BMPCC 4K Ground loop ? "Is it safe?"

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aknittel

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BMPCC 4K Ground loop ? "Is it safe?"

PostThu Jul 23, 2020 12:34 am

After extensive reading, I am totally on board a V – mount battery solution for the BMPCC 4K.

v-mount batteries (98W) with TWO d-taps should power the camera and a monitor (specifically SmallHD focus 7).

If I'm running a gimbal I can totally offload the weight on the gimbal or camera with a v- mount waist belt. When I’m on a stick, should be easy to rig the camera and v-mount battery with a passive v mount plate off the rig or tripod. Since there are no batteries needed plugged into the monitor, weight comes off the monitor as well.

So to build that flexibility, I'm considering the following.

SOONWELL B98-V 98Wh 14.8V Lithium-Ion Battery

Cheap passive V mount plate just for rigging the battery off a cage, rails, or waist belt

A dummy battery FOR THE SMALL HD that powers the monitor from the ONE of the D-TAPS on the
https://www.bhphotovideo.com/c/product/ ... ml/reviews

4. The OTHER d-tap ON THE BATTERY goes straight to the BMPCC D-TAP port

The BMPCC requires 22 watts when recording raw to external drive. So, I would guess the math is simple. If you have a 98 watt-hour battery, and the camera is drawing 22 watts + the monitor draw of 11 watts = 33 watts. 98/33 gives max almost 3 hours.


I've read these reports concerning issues with powering multiple components from ONE battery. Seems across the industry (BMC, Arri, etc.)

viewtopic.php?f=2&t=101481&p=596189&hilit=bmpcc+ground+loop#p596189

https://www.newsshooter.com/2020/02/15/ ... i-outputs/

viewtopic.php?f=2&t=107642

Seems like it’s just a matter of training oneself to plug the POWER taps in first, THEN the HDMI out from the BMPCC to the monitor. You can buy special “isolation” adapters for d-Taps but these seem expensive.

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Re: BMPCC 4K Ground loop ? "Is it safe?"

PostSat Jul 25, 2020 12:27 am

Bump. ANyone? BlackMagic?
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Re: BMPCC 4K Ground loop ? "Is it safe?"

PostSat Jul 25, 2020 5:40 am

The only way to be safe from a ground loop is to separate power or use a power distribution battery plate like the Wooden Camera battery distribution system.
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Re: BMPCC 4K Ground loop ? "Is it safe?"

PostMon Jul 27, 2020 12:02 am

That a $600 answer -- not great for Independent Filmmakers that just bought a $1200 camera and are spending $1000 just to rig it with acceptable power and a decent monitor.

I get we're supposed to rig the camera -- that's a bare bones solution, but I wish BMC would be a little more forthcoming with official statements about ground loops.

I've contacted TWO dual d-tap v mount battery suppliers -- Soonwell and IDX. Are they supposed to be able to answer for BMC if the power coming directly off their D-TAPS is also isolated and wont cause ground loops?

I can't believe the industry hasn't solved this as a standard for ANY camera -- RED, ARRI, whatever. "Solved" meaning it's a no-brainer issue and its fail safe and consumers (or prod) can't get themselves into trouble frying equipment just by plugging in rigs one way ot the other.

BMC can you PLEASE make a statement? Just tell us what we're supposed to buy, do, or avoid.
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Re: BMPCC 4K Ground loop ? "Is it safe?"

PostSat Aug 15, 2020 9:24 pm

Can we have a STATEMENT from BMC?

Just trying to keep my equipment safe and need a surefire procedure to power from ONE V-mount dtap monitor and camera. What is the best practice in workflow given this rig configuration.
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Re: BMPCC 4K Ground loop ? "Is it safe?"

PostSun Aug 16, 2020 4:27 pm

Can anyone from BlackMagic Design shed some light on this?

Can I raise a support case?
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Re: BMPCC 4K Ground loop ? "Is it safe?"

PostSun Aug 16, 2020 5:13 pm

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Re: BMPCC 4K Ground loop ? "Is it safe?"

