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Q. Should I use my PC's ACPI mode?

By Martin Walker
Published October 2003

MOTU 828 MkI Firewire audio interface.

I've just bought a PC laptop and a MOTU 828. I installed the software and have been using Emagic Logic 5.5. However when I turn off the computer's Advanced Configuration Power Interface (ACPI) it no longer recognises that the 828 is there. This is a common PC/Windows tweak for making audio apps run more efficiently, so I was hoping that the 828, or any other Firewire hardware for that matter, would be able to run with ACPI turned off. Can anyone enlighten me on this subject?

SOS Forum Post

SOS PC Notes columnist Martin Walker replies: It's strange that your MOTU 828 is no longer recognised, since Plug and Play will still detect exactly the same set of devices when you boot your PC, including the host controllers for both your USB and Firewire ports. It's then up to Windows to detect the devices plugged into their ports, and this ought not to be any different when running under ACPI or Standard mode.

MOTU have reported some problems running their 828 interface with Dell Inspiron laptops, due to IRQ sharing between the Firewire and graphics, and this could possibly be cured by changing to Standard Mode, but there's a much wider issue here. While switching from ACPI to Standard Mode has solved quite a few problems for some musicians in the past (those with M-Audio soundcards have certainly benefitted), it shouldn't be used as a general-purpose cure-all, and particularly not with Windows XP, which, I suspect, is installed on your recently bought PC laptop.

Since Windows XP needs very little tweaking compared with the Windows 9x platform, I would recommend all PC Musicians leave their machines running in ACPI mode on XP. I'd only suggest turning it off if there is some unresolved problem such as occasional audio clicks and pops that won't go away with any other OS tweaks, no matter how you set the soundcard's buffer size, or the inability to run with an audio latency lower than about 12ms without glitching.

Some modern motherboards now offer an Advanced Programmable Interrupt Controller (APIC) that offers 24 interrupts under Windows XP in ACPI mode rather than the 16 available to Standard Mode, so if yours provides this feature you should always stick to ACPI. Apparently it's also faster at task-switching, leaving a tiny amount more CPU for running applications. The latest hyperthreading processors also require ACPI to be enabled to use this technology, since each logical processor has its own local APIC. Moreover, laptops benefit from ACPI far more than desktop PCs, since it's integrated with various Power Management features that extend battery life. If I were you, I'd revert to ACPI.