RME Audio Fireface UFX+, User Manual

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112 User's Guide Fireface UFX+ © RME
AD/DA Offset under ASIO and OS X:
ASIO (Windows) and Core Audio (Mac OS X) allow for the
signalling of an offset value to correct buffer independent delays, like AD- and DA-conversion or
the Safety Buffer described below. An analog loopback test will then show no offset, because
the application shifts the recorded data accordingly. Because in real world operation analog
record and playback is unavoidable, the drivers include an offset value matching the Fireface's
converter delays.
Therefore, in a
digital loopback test a
negative
offset of about 3 ms occurs. This is no real
problem, because this way of working is more than seldom, and usually the offset can be com-
pensated manually within the application. Additionally, keep in mind that even when using the
digital I/Os usually at some place an AD- and DA-conversion is involved (no sound without...).
Note: Cubase and Nuendo display the latency values signalled from the driver separately for
record and playback. The values do not exactly equal the buffer size (for example 3 ms at 128
samples), instead a higher value is shown, including the time needed for the AD/DA-conversion.
Playback shows an even higher value – see Safety Buffer.
Safety Buffer
An additional small safety buffer on the playback side has proven to be very efficient and useful.
It is therefore implemented in all RME interfaces. Under Windows the Fireface UFX+ uses a
fixed additional buffer of 32 samples with USB and 64 samples with FireWire, under Mac 32
samples, which is added to the current buffer size. The main advantage is the ability to use
lowest latency at highest CPU loads. Furthermore, the fixed buffer does not add to the latency
jitter (see Tech Info), the subjective timing is extraordinary.
Core Audio's Safety Offset
Under OS X, every audio interface has to use a so called
Safety Offset
on record and playback,
otherwise Core Audio won't operate click-free. The Fireface uses a Safety Offset of 16 samples
with USB and 32 samples with FireWire. This offset is signalled to the system, and the software
can calculate and display the total latency of buffer size plus AD/DA offset plus 2 x Safety Offset
plus Safety Buffer for the current sample rate.
39.3 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 Fireface UFX+ hardware, detecting errors during transmis-
sion via USB 2 and 3, and displaying them in the Settings dialog. Additionally the UFX+ pro-
vides 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!