Usb Audio - RME Audio Fireface UCX User Manual

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 UCX 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.

32.3 USB Audio

USB audio is different from PCI based audio interfaces in several ways.
A Fireface can achieve a performance similar to a PCI or PCI Express card when used with an
optimal PC. Low CPU load and click-free operation even at 48 samples 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 (and with this the la-
tency).
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.
Like any audio interface the Fireface should have a data transmission to the computer as undis-
turbed as possible. The easiest way to guarantee this is to connect it 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:
User's Guide Fireface UCX © RME
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