RME Fireface UCX II 40-Channel USB-B Audio/MIDI Interface - Manual (Page 112)

112 User's Guide Fireface UCX II © RME
40.2 Latency and Monitoring
The term
Zero Latency Monitoring has been introduced by RME in 1998 for the DIGI96 series
of audio cards. It stands for the ability to pass-through the computer's input signal at the inter-
face directly to the output. Since then, the idea behind has become one of the most important
features of modern hard disk recording. In the year 2000, RME published two ground-breaking
Tech Infos on the topics
Low Latency Background , which are still up-to-date: Monitoring, ZLM
and ASIO , and Buffer and Latency Jitter , both found on the RME website.
How much Zero is Zero?
From a technical view there is no zero. Even the analog pass-through is subject to phase errors,
equalling a delay between input and output. However, delays below certain values can subjec-
tively be claimed to be a zero-latency. This applies to analog routing and mixing, and in our opin-
ion also to RME's Zero Latency Monitoring. The term describes the digital path of the audio data
from the input of the interface to its output. The digital receiver of the Fireface UCX II can't oper-
ate un-buffered, and together with TotalMix and the output via the transmitter, it causes a typical
delay of 3 samples. At 44.1 kHz this equals about 68 µs (0.000068 s), at 192 kHz only 15 µs.
The delay is valid for ADAT and SPDIF in the same way.
Oversampling
While the delays of digital interfaces can be disregarded altogether, the analog inputs and out-
puts do cause a significant delay. Modern converter chips operate with 64 or 128 times over-
sampling plus digital filtering, in order to move the error-prone analog filters away from the audi-
ble frequency range as far as possible. This typically generates a delay of one millisecond. A
playback and re-record of the same signal via DA and AD (loopback) then causes an offset of
the newly recorded track of about 2 ms.
Low Latency!
The Fireface UCX II uses AD and DA converters with an innovative digital filter, causing a delay
of only a few samples. With 5 samples AD and 6 samples DA the delay caused by the conver-
sion is only a fraction of previous generations. The exact delays of the UCX II converters are:
Sample frequency kHz 44.1 48 88.2 96 176.4 192
AD (5 x 1/fs) ms 0.11 0.10
AD (5 x 1/fs) ms 0.057 0.052
AD (6 x 1/fs) ms 0.034 0.031
DA (5.8 x 1/fs) ms 0.13 0.12 0.066 0.06 0.033 0.03
Buffer Size (Latency)
Windows: This option found in the Settings dialog defines the size of the buffers for the audio
data used in ASIO and WDM (see chapter 9).
Mac OS X: The buffer size is defined within the application. Only some do not offer any setting.
For example iTunes is fixed to 512 samples.
General: A setting of 64 samples at 44.1 kHz causes a latency of 1.5 ms, for record and play-
back each. But when performing a digital loopback test no latency/offset can be detected. The
reason is that the software naturally knows the size of the buffers, therefore is able to position
the newly recorded data at a place equalling a latency-free system.