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Design goal is to minimize mobo traffic to just the audio samples

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Dave's post is excellent. Realtime audio playback leaves no room for 'undo' and 'redo'. Obstacles interfering with sample streaming will manifest as increased jitter.

The part on avoiding PCI bus is explained as follows:

For best results, don’t play music from USB / Firewire hard drives or a Network (LAN / WiFi). This also applies to Windows on USB flash drives. Rationale:

■ All soundcards (USB, Ethernet or PCI) operate of PCI bus.
■ Best design is when soundcard has lions share of PCI bus use.
■ USB and Network drives (which also connect to PCI bus) go against this design principle as data traffic now competes for PCI bus use.

Technical background: PCI read or write transactions take place between master and slave devices (soundcard, CPU…). All devices connected to PCI bus can act as master or slave. During a read or write transaction, master takes control of bus and STOPS all other devices from transacting. This is where a conflict arises – soundcard wants more data for output but PCI bus is busy with a data transaction (from USB drive or Ethernet) thus forcing soundcard to wait. This all happens at frantic speeds but for ultra low latency playback, this conflict will have impacts. Hence, internal SATA drives are best where motherboard maintains dedicated bandwidth.


cMP, cMP² and other players that load into RAM before playback would eliminate this conflict as no wav data traffic occurs. It however doesn't end here as some devices and applications have a polling nature. This would create such conflict and is audible. Mixers for ASIO soundcards are terrible at this as the mixer application is constantly polling. Likewise, other devices (especially network related ones) do this by continuously polling (whatever related device) causing unwanted mobo traffic.

USB devices adds another level of complexity and worsens the noise floor by ±3db. Taking jitter measurements I was able to see this whenever I used the mobo USB port (1GB stick). Its presence alone caused the noise floor increase, i.e. no USB data activity. From a jitter viewpoint, this would be adding random jitter to your player. It could be that USB devices operate on a polling basis.

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