"The Intel 9xx family of Express Chipsets, including the Intel 925X chipset meets USB specifications. The Audio issues reported by Digidesign are not due to any direct USB issue, but appear to be a result of a documented errata (errata #7 (http://developer.intel.com/design/chipsets/specupdt/301474.htm) on the B-1 stepping of the ICH6, which is stepping of the I/O controller used with the 925X chipset. The B-1 stepping of the ICH6 controller is also used with the 915G and 915P Express chipsets. Errata #7 may impact devices connected to the ICH6, including PCI devices and External devices connected via the USB port, due to an internal arbitration issue that may result in up to 40 µsec delay.
Most USB devices are buffered adequately to withstand an additional 40 µsec delays, but some full speed USB 2.0 devices may experience underruns, depending on specific system configuration and instantaneous USB bandwidth consumption. In addition, USB devices that do not meet the USB specifications could also be impacted.
Details of the errata are given at the link above. The errata was fixed in subsequent steppings (B-2 and C-0) of the ICH6. The 925XE, 915GV, 915PL, 915GL and the 910GL do not utilize the B-1 stepping of the ICH6 and therefore are not impacted by this issue.
Suggested workaround:
For heavily loaded audio solutions, the best alternative is to utilize systems based on chipsets that utilize the B-2 or C-0 step of the ICH6 component, which has been better optimized for heavy audio traffic. Also, although Intel has not validated a high speed USB hub as a solution to this issue, we are aware of certain 3rd party system manufacturers that are using an external high speed USB hub to mitigate this issue on platforms with the B-1 step of the ICH6 component."
