Technical Background - RME Audio Fireface UFX User Manual

31.5 Technical Background

Compared to a usual computer, the front USB port of the UFX has a limited buffer to compen-
sate for interruptions to the write process. Unfortunately, these occur quite often with USB
sticks, and sometimes are of an astonishing duration. The effect of the buffer depends on the
amount of data to be transmitted, which is the combination of sample rate and channel count.
The higher the data rate, the shorter the available buffer time. When a medium causes a pause
that is longer than the buffer time, data is lost.
Information on the performance of the record media
To determine the highest record channel count of a hard drive or USB stick the UFX displays
several values. On the page Record/Playback:
USB Load: should exceed 80% only for a short time.
Errors: Each single error means a buffer with a bigger block of samples has been lost. Al-
though that is not always audible only a display of 0 is correct.
On the page USB Memory:
Max R/W Time (Read/Write Time): This constantly updated value is the quickest way to identify
slow media that suffer from interruptions of the write process. Hard drives often show values in
the range of 100 ms, while sticks reach up to 800 (!) ms.
Checking the performance of the record media
Every USB stick can be used for recording – but most are too slow to record more than a few
channels. While write rates higher than 10 MB/s are rare, simultaneous quick processing of
recorded data is even less frequently found. Therefore the toughest record situation for the
UFX, a recording of 34 channels at 96 kHz, is usually only possible by using hard drives.
To determine the maximum channel count of a medium, activate all 34 channels at a sample
rate of 96 kHz. This configuration causes the highest data throughput (9.79 MB/s) with the
smallest internal buffer (306 ms). Most hard drives manage an error-free recording until the
drive is full. In case the UFX shows errors, reduce the number of channels step by step until no
errors are shown anymore.
Note: 15 channels at 192 kHz equal 30 channels at 96 kHz and 60 channels at 48 kHz (8.64
MB/s, 347 ms buffer).
A more realistic example is the recording of all 30 inputs and of two submixes (34 channels) at
48 kHz. With 4.9 MB/s data transfer and 612 ms buffer time better USB sticks will already han-
dle the task without any problem. The display of USB Load, Max Write Time and errors quickly
reveals where the limit of the particular stick lies.
Note: Playback is usually not affected by the mentioned performance problems.
User's Guide Fireface UFX © RME
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