Alesis Hammerfall DSP System manual PCI Performance

30.7 PCI Performance

The HDSP MADI card's sheer number of audio channels makes it more demanding for a com- puter'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 MADI card will cause about

24.6MByte/s of traffic. So three cards should work in any modern computer without problems - but in fact they won't. That is, they can be installed and accessed, but audio will crackle like hell...

According to our research, even using two cards with all 128 channels of playback and re- cording is not possible on usual single-PCI-bus systems. The limit is around 80 channels in each direction, and varies depending on how many channels of inputs and outputs are active. Here's an example of Intel's popular 875 chipset, which uses the ICH5 as southbridge. The crackle-free maximum is:

•128 channels record with up to 96 channels playback

•64 channels record with up to 128 channels playback

If a PATA harddisk is used instead of SATA in this specific system, all 128 channels can be used in both directions without clicks. But neither type of disk will allow simultaneous recording and playback of so many tracks here.

With 80 channels transferring data each way, and a seemingly low PCI load of about 30.7 MByte/s, every additional system activity will cause a short interruption of the PCI bus activity. Be it disk or network access, depending on chipset and board architecture, this will cause a disturbance of the audio signal. These disturbances begin to appear on channel 64, and the stronger they get, the more lower channels will be affected also. Therefore, they may not be noticed immediately.

Although a single MADI card will usually work trouble-free on modern computers, some points 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 ex- tremely 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 aver- age data rate will be sufficient for many audio tracks.

Using two HDSP MADI cards to full capacity requires boards with a very high transfer rate be- tween 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 solu- tion 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 two MADI cards, but also provides ample CPU power, thanks to Dual Opteron CPUs.

User's Guide HDSP MADI © RME 75