PostWed Sep 16, 2020 12:08 am

Yes. The CORESWX solution is on the right track, but doesn't really solve the issue. A dummy battery is just that -- it STILL makes a connection direct to the camera.

I was watching this review of the SmallHD focus monitors:



Look what he is doing starting at 2:10.

I had the revelation that although there are TWO batteries involved here, the one that powers the monitor CAN BE FED and kept charged up by a V-MOUNT battery – basically a battery to battery connection via a D-TAP to DC input connection on the CORESWX NPF Flat pack.

https://www.coreswx.com/store.coreswx/product/npf-shd/

So I called CORESWX – they are local – and got a tech support guy who will tell me if this setup inherently avoids a ground loop, because i THINK it would effectively isolate the monitor from the V-Mount battery's other connection to the BMPCC connector.

Anyone care to comment if this is true?

So then I would choose a V-Mount with two D-TAPS, or one that mounts to a plate with two D-TAPS: one to the NPF Flat Pack battery that in turn powers the monitor and the other straight to the BMPCC.
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Re: BMPCC 4K Ground loop ? "Is it safe?"

PostThu Sep 17, 2020 7:48 pm

I was just about to create a thread asking a similar question because I recently ran into a power issue, and someone I know said it could've been a ground loop problem even though it wasn't the HDMI ports that were the victim, but instead my batteries. I figured it would be good input to this thread since I also prefer to power everything from one battery.

I posted a video explaining what happened (originally posted on Reddit, hence the "hey Reddit" at the start):
[I will update this with a link to the video when my account is allowed to post URLs. In the meantime, Googling "2 Gold Mount Batteries Died Out of Nowhere. Can anyone help me troubleshoot?" should yield the links to the two Reddit threads where I asked this question. One is under r/bmpcc and the other is under r/cinematography]

I recommend you watch it get the full picture of the issue and to see what my set-up looks like, but in short, what happened was that my 190wH D&O Lighting battery completely died during a shoot when I connected my IndiPro Tools Gold Mount plate to the monitor via the plate's d-tap. The cable used was a Lanparte d-tap cable. The camera was already connected to the plate, and the HDMI was already connected between the camera and the monitor. The monitor was the last thing that I had to connect and power. When I plugged in the d-tap cable from the plate to the monitor, the battery died.

Thinking this was a battery issue, I e-mailed D&O and asked for a replacement. They sent it to me, and all went well for one shoot. While prepping for the second shoot with the new battery, it completely died when I connected the 2 pin cable from the plate to the BMPCC4K. (Note: on this second occasion, the monitor was not yet connected via d-tap. So on the first occasion, the battery died when connecting the d-tap from the plate to the monitor. And on the second occasion, the battery died when connecting the plate to the camera).

I've posted this video on Reddit and gotten different answers to what the potential issue may be.
Here are the links to the threads in case you want to see what others had to say:

Thread 1:
[I will update this with a link to the thread when my account is allowed to post URLs. In the meantime, Googling "2 Gold Mount Batteries Died Out of Nowhere. Can anyone help me troubleshoot?" should yield the links to the two Reddit threads where I asked this question. One is under r/bmpcc and the other is under r/cinematography]

Thread 2:
[I will update this with a link to the thread when my account is allowed to post URLs. In the meantime, Googling "2 Gold Mount Batteries Died Out of Nowhere. Can anyone help me troubleshoot?" should yield the links to the two Reddit threads where I asked this question. One is under r/bmpcc and the other is under r/cinematography]

In summary:
- Someone said it's a damaged battery and that D&O batteries are unreliable and that I should use more reputable brands. Keep in mind that I used the first battery for a year with no issue, and then the replacement lasted only one shoot because it died during prep for the following shoot.
- Someone said the plate seems to be the culprit.
- Someone said the HDMI or d-tap cables I used could be the culprit.
- Someone said the battery fuse is gone, and it could've been a short in the d-tap terminals.
- Someone said it's a ground loop problem, but instead of my HDMI frying, my batteries did, and that I'm better off powering monitors separately from the camera (which I really don't want to do because I like having one battery powering everything, and I don't want to carry a bunch of different types of batteries).
- Someone said that I should always have everything connected BEFORE putting the battery on the plate because it can cause power spikes. This is something I never did, and think it could've honestly been the main problem, not the equipment itself, but my procedure....or it could be a mix of any of these suggestions. I don't know. I don't have the greatest electrical knowledge. I'm holding off on buying a new battery or a new plate or new cables until I have a better idea of why those two batteries died. Last thing I want is to buy a new battery and have it die on me, and be down a few hundred bucks.

