User Manual

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User's Guide Fireface UC © RME
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 rare, and usually the offset can be compen-
sated 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. While with our former cards these values equalled exactly the buffer size
(for example 3 ms at 128 samples), the Fireface displays an additional millisecond – the time
needed for the AD/DA-conversion. Playback even shows another millisecond added – see
Safety Buffer.
Safety Buffer
An additional small Safety Buffer on the playback side only has proven to be very efficient and
useful. The Fireface UC uses a fixed additional buffer of 32 samples, which is added to the cur-
rent 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, otherwise Core Audio
won't operate click-free. The Fireface UC uses a safety offset of 28 samples. This offset is sig-
nalled to the system, and the software can calculate and display the total latency of buffer size
plus AD/DA offset plus Safety Offset for the current sample rate.
35.3 USB Audio
USB audio is in several ways different from PCI based audio interfaces.
A Fireface UC 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 in-
deed possible on current computers. However, using older computers a simple stereo playback
will already cause a CPU load of more than 30%.
A computer blocked for a short time – no matter if ASIO or WDM – will loose one or more data
packets. Such problems can only be solved by increasing the buffer size (and with this the la-
tency).
The Fireface UC 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 spite of drop-outs, and to correct
the sample position in real-time.
Detailed information on this topic can be found in the Tech Info USB Audio by RME – Technical
Background on our website:
http://www.rme-audio.com/english/techinfo/usbudio_rme.htm