User Manual

User's Guide Fireface 800 © RME
57
30.2 Latency and Monitoring
The term Zero Latency Monitoring was 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 interface
directly to the output. Since then, the idea behind has become one of the most important fea-
tures 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 Driver CD and the RME web-
site.
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. 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 800
can't operate 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
audible frequency range as far as possible. This typically generates a delay of one millisecond.
A playback and re-record of the same signal (loopback) via DA and AD then causes an offset
of the newly recorded track of about 2 ms.
The exact delays of the Fireface 800 are:
Sample frequency kHz 44.1 48 88.2 96 176.4 192
AD (43.2 x 1/fs) ms 0.98 0.9 0.49 0.45
AD (38.2 x 1/fs) ms 0.22 0.2
DA (43.5 x 1/fs) ms 0.99 0.9 0.49 0.45 0.25 0.23
Buffer Size (Latency)
This option in the Settings dialog defines the size of the buffers for the audio data used in ASIO
(see also chapter 8.1 and 9.4). A setting of 64 samples at 44.1 kHz causes a latency of 1.5 ms,
for record and playback each. In the above example this latency stayed completely unnoticed,
the newly recorded track was shifted only by the amount of the AD- and DA-converter's latency.
The reason for this 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. Since the soft-
ware can not determine the amount of external latency, the AD-/DA-converters etc. are not part
of the automatic correction. The user has to add an offset in the software manually to compen-
sate such delays.
Safety Buffer
FireWire audio differs significantly from RME's previous DMA technology. DMA access is not
possible at all with FireWire. To be able to transmit audio reliably at lower latencies, FireWire
requires a new concept – the Safety Buffer. The Fireface 800 uses a fixed additional buffer of
64 samples (on the playback side only), 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.
Disadvantage: the safety buffer's 64 samples needs to be considered when compensating off-
sets manually.