[sdiy] How to design out "usb noise"?

[Mattias Rickardsson]

If I'm not mistaken, this is the ground loop that typically brings in
computer/USB noise into audio:

a) Computer
b) USB cable to synth
c) Digital system in synth
d) Digital power domain in synth
e) Analog power domain in synth
f) Analog system in synth
g) Analog audio cable to audio interface
h) Analog system in audio interface
i) Analog power domain in audio interface
j) Digital power domain in audio interface
k) Digital system in audio interface
l) USB cable to computer
a) Computer (again)

If the ground is disconnected between any two stages in this loop, the
noise can go away.

Isolating USB itself at b) or l) has been mentioned, but can be problematic
for higher bandwidth connections, like when streaming many channels of
audio along with the MIDI data.

Isolation between d) and e), together with data isolators between c) and
f), is an alternative when building synths. But it seems very rare.

Likewise, audio interfaces could have isolation between i) and j), as well
as between h) and k). Do any manufacturers do it like this?

It should be mentioned that g) can also play a role here. A balanced audio
connection can suppress enough of the noise in some cases where the rest is
designed well - for instance by introducing a resistance instead of tight
ground connection somewhere in the ground loop.

But it's all a very very hairy issue.

/mr


Den fre 2 feb. 2024 12:30René Schmitz <synth at schmitzbits.de> skrev:

>
> Have a seperate USB uC as a front end that has an insulation barrier to
> everything else.
>
> Or seperate the Audio Output. (Transformer, Optically)
>
>
> Best,
>
>   René
>
>
> Am 02.02.2024 um 12:01 schrieb Vladimir Pantelic via Synth-diy:
> > have a look at e.g.
> >
> > https://waldorfmusic.com/legacy-nw1/
> >
> > they explicitly state:
> >
> > "...USB port with *galvanic isolation* for wavetable transfer and text
> > input.."
> >
> >
> > On 01.02.24 23:06, Mike Beauchamp wrote:
> >> I recently stumbled across this post on reddit of someone showing usb
> >> connection causing some serious noise:
> >>
> https://www.reddit.com/r/synthesizers/comments/1aghqsa/how_to_get_rid_of_usb_noise/
> >>
> >> Some comments suggest to use DIN MIDI instead of USB MIDI as a
> >> solution, but it got me wondering if there's a best practice for
> >> implementing MIDI OVER USB output that wouldn't cause this. MIDI spec
> >> includes schematics for hardware implementation of DIN MIDI
> >> connections as we know, including the use of opto-isolator, but I
> >> can't find the equivalent for the USB connection.
> >>
> >> A typical USB-B connector has the following pins:
> >>
> >> 1. VBUS
> >> 2. D-
> >> 3. D+
> >> 4. GND
> >> 5. SHIELD
> >>
> >> How do you think these connections should be handled inside the
> >> synth, assuming a hostile environment where connections are also
> >> being made to/from a PC with audio cables, MIDI cables, power
> >> adapters to different outlets/circuits, etc.
> >>
> >> Mike
> >>
> >>
> >>
> >>
> >>
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