
Can you send video (assuming that is why you use hdmi) via HDMI. 
But select the output audio device as headphones. Use regular line cable and ground lift as appropriate? 

Not really helping you solve the problem, but maybe working around it? 
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