Anyway, if someone knowledgeable could offer input into what I did wrong or where they think the issue lies, that would be greatly appreciated. And hopefully to anyone that stumbles upon this thread, they get some good insight when building out their BMPCC 4K.
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Re: BMPCC 4K Ground loop ? "Is it safe?"

PostFri Sep 18, 2020 7:46 am

If the battery died quickly, it surely is not a ground loop problem, or your monitor and camera would be totally fried. If it is a short, your battery plate did a nice job protecting.

Just a month ago I put three Eneloop AA batteries in a doorbell. I put one the other way around. The plastic casing was on fire in under a second.

Now imagine what happens with a 15V (?) 190wH battery ...
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Re: BMPCC 4K Ground loop ? "Is it safe?"

PostSun Sep 20, 2020 5:09 pm

BUMP!

Anyone from BMC want to comment on my "solution" with CORESWX? Is a seperate battery for the monitor in turn charged from the same V-Mount battery that is using the power input in the BMPCC avoid a ground loop?

Why is this so hard to get engineers to comment from BMC??? Doesn't it behoove you to test these configurations since it's so obvious that people have to rig the BMPCC for more power for any professional use?

Don't you want to avoid warranty claims?
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Re: BMPCC 4K Ground loop ? "Is it safe?"

PostWed Sep 30, 2020 11:44 pm

aknittel wrote:BUMP!

Anyone from BMC want to comment on my "solution" with CORESWX? Is a seperate battery for the monitor in turn charged from the same V-Mount battery that is using the power input in the BMPCC avoid a ground loop?

Why is this so hard to get engineers to comment from BMC??? Doesn't it behoove you to test these configurations since it's so obvious that people have to rig the BMPCC for more power for any professional use?

Don't you want to avoid warranty claims?


I'm with you. My camera is at Blackmagic right now because my v-mount fried the AC port, smoke out of the camera and everything. I asked if I could be put in contact with the techs but they only referred me to the forums to ask what other users are doing. Check out my thread called "AC port on my bmpcc4k fried today"

I have absolutely no idea why my camera went up in smoke, and I have no idea if when I get the camera back and I plug everything back in that it won't immediately happen again. I wish they had some kind of diagram or approved list of ways to do this because powering these cameras via v-mount is extremely common.
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Re: BMPCC 4K Ground loop ? "Is it safe?"

PostThu Oct 01, 2020 3:37 pm

If the techs at BM don't know, how can they assume the forum will know ?

There should be an official document describing best practice and what not to do.
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Re: BMPCC 4K Ground loop ? "Is it safe?"

PostFri Oct 02, 2020 2:50 am

Available options are:
Use separate battery for monitor. Safest and simplest option.

Use special splitter box with opto-isolator chip similar as described here viewtopic.php?f=2&t=101481&p=596189#p596189 Very rare, expensive and low wattage solution. opto-isolator is very rare thing. It is NOT the same as voltage regulator or step up/down converter used in most battery plate splitters..

Somehow connect all (-) wires before (+) wires. Disconnect all (+) wires before (-) wires. It is possible with some custom made on/off switch on positive (+) wire of DC cable or with build-in ON/OFF switch on some batteries. More info described in this thread viewtopic.php?f=2&t=107642 Still risky option.

Use quality HDMI cables. Damaged or low quality insulation may produce voltage leak and in some critical moment may damage HDMI chip.
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Re: BMPCC 4K Ground loop ? "Is it safe?"

