In the past direct USB connection did not work. What did work, though, was using the emulated WDM audio driver of the virtual machine even with something like ASIO4ALL. Of course latencies were rather problematic.

> USB passthrough gives the virual machine full access to the usb device

Theoretically yes .. but does it work for you ?! There must be a reason why it doesn't work for you.
The only obvious reason that I see is the virtualization layer that at the end is there.
Do you know how USB passthrough is being implemented ? Surely not.
All we see is, that it does not work.

> Audio drops could maybe happen, but all the processing of the sound happens in the UFX so why should that be a problems?

Sorry, but you completely forget the PC side and how a PC works.

> Latency could of course be an issue, since the audio have to travel/routed 1 more layer(?) between the virtual machine,
> the esxi server and the hardware - but that I can accept, since I don't see any reasons for the delay to more than 1-2ms.

I didn't talk about RTL (round trip latency) between recording interface and PC.
I talk about the latencies inside the computer that can i.e. be measured using LatencyMon.

If a CPU is not able to process audio data in time, then you get audio drops.
Reasons for this can be CPU load and a high DPC load and bad written programs and drivers.
It can be a pain already on baremetal.
What I mean is, this won't become any better by a virtualization layer in between.
And usually you also do not give one VM all CPU cores, or all your other VMs get issues.

People working in the IT know
1. that for performance critical server you have to install on bare metal.
And for audio processing I would definitively raise this as a strong requirement.
2. even for some applications the support is excluded if you run them on virtualized environment.
Also these exclusions have reasons.

And as time and performance critical recording / audio processing is .. you can be sure that virtualization is not the way to go. It's a bad idea to base a recording system on a VM, you also do not build a house onto sand.

> See that this post never got any answer: https://www.forum.rme-audio.de/viewtopic.php?id=24448

Most likely it simply was overlooked as there is not good answer for it besides "better don't do it".

AFAIR the Apple issues last seen were also there when only playing back audio.

I had myself issues with a Lenovo Laptop that required a ridicolous high ASIO buffersize
of 256 ASIO buffers to be able to playback only a stereo track.
The issue was the GPU inside of the CPU.
Luckily I had a dedicated GPU. When configuring Firefox to use the dedicated nVidia GPU all issues were gone.
Audio playback worked with minimum ASIO buffersize (48 for the old UFX) and I could browse the internet without issues.

Windows and also MacOS X are no realtime operating systems. Thus audio can be a fragile thing at times.
The only thing that brings stability is to follow "best practises" to avoid some things that have negative impact.

UFX is connected with USB to the server, which runs vmware esxi 6.7. I then enable usb passthrough on that usb-device to my (virtual) windows-machine.

Since i'm running esxi (not workstation or fusion), i log into the browser and open a 'console view' to that machine.
Have also tried to use Teamviewer just to test - same conclusion.

Thanks for your input and suggestions, snoskit. Appreciate the effort!

Tested high latency: no difference
VRMC: no difference
USB2 or USB3: no difference
VMXNET: more unstable in general, no difference to the sound problem

Also tried a new server, xeon 8cores, 32gb ram - same experience. Finds and can controll the soundcard but no sound out over usb.

Ok. Giving up on this now.. will try hyper-v instead.

The basic technique here of connecting usb to vm's is still very useful know about

I use it a daily basis for studio work, to resurrect a Roland XV5050's editor software - usb into daw pc - redirected into windows xp vm via vmware workstation - Roland software runs fine inside vm

Good luck

Why so negative?

If you don't try something 'new', you'll never learn anything. (I know, sounds like a cliche..) I learned several things today and dusted off an server, installed win server 2019 and are playing with hyper-v now. This IS a good result for me.

People are different.