Technical Background; Usb Audio - RME Audio Digiface Ravenna User Manual

25. Technical Background

25.1 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 circumvented.
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 reputa-
tion 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 Ravenna hardware, detecting errors during transmission
via USB 2 and 3, and displaying them in the Settings dialog. Additionally the Digiface Ravenna
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 ap-
plies: 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!
Digiface RAVENNA USB 3 Compatibility Information
 Fully compatible to Intel's USB 3 implementation which - on current Windows and Mac com-
puters - 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 de-
pend on the firmware version, driver version and the PCB layout of the respective extension
card/motherboard.
 Compatible to Fresco USB 3 chip.
 Compatible to Via VL912, very often used in USB 3 hubs
 ASMedia – mostly not compatible
 Etron EJ168A - not compatible
 Texas TUSB7340 - not compatible
 Via VL800/805 - not compatible
The Digiface Ravenna is capable of using two different transmission modes to record data. It
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 ex-
pected!), 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 Ravenna work at all, isochronous mode must be
used. To make this standard mode available the Settings dialog includes an option to activate
Isochronous Streaming.
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User's Guide Digiface Ravenna © RME