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20. Technical Background
20.1 USB Audio
USB audio is in several ways different from PCI based audio interfaces. Thanks to the high
speed of USB 3, typical problems of streamed (isochronous) data transmission can be circum-
vented. RME's proprietary USB 3 technology makes it possible to achieve performance similar
to PCI Express, IF all participating components work perfectly. At this time USB 3 has got a bad
reputation because many layouts, cables and connectors are designed badly and lower the
maximum throughput. Typical problems are not or randomly detected devices, devices that fail
as soon as data is transmitted, transmission errors and following (as required by the USB 3
standard) the reset of the host controller, guaranteeing a drop out for several seconds.
It is therefore not possible to guarantee reliable and highest performance of USB 3 operation
with any computer and cable just because they claim to support USB 3. The real performance
has to be evaluated on a per USB 3 port (!) and per cable base.
To make this as easy as possible, and to quickly signal problems, RME added an extensive
error detection and analysis into the Digiface AVB hardware, detecting errors during transmis-
sion via USB 2 and 3, and displaying them in the Settings dialog. Additionally the Digiface AVB
provides a special mechanism to continue recording and playback in case of drop-outs, and to
correct the sample position in real-time.
While some errors shown here might not cause audible
clicks and do not stop record/playback, the golden rule
applies: only zero errors are acceptable. If errors show up
use a different USB 3 port and exchange the USB 3 cable.
Never use simple USB 3 cable extensions!
Current Digiface USB 3 Compatibility Information
Fully compatible to Intel's USB 3 implementation which - on current Windows and Mac
computers - is part of the chipset. USB 3 sockets that are connected via an internal cable
(not directly soldered onto the motherboard) can cause transmission errors (these are
shown in the Settings dialog).
Fully compatible to AMD's USB 3 implementation. USB 3 sockets that are connected via an
internal cable (not directly soldered onto the motherboard) can cause transmission errors
(shown in the Settings dialog).
Compatible to NEC / Renesas USB 3. Real-world performance and error-free operation
depend on the firmware version, driver version and the PCB layout of the respective exten-
sion card/motherboard.
Fully compatible to Fresco USB 3 chip.
Fully compatible to Via VL912, very often used in USB 3 hubs
ASMedia - not compatible
Etron EJ168A - not compatible
Texas TUSB7340 - not compatible
Via VL800/805 - not compatible
The Digiface AVB is capable of using two different transmission modes to record data. The
Digiface AVB defaults to Bulk Mode, which improves reliability of USB 3 audio tremendously
and helps with both less than perfect cabling as well as critical layouts around the USB 3 chip.
The standard's default is Isochronous Streaming, which is unreliable as per the standard (errors
are to be expected!), unreliable in real-world operation, and not correctly supported by popular
USB 3 chips. In very seldom cases, bulk mode can not keep up with the strict timing required
for continuous audio streaming. Then, to make the Digiface AVB work at all, isochronous mode
must be used (but is still prone to clicks and drop-outs). To make the standard mode available
as reference the Settings dialog includes an option to activate Isochronous Streaming.
User's Guide Digiface AVB © RME
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