PostFri Oct 02, 2020 6:15 am

The problem is, when connecting battery to a device there can be a current spike as the capacitors load etc. also long cables can have capacitance and cause voltage/current spikes when connecting.

Normally this is not a problem, it has been taken into account when designing the device.

For some reasons it seems that the engineers have not always taken into account the situation when more devices are connected to the same battery. They really should consider this quite normal situation. I am not sure if this is the case with P4K, maybe it is just fine regarding this also, and it would be good to get BMD comment on this.

The Problem can be, if HDMI is connected between camera and monitor, camera external power is already connected, and same external power is used for the monitor. On this situation when connecting the power tab to the monitor, if + pin connects first, the the GND is already connected via HDMI cable, and all the current will go trough the HDMI cable before the GND pin is connected on the power connector. Just the fraction of the second can be enough to burn the HDMI GND connection, if it is not designed to take this.

There is the above mentioned solutions, but really I would hope the design engineer take this into account when the device is on design phase, and:

- design the HDMI etc. GND connections strong enough to take the connection current spike.
- isolate the GND so that it is not a problem
- use power connector that makes sure GND is always first connected.
- have some smarter system verifying GND is properly connected, before connecting the power to the device.

Otherwise if using same unisolated power source for multiple devices, like connecting one V-lock battery to monitor and camera directly, is just an accident waiting to happen.

The question I have is, is the P4k designed so that it can handle the ground loops?

Ps. This is what happens when connecting a power cable to a device that has empty capacitors. In this case it is very clear as the capacitors are big and this is a 50V bicycle battery, but the same happens in smaller scale with smaller batteries and capacitors. If that current goes on a route it is not designed to go...



Pss. There can be also other problems related to ground loops, like disturbance on video or hum on audio.
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Re: BMPCC 4K Ground loop ? "Is it safe?"

PostFri Oct 02, 2020 8:13 pm

aknittel wrote:BUMP!

Anyone from BMC want to comment on my "solution" with CORESWX? Is a seperate battery for the monitor in turn charged from the same V-Mount battery that is using the power input in the BMPCC avoid a ground loop?

Why is this so hard to get engineers to comment from BMC??? Doesn't it behoove you to test these configurations since it's so obvious that people have to rig the BMPCC for more power for any professional use?

Don't you want to avoid warranty claims?


Have you contacted BM directly? When I have a question, that's what I've always done and never had a problem getting a helpful reply. Though BM does get on these forums this isn't a direct support site for questions, it's a user site to gather information from other users.
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Re: BMPCC 4K Ground loop ? "Is it safe?"

PostSat Oct 03, 2020 8:23 pm

I am curious. Has any one had this issue when they had all the cables hooked up before turning on the power on their battery plate?
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Re: BMPCC 4K Ground loop ? "Is it safe?"

PostMon Oct 19, 2020 7:28 pm

As far as contacting BM, can someone use a support request for that WITHOUT buying the camera first?

Im happy to do that.
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Re: BMPCC 4K Ground loop ? "Is it safe?"

PostTue Oct 20, 2020 7:23 am

lost_soul wrote:I am curious. Has any one had this issue when they had all the cables hooked up before turning on the power on their battery plate?

If on/off switch in your battery plate wired to (-) cable, soon or late it will cause permanent HDMI damage when you turn battery on. I personally damaged HDMI port on BMMCC in exact same way when experiment with my early battery enclosure prototype connected to camera and monitor. In final version of battery enclosure i changed cables layout and place on/off switch to (+) wire. So negative always connected before positive when you turn battery on.

It is unknown if all battery plates from other manufacturers use on/off switch connecterd to (+) wire.
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Re: BMPCC 4K Ground loop ? "Is it safe?"

PostWed Jan 13, 2021 9:39 pm

Sorry for bumping this up, I just had a brilliant idea ;) . I am using a p4k with a bm5 ii monitor that are both powered with one single v mount battery through one d tap port via d tap splitter. Sooo in order to eliminate this potential threat, I'm thinking about using the bm's bi-direction micro converter which will be powered by a power bank instead of using the same v mount battery. basically camera HDMI out to converter HDMI into SDI out to monitor SDI into SDI out to converter SDI into HDMI out to accsoon cineeye 2 HDMI in.

