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USB Isolator advice needed


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I'm using a Focusrite Scarlett 2i2 USB audio interface to connect microphone audio to the computer for processing (Audiomulch/Ozone 5) and then output through the Focusrite to a radio transmitter. There's audible 60hz hum on the audio at the transmitter. I've been unable to tame the hum via better grounding, use of balanced connection, etc. I'd prefer to isolate the USB port and only use an audio isolation transformer as a last resort.

 

I purchased a ADMU4160 based isolator supposedly rated at 350ma (Focusrite draws 310ma). I've been unable to get Windows applications or Windows Sound to recognize my Focusrite when it's connected through this isolator, even though it shows up fine in Device Manager. I've also gone through a reinstall of the driver with no change.

 

So I'm wondering if anybody might have some advice on how to resolve the issue with the Focusrite/isolator not being recognized as an available soundcard by Windows applications. Also, can anyone recommend an alternate usb isolation product that would work reliably under Windows?

 

Thanks for any help.

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Thanks for the suggestion but I don't think that will have an effect on 60hz hum.

 

Goto Control Panel/Hardware/Look for Sound Device or other devices with a yellow check/right click/reinstall software

and it should work unless there is something else wrong

fmak

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The USB isolator you have is only good for up to full speed mode (12Mbps), but your Focusrite almost certainly uses high speed mode (480mbps). You can't run a high speed device through a full speed isolator, it won't even see it.

 

There are no USB high speed isolators. The iFi mentioned is not really an isolator, it may offer some degree of help with ground loops, the Wyred offers no advantage for ground loops.

 

There IS a way to get true galvanic isolation at high speed, Adnaco makes a PCIe over fiber optic system, one end plugs into a PCIe slot on your PC, in the other end you put a PCIe to USB board. This works well at high speed, but is NOT cheap.

 

John S.

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The USB isolator you have is only good for up to full speed mode (12Mbps), but your Focusrite almost certainly uses high speed mode (480mbps).

 

John, could you explain why you say this? It seems that 96K sampling @ 24 bits, 2 channels will easily be handled with 12Mbps mode. My math says it is only 4.7Mbps. But if the Focusrite specifies USB 2.0, as opposed to USB 1.1, does that imply it exceeds the 12Mbps mode?

 

thanks!

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John, could you explain why you say this? It seems that 96K sampling @ 24 bits, 2 channels will easily be handled with 12Mbps mode. My math says it is only 4.7Mbps. But if the Focusrite specifies USB 2.0, as opposed to USB 1.1, does that imply it exceeds the 12Mbps mode?

 

thanks!

 

USB runs at specific speeds, it is not a continuum. Full speed is 12MBPS, high speed is 480MBPS. If you are not using the full bandwidth of speed you are set for the packets contain less than full payload. Full speed and high speed use a completely different electrical protocol, so a device that is designed for just full speed cannot talk AT ALL with a device designed for high speed, they don't even see each other. Even if the high speed device is running at 3Mb per second of data, it is still using the high speed protocol and actually sending bits over the wire at 480, it's just that most of those bits are unused by the application.

 

The Focusrite 2i2 is designed for 2 channels in and two channels out at 24/96. That is actually 4 channels going over the USB bus at a given time. You can't do that with full speed mode, so it HAS to use high speed mode. The hardware in the box is designed to use high speed mode, so that is what it will use, even if you are only using two out channels, which could theoretically work at full speed, the hardware is built for high speed, so that is what it uses, even if it is only using a small fraction of the high speed bandwidth.

 

Thus any of the full speed only isolators will not work with this device.

 

John S.

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Perhaps it is an obvious question, but why it is that isolators only seem to be available for 12MB/s "full-speed" USB?

I mean, we have optical connections capable of signalling at 10Gb/s+ so it does not seem like there would be a technical limitation.

 

Analog Devices makes a chip that is a USB full speed isolator, NOBODY makes a chip that is a high speed isolator. If one of the chip companies did a high speed isolator chip, there would be lots of products using it.

 

It certainly can be done by buying parts and putting an isolator together, but it is not cheap or easy. Part of the problem is that USB2.0 is not a simple single interface, it is a complex marriage of two different interfaces, you can't just have a simple receiver -> isolator -> transmitter, you need a receiver that actually knows the protocols to switch between the two types of interface.

 

It should be possible to do it with two high speed phys and a processor that understands USB protocol, but nobody has actually sat down and done this, it's a fair amount of work to do. And the hardware itself would not be cheap either.

 

It looks like nobody considers it worth while enough to spend the time, effort and money to do it.

 

John S.

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Analog Devices makes a chip that is a USB full speed isolator, NOBODY makes a chip that is a high speed isolator. If one of the chip companies did a high speed isolator chip, there would be lots of products using it.

 

It certainly can be done by buying parts and putting an isolator together, but it is not cheap or easy. Part of the problem is that USB2.0 is not a simple single interface, it is a complex marriage of two different interfaces, you can't just have a simple receiver -> isolator -> transmitter, you need a receiver that actually knows the protocols to switch between the two types of interface.

 

It should be possible to do it with two high speed phys and a processor that understands USB protocol, but nobody has actually sat down and done this, it's a fair amount of work to do. And the hardware itself would not be cheap either.

 

It looks like nobody considers it worth while enough to spend the time, effort and money to do it.

 

John S.

 

What about Chord's upcoming 2Qute and Hugo TT which offer "galvanically isolated" USB 2.0 port? Did they come up with their own design, or is it marketing fluff?

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What about Chord's upcoming 2Qute and Hugo TT which offer "galvanically isolated" USB 2.0 port? Did they come up with their own design, or is it marketing fluff?

 

You don't have to isolate BEFORE the USB receiver, you can isolate AFTER the USB receiver. The USB receiver chip is directly connected to the USB bus, but all the signals coming out of and into the receiver go through isolators. As long as you either have two power supplies or power the receiver (and input side of the isolators) from VBUS you have full galvanic isolation. Of course this has to be built in to the DAC, it's not a separate box that you can add to any USB DAC out there.

 

This quite easy to do, but you have to be careful of the implementation. ALL digital isolators add a lot of jitter to the signal, so you need to reclock the signals after the isolators. This means the low jitter master clock has to be on the DAC chip side, and that same clock gets fed back through an isolator into the "dirty" side and on to the USB receiver.

 

It's surprising how many designs get this wrong, they don't reclock or they put the master clock on the dirty side, both of which add a lot of jitter to the clock.

 

John S.

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ALL digital isolators add a lot of jitter to the signal, so you need to reclock the signals after the isolators. This means the low jitter master clock has to be on the DAC chip side, and that same clock gets fed back through an isolator into the "dirty" side and on to the USB receiver.

 

It's surprising how many designs get this wrong, they don't reclock or they put the master clock on the dirty side, both of which add a lot of jitter to the clock.

 

John S.

The Regen will reclock though; right?

W10 NUC i7 (Gen 10) > Roon (Audiolense FIR) > Motu UltraLite mk5 > (4) Hypex NCore NC502MP > JBL M2 Master Reference +4 subs

 

Watch my Podcast https://www.youtube.com/channel/UCXMw_bZWBMtRWNJQfTJ38kA/videos

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The Regen will reclock though; right?

 

In the previous post I was talking about reclocking the digital signals after isolators built in to the DAC, using the low jitter master clock in the DAC.

 

The regen is regenerating the USB signals before the USB receiver, using it's own low jitter clock. Technically you can call them both "reclocking" but they are actually very different things.

 

BTW the regen is NOT galvanically isolating, as posted earlier that is a very difficult thing to do with an external device running in high speed mode.

 

 

John S.

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