User Manual

110
User's Guide Fireface UFX II © 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 seldom, and usually the offset can be compensated
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 II 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. A good performance
and click-free operation at a low buffer size are indeed possible on current computers. How-
ever, using older computers a simple stereo playback will begin to cause a CPU load of more
than 30%.
A computer blocked for a short time – no matter if ASIO or WDM – will lose one or more data
packets. Such problems can only be solved by increasing the buffer size (latency).
The Fireface features a unique data
checking, detecting errors during
transmission via USB and displaying
them in the Settings dialog.
Additionally the Fireface provides a special mechanism to continue recording and playback in
case of drop-outs, and to correct the sample position in real-time.
Best USB 2 performance is achieved by connecting the UFX II to its own bus, which should be
no big problem as most USB 2.0 interfaces are a double bus design. A check in the Device
Manager can be done as follows:
Connect the UFX II to a USB 2 port
¾ Start the Device Manager, View set to Devices by Connection
¾ Select ACPI x86-based PC, Microsoft ACPI-Compliant System, expand PCI Bus