Will this solve the issue of the ground loop since the converter is using a different power source?
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Re: BMPCC 4K Ground loop ? "Is it safe?"

PostThu Jan 14, 2021 3:37 am

only if the converter cuts the ground, I do not think it does that, but maybe you could use ground separator on SDI, not sure if such exist, but this solution is getting really complex.

The simplest solution would be if there is power plugs that make sure that ground connects first. something like this, but reversed, i.e the outer part (GND) should connect fist, the inner part would need the isolator on the tip. Would be good that BMD verifies that the plug solves the issue and make it available.

But as I have not found such plugs, maybe not possible to make, so it is back to design table, the connectors should be such that ground connects always first.

(the one on the picture would be bad as it makes sure + connects fist and that is exactly the problem)
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Re: BMPCC 4K Ground loop ? "Is it safe?"

PostThu Jan 14, 2021 4:03 am

Zinlioren wrote:Sorry for bumping this up, I just had a brilliant idea ;) . I am using a p4k with a bm5 ii monitor that are both powered with one single v mount battery through one d tap port via d tap splitter. Sooo in order to eliminate this potential threat, I'm thinking about using the bm's bi-direction micro converter which will be powered by a power bank instead of using the same v mount battery. basically camera HDMI out to converter HDMI into SDI out to monitor SDI into SDI out to converter SDI into HDMI out to accsoon cineeye 2 HDMI in.

Will this solve the issue of the ground loop since the converter is using a different power source?


I got a kick out of reading this, but I have a serious question. Is there a reason why you wouldn't just use a separate battery to provide power to the monitor? Maybe I'm missing something, but your proposed solution makes it sound like you're an admirer of Rube Goldberg :)
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Re: BMPCC 4K Ground loop ? "Is it safe?"

PostThu Jan 14, 2021 9:32 am

I was looking for solution as would need it for my own use also. Something like this would likely work pretty nicely

This one operated from 9 to 36 V input and provides 15V 1A isolated out.
https://www.digikey.fi/products/fi?keyw ... -Q24-S15-D

Or maybe a bit higher power version.

This one operated from 9 to 36 V input and provides 15V 2A isolated out.
https://www.digikey.fi/product-detail/f ... ND/9859975

There is many available and maybe requires some testing to find good ones. The power and voltage needs to be selected suitable for the application and ideally the circuitry should look something like this

Screenshot 2021-01-14 at 11.35.59.jpg
Screenshot 2021-01-14 at 11.35.59.jpg (80.66 KiB) Viewed 1999 times


There is also models where wires are attached with screws, likely also ones that need no additional components, but in general better not use these unless understanding what doing.
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Re: BMPCC 4K Ground loop ? "Is it safe?"

PostThu Jan 14, 2021 10:47 am

Kim Janson wrote:I was looking for solution as would need it for my own use also. Something like this would likely work pretty nicely

This one operated from 9 to 36 V input and provides 15V 1A isolated out.
https://www.digikey.fi/products/fi?keyw ... -Q24-S15-D

Or maybe a bit higher power version.

This one operated from 9 to 36 V input and provides 15V 2A isolated out.
https://www.digikey.fi/product-detail/f ... ND/9859975

There is many available and maybe requires some testing to find good ones. The power and voltage needs to be selected suitable for the application and ideally the circuitry should look something like this

Screenshot 2021-01-14 at 11.35.59.jpg


There is also models where wires are attached with screws, likely also ones that need no additional components, but in general better not use these unless understanding what doing.


Thx for the detailed info :) I guess the best solution for me might be just to use a different power source for the monitor then. I can feel my brain cells evaporating reading this.
Last edited by Zinlioren on Thu Jan 14, 2021 10:50 am, edited 1 time in total.
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Re: BMPCC 4K Ground loop ? "Is it safe?"

