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

[mskala at northcoastsynthesis.com]

On Sun, 4 Feb 2024, brianw wrote:

> You cannot optoisolate a (DC) power source. You can only optoisolate
> (AC) signals.

Well, you literally could optoisolate power with a bunch of LEDs and solar
cells, but that's not what I had in mind and it wouldn't necessarily work
well for noise.

When I said "optoisolate it" I thought it was clear that "it" meant the
data.  The point is to not have a connection between the USB power and the
synth power.  Whatever receiver electronics need to be powered by USB
power can be, but then that stuff transmits only data and *not* power
across the optoisolator to the rest of the synth, which is on its own
power with some effort to keep that power clean.  The synth's power is not
coming from the USB cable.  The original question was pretty clearly
describing a synth with its own power system that needs a USB cable
plugged in to receive data, but then when that cable gets plugged in, oh
no!, everything gets noisy because of the ground loop created.  It's a
losing battle trying to clean that situation up unless the ground loop can
be broken and the noise voltage on the USB +5V prevented from influencing
other power lines.

> even filtering for (DIN) MIDI, and if you follow those instructions
> you'll have solved a lot of the potential problems. But you still need
> to keep the CPU from introducing noise on the ground and power rails.

Ideally, any CPU involved would as you say be on the USB side of the
isolation.  However, even just the USB transceiver being on the USB-power
side, with most of the digital stuff being on synth power, would probably
be enough (requiring two-way optoisolation because USB needs to transmit
in order to receive).  The very common complaint of synths becoming noisy
after connection to USB isn't caused by a USB-receiving CPU built into
the device.  It's caused by garbage coming through the USB power lines
from the much larger computer on the host end of the USB cable.  Keeping
those noisy power lines separate from the synth's own power lines is a big
step forward, even if some digital electronics built into the synth still
end up being powered from the synth's own power.

-- 
Matthew Skala
North Coast Synthesis Ltd.