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PC AUDIO PCI Express is coming to a PC near you, whether you want it or not. So best to know what we’re dealing with. Text: Martin Walker
In Issue 44 I explained why dual-core processors can be so beneficial, but if you’re thinking of upgrading your PC or buying a completely new one to take advantage of their processing power, there’s another big decision to make – whether or not to indulge in PCI Express (PCIe) slots. Although these first started appearing on a few PC motherboards back in 2004, there’s still a lot of confusion about them. So here’s a little bit of background. The classic PCI bus is 32 bits wide and runs at 33MHz, providing a peak bandwidth of 132MB/ second. However, this bandwidth must be shared between all the devices attached to the bus, and on modern motherboards this not only includes those plugged into the various expansion slots, but also the hard drives, optical drives, video, and any further devices plugged into the USB and Firewire ports. Nevertheless, it’s possible to run up to 32-input and 32-output channels of 24-bit/192k audio on the PCI bus, and the huge number of very capable music PCs prove that PCI is quite sufficient for many of us. PCI Express is a new higher speed bus that provides each device with direct access, rather than having to fight for its share of PCI bandwidth with other devices, and it also supports hot-plugging. Even better, since a single-switch chip is in control of resource-sharing decisions it can also prioritise data packets, so that (for instance) real-time streaming of audio and video data can take priority over other less time-critical data. Each connection between a PCIe device and the PCIe switch controlling I/O traffic is termed a ‘link’, and is composed of one or more ‘lanes’ that can send/receive one byte in each direction simultaneously. The simplest PCIe expansion slot supports a x1 link that has one dedicated lane capable of transmitting 2.5 gigabits per second simultaneously in each direction. With error correction it takes 10 bits to transmit an 8-bit byte, so this translates to 250 megabytes per second. PCI Express motherboards generally offer a x16 graphics slot running 16 lanes in parallel, to boost their bandwidth to a massive 4.0 gigabytes/second. A PCI to PCIe ‘bridge’ is needed to provide support for older PCI devices as well as the new PCI Express ones, and such bridging can either be on the motherboard (so it can offer PCI slots AT 84
as well as the newer PCIe ones) or even on a PCIe card (some early PCIe cards may fit in the shorter PCIe expansion slots, but they are actually PCI designs with an on-board bridge). In practice there are currently very few PCIe peripherals available, apart from (surprise, surprise) yet faster graphics cards to fit in the x16 slot. However, these are rarely of much use to musicians unless they also play games or indulge in 3D modelling, and may even cause problems by requiring noisy cooling fans. A few PCIe soundcards are starting to appear, but they’re mostly designed specifically for Macs (Apple’s new Power Macs completely abandon the previous PCI, PCI-X, and AGP slots of previous models in favour of PCIe slots, forcing soundcard manufacturers to adopt the new technology). Ultimately, however, I’m sure that PCI Express will have lots to offer the musician, since its 250MB/second bandwidth eclipses the 50MB/second of Firewire400 and 100MB/second of Firewire800. A new PC featuring an Intel Pentium D dualcore processor automatically gives you PCIe slots (and on most motherboards a few ‘legacy’ PCI slots as well). The earlier Intel 915, 925X, and 925XE chipsets should be avoided, as they caused a lot of problems with legacy PCI soundcards, but the newer 945/955X ones have proved to be very compatible, so a modern PC with a Pentium D processor and 955X chipset should give you the benefits of both dual-core performance and the future-proofing of having some PCI Express slots to house any exciting new soundcards that appear in this format. My own tests suggest that while Intel’s Pentium D range is fully compatible with legacy PCI soundcards, their performance lags behind AMD’s Athlon X2 range when running audio applications. Unfortunately, AMD dual-core system builders face a trickier set of choices. The easiest is to ignore PCI Express and install an Athlon X2 processor into an older motherboard that previously supported singlecore Athlon 64 processors – most Socket 939 motherboard manufacturers provide BIOS updates that add support for the new X2 range, and suitable chipsets include nVidia’s nForce3 and Via K8T800 Pro (the Asus A8V Deluxe motherboard is one of the most popular). However, if you want PCIe slots, then be wary
“I’m sure that PCI Express will have lots to offer the musician, since its 250MB/second bandwidth eclipses the 50MB/second of Firewire400 and 100MB/ second of Firewire800.” – each AMD-based motherboard seems to implement PCI to PCIe bridging in a slightly different way. In fact, many musicians have had problems after fitting their existing PCI soundcards into the legacy PCI slots on a PCI Express motherboard, with a variety of symptoms ranging from unexpectedly high CPU readings, to incurable audio stuttering, to having to run their audio interfaces with extremely high buffer sizes resulting in latencies approaching 50ms. The USB and Firewire ports all hang off the PCI bus, so devices plugged into these can be affected as well. I can’t recommend any AMD chipset, such as Via’s K8T890 or nVidia’s nForce4, to musicians when coupled with a single-core Athlon 64 processor, but thankfully the nForce4 problems are masked (but not completely cured) after fitting a dual-core Athlon X2. So this combination gets a cautious thumbs-up, although it still doesn’t work with UAD-1 cards as I write this. In short, if you’re about to buy or build an AMD dual-core PC, be wary. Perhaps Apple has the right approach in completely abandoning PCI in favour of PCIe – this neatly avoids any compatibility problems, but as usual it angers those who have invested a great deal of money on multiple PCI soundcards and DSP cards like the Powercore and UAD-1. In the meantime, PCI Express slots will start to appear in plenty of musician’s PCs (whether or not they get used), since dual-core is just too enticing to ignore.