PostThu Jan 14, 2021 10:48 am

robedge wrote:
Zinlioren wrote:Sorry for bumping this up, I just had a brilliant idea ;) . I am using a p4k with a bm5 ii monitor that are both powered with one single v mount battery through one d tap port via d tap splitter. Sooo in order to eliminate this potential threat, I'm thinking about using the bm's bi-direction micro converter which will be powered by a power bank instead of using the same v mount battery. basically camera HDMI out to converter HDMI into SDI out to monitor SDI into SDI out to converter SDI into HDMI out to accsoon cineeye 2 HDMI in.

Will this solve the issue of the ground loop since the converter is using a different power source?


I got a kick out of reading this, but I have a serious question. Is there a reason why you wouldn't just use a separate battery to provide power to the monitor? Maybe I'm missing something, but your proposed solution makes it sound like you're an admirer of Rube Goldberg :)


Ahahaha. Didn't meant to be like him xD (but I will admit that I absolutely love his work). Welp, as for now I only have 6 npf970 that I need to power one slider, three yn360 light wands at the same time occasionally, but I have 4 V-mount batteries and a fast charge station, so it makes more sense to me to use the v mount to power as much stuff as I can to save some npf batteries.
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Re: BMPCC 4K Ground loop ? "Is it safe?"

PostThu Jan 14, 2021 7:18 pm

Isolation is a natural by-product of using and induction transformer. Upstream there's some additional hardware but the end of the circuit is a textbook transformer.

Soundwave.jpg
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Functionally, this isn't any different than any DC-DC transformer you'd find in a number of HDMI/SDI smokin' battery plates. Extra precaution needs to be taken to lift common chassis sources as well. Consider for a moment that grounds are also a safety measure to protect humans and their electronics before lifting them superstitiously or without care.

Good Luck
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Re: BMPCC 4K Ground loop ? "Is it safe?"

PostTue Apr 13, 2021 10:09 pm

@Zinlioren the only thing I can say about not requiring a separate battery is that it saves on weight. I run the camera on a gimbal ALL THE TIME, even on a tripod - it's on the gimbal. When it's on a tripod I clip the v-Mount battery on the tripod.

Now when I take it OFF the tripod, the v-MOUNT slips into my belt, or on the vest of the Steadicam mount for the gimbal. If I add yet ANOTHER v-MOUNT to all that, it just adds more total weight.

I don't need a separate battery for the monitors I'm using. Their power draw is low enough.
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Re: BMPCC 4K Ground loop ? "Is it safe?"

PostWed Apr 21, 2021 12:35 pm

If I never had a ground loop issue with the original Pocket in the exact same setup, is there reason to be concerned with the BMPCC 4K? Is there a difference between how the two cameras handle power or HDMI? The only difference I can think of with my setup is the adapter from full sized HDMI to micro HDMI.

On the Pocket, I powered everything from a V-mount battery plate, never hot plugging. There's an on-off switch on the plate that I keep off unless everything is connected and the battery mounted. I also remove the battery if I'm plugging or unplugging cables. Camera was powered from 12v barrel connector. EVF was powered with D-tap to XLR cable. And even Tascam DR-70D was powered from USB, though I won't be doing that on the BMPCC 4K.

Is there reason to be concerned if I change cameras?
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Mark Foster

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Re: BMPCC 4K Ground loop ? "Is it safe?"

PostWed Apr 21, 2021 2:13 pm

this is also true for HDMI and also applicable to almost all cameras (BMD, RED, canon etc):

https://www.arri.com/service/search/en/ ... DI+Outputs
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Re: BMPCC 4K Ground loop ? "Is it safe?"

PostWed Apr 21, 2021 4:24 pm

Mark Foster wrote:this is also true for HDMI and also applicable to almost all cameras (BMD, RED, canon etc):

https://www.arri.com/service/search/en/ ... DI+Outputs


Sounds like if I follow Arri's rules (that I was already following) I should be fine.
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Re: BMPCC 4K Ground loop ? "Is it safe?"

