PCI audio

PCI audio chips started to emerge during 1996 and are either integrated on the motherboard or on a card in a PCI expansion slot. By mid-1998 a trend towards PCI cards providing enhanced features for both gaming and music applications had become firmly established. As greater demands are made on audio processing, traditional cards fall short due to the physical constraints of the ISA bus. The problem is bandwidth. In quantitative terms, while ISA's theoretical maximum is a mere 8 MBps, the PCI bus can theoretically support data transfers as fast as 132 MBps. This limits audio to just 16 channels. Whilst this is enough for most games, for professional audio applications 32, or better still 64, channels are preferred. Some ISA cards implement proprietary technology to increase throughput, but it is in everyone's best interests that the industry move towards a standard.

PCI-based cards deliver greater performance, offering the performance required by advanced features like mixing multiple audio streams and processing 3D positional streams. Due to high overheads inherent with ISA technology, it is estimated that up to 20% of a CPU's capacity can be blocked when playing a 16-bit stereo sample at 44.1kHz. PCI significantly reduces the performance bottleneck, freeing up the CPU to focus on other tasks like 3D graphics, game logic, and game physics. Overall, PCI may be as much as 10 to 20 times as efficient as ISA for processing audio streams.

PCI support has been around since 1993, yet, despite the benefits it offers, it took a further 5 years for PCI audio to emerge in a serious way. There are a number of reasons for this:

Now, however, PCI sound cards are often less expensive than their ISA counterparts. This results partly from the speed and elegance of the PCI bus. An ISA sound card that includes wavetable synthesis typically includes 1MB to 4MB of expensive ROM to hold its wavetable synthesiser's set of sample instrument sounds (often called a patch set or wave set). In contrast, many PCI cards eschew the ROM approach in favour of loading their patch sets into system RAM . The speed of the PCI bus enables this approach because it gives sound cards the ability to access the samples in system memory quickly.

An interesting feature of the new crop of PCI audio cards was their ability to provide real-mode DOS Sound Blaster compatibility for the huge number of DOS games still in existence. It's significantly more complicated to provide this compatibility with a PCI bus-based audio card than with a PCI audio chip integrated on the motherboard. They also allow multiple speaker connection; soon it'll be possible to add as many as eight speakers to a PC in a so-called 7.1 format (seven separate positional audio channels plus one subwoofer) - a capability provided by the "Environmental Audio" of the Sound Blaster Live! board which came to market in the summer of 1998.

While PCI audio was a huge advance, initially there was one serious problem that had be resolved to ensure that users didn't encounter unpleasant experiences with their PCI audio subsystems. The problem was actually caused by certain graphics subsystems, yet it could affect the playback quality of the PCI audio subsystem. Some graphics drivers continually performed retries of data transfers to the graphics chip - where the data is transferred through, and buffered by, the system's PCI chipset - during periods when the graphics chip was unable to accept data. Apparently, this behaviour enhanced graphics benchmark scores slightly, but it could also prevent other PCI bus devices from receiving their data through the chipset output buffers for a fairly lengthy period - long enough to cause an audible interruption of an audio stream.

Notwithstanding these problems, with Microsoft's PC 98 specification calling for new systems to stop using ISA boards by January 1999, it was evident that ISA's days were numbered.


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