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

Thu Feb 8 13:59:28 CET 2024

I think that's a pretty good summary of the problem MR.  I would just 
add that often the noise source isn't "USB noise" but can be due to 
pulsed supply current drawn by the application device itself from the 
USB power source.  A classic example of this is "DSP block processing 
whine", where a DSP might process audio in 2ms blocks.  The micro might 
be working flat out for 1600us out of every 2ms and then idling for the 
remaining 400us of each audio frame.  This heavily pulsed current can 
cause significant voltage drops along the supply wiring (USB cable) but 
also along the ground of the analogue audio connection cable!  
Essentially some of the ugly pulsed supply current decides it would 
rather find it's way back to the PC via the analogue audio cable ground 
rather than along the USB power cable ground, and this results in an AC 
voltage difference between the audio grounds at either end of the 
analogue signal cable.

This kind of whine can be quite hard to deal with because the frequency 
is so low.  It's easy to stop high frequency (RF) currents going where 
you don't want them just by adding some common-mode chokes and tiny 
ceramic capacitors, etc, but if the main micro is processing audio in 
2ms blocks that results in an ugly 500Hz current waveform that is quite 
hard to filter out!  You either need a *LOT* of local stored charge or a 
big inductor to smooth out the ripple of a 500Hz pulse waveform, 
otherwise it's influence starts to spread across the board!

-Richie,

On 2024-02-02 19:14, Mattias Rickardsson wrote:
> 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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>> --
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