RME Fireface UCX II 40-Channel USB-B Audio/MIDI Interface - Manual (Page 113)

User's Guide Fireface UCX II © RME
113
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 rec-
ord and playback is unavoidable, the drivers include an offset value matching the Fireface's con-
verter delays.
Therefore, in a
digital loopback test a negative offset of about 3 ms occurs. This is no real prob-
lem, because this way of working is seldom, and usually the offset can be compensated manual-
ly 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...).
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 UCX II uses a
fixed additional buffer of 32 samples, under Mac 24 samples, which is added to the current buff-
er size. The main advantage is the ability to use lowest latency at highest CPU loads. Further-
more, 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 24 samples.
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. The option
Short Safety Offset in the Settings dialog reduces this to 12 samples each.
40.3 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. However,
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 UCX 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 UCX II to a USB 2 port
 Start the Device Manager, View set to Devices by Connection
 Select ACPI x86-based PC, Microsoft ACPI-Compliant System, expand PCI Bus
This branch normally includes two entries of a
USB2 Enhanced Host Controller . A USB Root
Hub can be seen, which then connects all USB devices, including the UCX II. By reconnecting to
a different port this view shows at which of the two controllers the UCX II is connected. With
multiple devices it can be checked if they are connected to the same controller.