User's Guide HDSP AES-32 - RME

30.7 PCI – Performance and Bus Load
The HDSP AES-32 card's sheer number of audio channels makes it more demanding for a
computer's PCI bus performance than any other audio card. Furthermore, measurements of
pure data throughput are not sufficient for measuring realtime audio performance or compatibility.
Large amounts of data may be transferred in short and fast bursts with small interruptions,
which will result in a relatively high data rate when measured averaged, but audio signals will
suffer clicks and dropouts because of the interruptions.
Theoretically, PCI can transfer up to 133 MByte/s. A single HDSP AES-32 card will cause about
6 MByte/s of traffic. Therefore up to three cards can be used in any modern computer without
problems. 48 channels I/O is no real challenge anymore, even notebooks can often handle this.
On a standard computer with modern single PCI bus, about 100 audio channels can be transmitted
per direction (record/playback). Exceeding this limit, any system acitivity - even outside
the PCI bus - causes drop outs. Transferring these experiences to the HDSP AES-32 means
that besides the number of channels the bus load has to be taken into account too. One channel
at 96 kHz causes the same load to the system as two channels at 48 kHz! So at 192 kHz, a
HDSP AES-32 causes a bus load equalling a MADI card with 64 channels I/O at 48 kHz!
The usage of multiple AES-32 in DS and QS operation can therefore be problematic. Some
examples:
• 2 HDSP AES-32 will most likely not run stable at 192 kHz at full track count. 2 x 16 channels
192 kHz equal 2 x 64 channels at 48 kHz = 128 channels per direction.
• 2 HDSP AES-32 at 96 kHz should operate reliable at full channel count. 2 x 16 equals 2 x 32
= 64 channels per direction.
• 3 HDSP AES-32 at 96 kHz can't operate at full channel count (3 x 16 equals 3 x 32 = 96
channels per direction). This limit may be different for different computers.
Using many channels at high sample rates, the follwing should be noted:
• The PCI-bus should be kept free of other devices. This also applies to on-board components
such as modems, USB devices or network adapters.
• If disturbances occur during hard drive activity: Modern SATA hard drives often feature extremely
high peak data rates, which are unnecessary for audio playback and recording, but
tend to disturb the PCI bus. In this case, taking the step back to PATA controllers and drives
can turn out to be a big step forward. On Macs, keeping audio files on an external FireWire
drive may be worthwile. Even FireWire 800 efficiently limits peak performance, but the average
data rate will be sufficient for many audio tracks.
Using two HDSP AES-32 cards at 192 kHz, or three at 96 kHz, requires boards with a very high
transfer rate between north bridge and south bridge, in order to also allow the processing of
other data (hard drive...) in real-time. The 266 MByte/s of an Intel 875 chipset are simply not
sufficient. The solution is found in motherboards with server chipsets that usually incorporate
several separated PCI busses. One good example is the Tyan 8KW (S2885), which not only
operates flawlessly with thre HDSP AES-32 cards, but also provides ample CPU power, thanks
to Dual Opteron CPUs.
76 User's Guide HDSP AES-32 © RME