User`s guide

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User’s Guide DMC-842 © RME
19.3 Latency and Monitoring
The term Zero Latency Monitoring has been introduced by RME in 1998 for the DIGI96 series
and describes the ability to pass-through the computer's input signal at the interface directly to
the output. Since then, the idea behind has become one of the most important features of mod-
ern 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, 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 er-
rors, equalling a delay between input and output. However, delays below certain values can
subjectively be claimed to be a zero-latency. This applies to analog routing and mixing, and in
our opinion also to RME's Zero Latency Monitoring. RME's digital receiver's buffer and the out-
put via the transmitter cause a typical delay of 3 samples. At 44.1 kHz this equals about 68 µs
(0.000068 s), at 192 kHz only 15 µs.
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 au-
dible frequency range as far as possible. This typically generates a delay of about 40 samples,
equalling one millisecond. A playback and re-record of the same signal via DA and AD (loop-
back) then causes an offset of the newly recorded track of about 2 ms.
Low Latency!
The DMC-842 uses a high-class DA-converter from Texas Instruments, offering exceptional
Signal to Noise and distortion figures. Its digital filter delays the analog output signals by 29
samples. The exact delays caused by the DA-conversion of the DMC-842 are:
Sample frequency kHz 44.1 48 88.2 96 176.4 192
DA (29 x 1/fs) ms 0,66 0,60 0,33 0,30 0,16 0,15
These values are about a quarter of those available from former DA-converters. They represent
an important step in further reducing the latency in the computer-based recording studio. At
Quad Speed the added latency can simply be ignored. And with the ADI-8 QS, a matching AD-
converter with only 5 up to 10 samples delay is ready to turn 'analog digital monitoring' into real
analog-style monitoring.