Intel Fireface 800 - Manual (Page 89)

User's Guide Fireface 800 © RME
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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
FireWire audio differs significantly from RME's previous DMA technology. DMA access is not
possible here. 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.
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 uses a safety offset of 64 samples. This offset is signalled
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 FireWire Audio
FireWire audio is in several ways different from RME's earlier PCI audio interfaces. First of all,
our cards have a PCI interface which has been developed by RME and optimized for audio.
FireWire on the other hand, uses OHCI-compatible controllers that have not been optimized for
audio, no matter from which manufacturer they are. Our PCI data transmission is per channel,
while FireWire is working interleaved, i.e. it transmits all channels simultaneously. With the
Hammerfall, drop-outs thus occur only on the last channels, which is not always noticeable,
while a drop-out with FireWire always concerns all channels and is thus perceived much
clearer. Apart from this, RME's PCI audio cards establish a direct connection with the applica-
tion under ASIO (Zero CPU load), which is principally not possible with FireWire, because
communication has to be established by the operating system's FireWire driver. Compared to
our PCI cards, the FireWire subsystem creates an additional CPU load at lower latencies.
One Fireface 800 can achieve a performance similar to a PCI card with an optimal PC. An 'op-
timal' PC has an undisturbed PCI bus. Intel's motherboard D875PBZ e.g., has network, PATA
and SATA connected directly to the chipset. No matter what you do with the computer, FireWire
audio is not being disturbed. The same holds true for the ASUS P4C800, as long as you leave
the additional SATA controller (PCI) unused.
Due to insufficient buffering within FireWire controllers, single peak
loads on the PCI bus can already cause loss of one or more data
packets. This is independent of the manufacturer and no RME
problem. The Fireface 800 features a unique data checking, detect-
ing errors during transmission via PCI/FireWire and displaying them
in the Settings dialog. Additionally the Fireface provides a special
mechanism which allows to continue record 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
FireWire Audio by RME – Technical Background on our website:
http://www.rme-audio.com/english/techinfo/fwaudio_rme.htm