PostWed Apr 21, 2021 4:28 pm

Connect HDMI cable before DC cables.
Disconnect DC cables before HDMI cable.


To avoid problems and damages - negative pin (-) or (ground) should be touched first. HDMI cables designed in the way like this (-) always touched before other pins.
DC cables and sockets are designed very random ways and in many cases (+) touches before (-) This may cause HDMI port damage if monitor and camera powered from same battery.

But Arri sugegst the opposite way here https://www.arri.com/en/learn-help/lear ... ion-196368
Last edited by Dmitry Shijan on Thu Apr 22, 2021 9:43 am, edited 1 time in total.
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Re: BMPCC 4K Ground loop ? "Is it safe?"

PostWed Apr 21, 2021 7:20 pm

Dmitry Shijan wrote:Connect HDMI cable before DC cables.
Disconnect DC cables before HDMI cable.

To avoid problems and damages - negative pin (-) or (ground) should be touched first. HDMI cables designed in the way like this (-) always touched before other pins.
DC cables and sockets are designed very random ways and in many cases (+) touches before (-) This may cause HDMI port damage if monitor and camera powered from same battery.


The connection order you have specified here is the exact opposite of Arri’s suggested order above.

The lack of clarity on this issue is maddening - I’ve seen experienced folks suggest either order while providing similar explanations (positive pin touches ground momentarily during connection or positive pin touches first, completing the power circuit through the video cable).

Someone reading this thread from beginning to end would have no idea which advice is correct.
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Mark Foster

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Re: BMPCC 4K Ground loop ? "Is it safe?"

PostWed Apr 21, 2021 8:58 pm

Dmitry Shijan wrote:Connect HDMI cable before DC cables.
Disconnect DC cables before HDMI cable.....


no, the other way around!

first connect POWER then VIDEO
then first disconnect VIDEO and then POWER




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Howard Roll

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Re: BMPCC 4K Ground loop ? "Is it safe?"

PostWed Apr 21, 2021 9:40 pm

Arri's only been making video cameras for a couple years longer than Blackmagic and they haven't quite figured it out yet either, Grass Valley, Ikegami, Sony, Hitachi, et al. are having a laugh.

Good Luck
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Mark Foster

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Re: BMPCC 4K Ground loop ? "Is it safe?"

PostWed Apr 21, 2021 10:34 pm

Howard Roll wrote:Arri's only been making video cameras for a couple years longer than Blackmagic and they haven't quite figured it out yet either, Grass Valley, Ikegami, Sony, Hitachi, et al. are having a laugh.

Good Luck


have already seen RED and SONY venice destroyed by ground loop
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Re: BMPCC 4K Ground loop ? "Is it safe?"

PostWed Apr 21, 2021 10:36 pm

Howard Roll wrote:Arri's only been making video cameras for a couple years longer than Blackmagic and they haven't quite figured it out yet either, Grass Valley, Ikegami, Sony, Hitachi, et al. are having a laugh.

Good Luck


it's really 12G that has caused it. There's a giant thread on Reduser asking why Komodo has this massive problem and other RED cameras don't. Pretty sure Komodo is the first 12G SDI port they've done too. Monstro only has 3G SDI.

To get the 12G bandwidth the usual ESP type protections circuits have had to be removed because they interfere with the higher frequency signals.

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Howard Roll

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Re: BMPCC 4K Ground loop ? "Is it safe?"

PostThu Apr 22, 2021 12:49 am

BM
John Brawley wrote:To get the 12G bandwidth the usual ESP type protections circuits have had to be removed because they interfere with the higher frequency signals.


It's a cost based decision, besides none of the usual BM burnouts are even 6G. The BMCC, BMPCC, P4k, Micro Cinema, are all 3G at best. Sure 12G is harder/more expensive but that has nothing to do with the P4K.

Good Luck
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Re: BMPCC 4K Ground loop ? "Is it safe?"

PostThu Apr 22, 2021 5:19 am

It is very weird the problem is not being addressed by the camera and accessory manufacturers. They just assume all devices will be powered from separate battery and not their responsibility if someone uses 3rd party power solution.

