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Galvanically-Isolated USB (+ longer distances) - USB over Ethernet
Galvanically-Isolated USB (+ longer distances) - USB over Ethernet
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Old 2nd November 2019, 02:01 PM   #1
phofman is offline phofman  Czech Republic
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Join Date: Apr 2005
Location: Pilsen
Default Galvanically-Isolated USB (+ longer distances) - USB over Ethernet

For measuring it may be important to have the measuring device galvanically isolated from the analyzing computer. Some devices use internal I2S isolators (e.g. RTX6001, some RPi DACs) but most USB audio devices don't.

USB isolators up to full speed (12Mbps) are inexpensive (ADUM etc.), but full speed does not allow duplex higher samplerates. High-speed (480Mbps) isolators are still quite expensive (150USD+).

A dirt-cheap galvanically-isolated data-transfer technology is ethernet. I tested USB-IP protocol offered by linux and it behaves great. RPi4 hosting a USB soundcard, linked with 100Mbps ethernet to a PC. When RPi exports the USB device over network and the PC attaches it, a USB soundcard appears in the PC OS which can be used by any software. Since the USB-IP protocol transfers raw USB communication, the actual USB device drivers are applicable only on the PC side.

Another advantage is the max ethernet distance is much higher than for USB, it may come handy for some use cases.

Duplex 192kHz/24bit/2ch generated 12Mbps of ethernet traffic in both directions (9.2Mbps raw data + overhead). Load on RPi was 20% of one core for each direction, the more powerful PC was at 2 x 8%.

I tried to measure how USB over network affects latency with jack_delay jack delay [Linux-Sound] but could not get any statistically significant difference compared to direct USB connection (ping between the PC and RPi is 300us avg. on my home network).

Since the USB-IP stack has been in linux for many years, even the cheapest ARM boards with 100Mbps ethernet supporting older kernel will do easily (e.g. Orange Pi Zero for 12USD incl. shipping + any USB charger). Or a dirt-cheap router with USB port, running properly-configured OpenWRT.

There are proprietary USB-IP solutions for windows too but they are not possible to use on the inexpensive ARM boards out of the box.

I can imagine the "smart" USB RPi4-based soundcard having the actual USB soundcard connected via the isolated ethernet + small board, as a low-cost galvanic-isolation option. The output/input frontend would be isolated in any case.
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Old 5th November 2019, 12:35 PM   #2
phofman is offline phofman  Czech Republic
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Join Date: Apr 2005
Location: Pilsen
A refurbished x86 thin client with class II adapter would be the most economical solution - quality good-brand (basically immortal) HW, gigabit ethernet, very reliable power adapter, reliable storage (CF card) all included in price (SD cards for ARM boards are either cheap and unreliable, or a bit costly), solid metal case, using latest linux kernel, trivial installation, wake-on-lan support for fixed installation, etc.

In my projects I have very good experience with Futro S900 - 9EUR + shipping Fujitsu Futro, AMD G-Series G-T44R, S900 Thin Client, 2GB Flash, 2GB DDR3, Citrix | eBay . It would work like a charm in this application.
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