DirectSound Buffers on ISA Cards

When DirectSound was first introduced, all audio devices operated on the ISA bus. The ISA bus was sufficient to stream 16-bit, 22 kHz stereo data from the CPU to the sound card without unduly affecting overall system performance, but attempting to send more data often placed an undue burden on the bus. This limitation had a strong influence on audio hardware design.

To mix DirectSound buffers, a chip on the sound card has two choices: it can reach across the ISA bus into system memory, or it can have dedicated memory on the card itself. Owing to the ISA bus's limited bandwidth, reading audio data from system memory is not practical. Therefore, ISA-based sound cards that do DirectSound acceleration almost always have dedicated on-card RAM.

When such a sound card is used to accelerate DirectSound, the sound data must first be copied from system memory, across the ISA bus, and into the dedicated memory on the card. This copying can take some time. However, once the data is on the card, there is almost no cost to the system for starting, stopping, and mixing sounds; all that is handled by the audio processor chip accessing its own dedicated memory.

ISA-based DirectSound accelerators support static buffers on the card, but not streaming buffers, because it is impractical to stream data across the slow ISA bus. DirectSound buffers created with the DSBCAPS_STATIC flag are located in card memory if it is available. Others are created in system memory.