The solution is simple:

1. making sure that HDMI, Audio, SDI etc ground can carry the load if needed or

2. making sure the device can not be powered on unless power is properly connected or

3. connecting cable ground internally after the device is powered or

4. Isolating the cable grounds or

5. making sure the power connector is such that ground connects always first or

6. providing a separate ground point to connect device grounds together with a separate ground wire or

7. isolated DC/DC converter on device (they likely have DC/DC converter anyway, isolated is just a bit more expensive)

8. many of the above combined and possibly something else,

but for sure there is solutions for this real problem.

The 3rd party solution is the ground isolated DC/DC converters, but as mentioned above they can cause interference as they are cutting the power high frequency to do the isolation and voltage conversion. Good quality converters should though not cause problems.

The rules on what order connect cables will not work, or are at least too risky. At some point a connector comes loose in middle of shoot and needs to be reconnected or a monitor needs to be changed and the rules are forgotten.
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Dmitry Shijan

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Re: BMPCC 4K Ground loop ? "Is it safe?"

PostThu Apr 22, 2021 9:26 am

Mark Foster wrote:
Dmitry Shijan wrote:Connect HDMI cable before DC cables.
Disconnect DC cables before HDMI cable.....


no, the other way around!

first connect POWER then VIDEO
then first disconnect VIDEO and then POWER
.


Interesting. So so yep, Arri suggests like this. https://www.arri.com/en/learn-help/lear ... ion-196368

When connecting cables: FIRST connect power cables, THEN the BNC.
When disconnecting: FIRST disconnect the BNC, THEN power.

But that is strange because it is not about POWER or VIDEO. It is about connecting ground (-) pin first of all.
Ground is common for POWER and VIDEO and camera metal body as well.

You may also make a DIY DC cable with on/off switch located on (+) wire. So it will help to connect (-) before (+) as well.
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Re: BMPCC 4K Ground loop ? "Is it safe?"

PostThu Apr 22, 2021 1:11 pm

You just can't make an HDMI connection capable of carrying any current ... just doesn't exist.

- The shield is often connected through a RC network to prevent current from flowing for EMI compliance and avoid noise pickup/radiation.
- The other "GND" lines (Drain) are designed as signal reference, specifically to carry no currents so the potential is identical on both ends.
- Even if those were connected, you still have the bigger issue is that the data lines, when driving a "0" on the line are effectively paths to ground (through a tiny NMOS transistor) and if for some reason (bad psu connection) they start carrying significant current, they will burn up in a jiffy. Those are driven by transistors designed to toggle billions of times per seconds, it's just impossible to make them resiliant to high currents.

You can use isolated DC/DC conversion internally but not only is that costly, it's also _bulky_. Take even a basic BMPCC4k, power is up to 30-40W, it would take a significant amount of space inside to fit the magnetics required for an isolated DC/DC with enough power margin. They are also much less efficient than their non-isolated counterparts. (more heat / more power draw).

It's actually way easier to do externally since you can design magnetics with single primary and several secondaries for several outputs as a non-regulated isolated DC/DC. The output voltage will vary a bit depending on load but since this will be post-regulated anyway by another DC/DC, it doesn't matter ( just need to filter out the high frequency content that could leak through ).
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Re: BMPCC 4K Ground loop ? "Is it safe?"

PostThu Apr 22, 2021 1:15 pm

@Dimitry:

If you connect power first (with nothing else plugged in the cam), the cam will be in a safe state because power always "flows back" through the path of least inductance, which will be the GND lead of the power cable.

As long as the power cable stays plugged, it'll be the preferred current return path (mostly) and you can plug/unplug accessories.

And since when plugging it, nothing else it plugged in the cam, doesn't matter if + or - connects first ... current will only flow when both are connected (since that's the only path it could take).


The situation you want to avoid at all cost is having the power lead + cable connected and _not_ the - power lead cable but have another path through HDMI / SDI / whatever that can lead back to the PSU ground.
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