How to get rid of analog video noise?

[TiredOldDad]

Here's the scenario:

I have a Roku Express+ streaming device installed in the car and is powered through a 5V micro-USB.. It also has a composite video (and line audio) output which is connected to my Kenwood head unit's analog A/V input (Video 1).



Now, when the Roku is powered using any 12V to 5V USB power supply, I get some subtle, static-like noise on the video display. However, if the Roku is powered from a USB power bank, the video is crystal clear. Whether the engine is running or not, doesn't make any difference.

I have a hunch that there's likely a ground loop going but I don't know how to prevent it. Any help is appreciated.

[Renate]

You don't say what kind of 12V/5V converter you are using. Some dollar store "cigarette lighter" adapter?
Most of those little adapters are really cheesy. You're probably getting a lot of switching noise through there.
Also, it might be better to get your power for the Roku from as close a source to the Kenwood head unit as you can to cut down on ground currents.

[samnmax]

You can try ground loop isolators, or you can power the Roku with an isolated 5V DC-DC converter.

[nali]

As mentioned above the 12V-5V converter could be responsible for the noise. If it's anything like the cheap eBay buck converter I bought, they're noisy AF, and looking at the PCB layout it just looks like someone has plonked compoinents down so they look nice and joined up the dots.

You could check the ground loop by powering your adapter from a seperate battery if possible.

[Manul]

You could do an experiment and power your 12V to 5V converter from an isolated source, like a spare lead acid battery.

[BradC]

I tried something similar once only to find out the DC/DC converter was a switched ground rather than common ground. Once I plugged the A/V cables in and completed the earth the device suddenly had 12V into the 5V input and let the smoke out.

On the bright side, there was no analog video noise.

[TiredOldDad]

Appreciate the help. I'll try to address each reply...

You don't say what kind of 12V/5V converter you are using. Some dollar store "cigarette lighter" adapter?
Most of those little adapters are really cheesy. You're probably getting a lot of switching noise through there.
Also, it might be better to get your power for the Roku from as close a source to the Kenwood head unit as you can to cut down on ground currents.

I've tried all kinds of 12V-to-5V adapters, from branded Samsung to the cheesy stuff... same outcome.

But you may be on to something on the power source. The Roku is powered from the accessory (ACC) line. The same goes with the HU's ACC input (so it turns on and off with the ignition key). However, the HU's main power input is directly connected to the battery (B+) and fused. The problem is, without designing an elaborate switched relay, connecting the Roku's power to this terminal means it will draw power continuously from the battery.

You can try ground loop isolators, or you can power the Roku with an isolated 5V DC-DC converter.

This was my first go-to but I found out that the ones that I bought only have isolated USB data lines, which is useless as the Roku just needs the 5V and Gnd for power. I tried it nevertheless but it's still the same outcome.

As mentioned above the 12V-5V converter could be responsible for the noise. If it's anything like the cheap eBay buck converter I bought, they're noisy AF, and looking at the PCB layout it just looks like someone has plonked compoinents down so they look nice and joined up the dots.

You could check the ground loop by powering your adapter from a seperate battery if possible.

You could do an experiment and power your 12V to 5V converter from an isolated source, like a spare lead acid battery.

Yes, powering the Roku from a portable USB power bank solved the issue. But this is not sustainable and can be rather cumbersome to do.

But it did occur to me that the static could be caused by buck converter noise. How does one build a voltage dividing network to get 12V down to 5V (without switching) to isolate if the noise is from the switching circuit?

I tried something similar once only to find out the DC/DC converter was a switched ground rather than common ground. Once I plugged the A/V cables in and completed the earth the device suddenly had 12V into the 5V input and let the smoke out.

On the bright side, there was no analog video noise.

Yeah, at face value, I "think" that I need the ground on the Roku to be floating and isolated from the HU ground. Then again, having different ground references can prove be a bad idea and this is not something I want to find out the hard way.

[BrokenYugo]

But it did occur to me that the static could be caused by buck converter noise. How does one build a voltage dividing network to get 12V down to 5V (without switching) to isolate if the noise is from the switching circuit?

You'd want to use a linear regulator for that, something like the LM7805 might work with a reasonable heatsink, depending on the current consumption of the Roku.

[Renate]

I've used Velleman buck converter modules. At least they have a little voltage trimmer. I've used them for 5V for Raspberry Pi and for ~4V for Android battery replacement.
https://www.velleman.eu/products/view/?id=435566

Edit: I just did another battery-ectomy for my Walmart tablet and it's running on 4V from a Velleman on my dashboard.

[TiredOldDad]

You'd want to use a linear regulator for that, something like the LM7805 might work with a reasonable heatsink, depending on the current consumption of the Roku.

You're right. I'll look for a finished product or build one myself. The Roku doesn't need a whole lot of current.... just 5V, 1A, I believe. I'll double check.

Thanks!

[TiredOldDad]

Found one online. I can probably jerry-rig this to a USB pigtail to power the Roku. This will take a while. I'll revert back whatever the outcome may be.

DC-DC L7805 LM7805 7805 Tri-Ended Regulator Module 5V Regulator Power Supply Module

[Ed.Kloonk]

You can try ground loop isolators, or you can power the Roku with an isolated 5V DC-DC converter.

That's what I'd try. Run the device off a battery bank.

The other suggestion I had was try a video balun. We used to use these for running long distances but they may help with ground mis-matches, if that's what is causing it.

https://learncctv.com/balun-for-cctv/