How may we help you?

To achieve stability and low latency, the driver needs to process audio in real time. While the driver itself is not CPU-intensive, it must be scheduled for execution by the OS as soon as data is available. The same applies to DAWs. To ensure that latency-sensitive applications are scheduled promptly, they run with high (or even real-time) process and thread priority.

Windows provides a special service for multimedia applications called the Multimedia Class Scheduler Service (MMCSS). Instead of manually managing scheduling priorities, each thread in the application can register with MMCSS, which automatically raises the thread’s priority. Most of the time, this works well. However, there is a catch: if a thread exceeds a given time quota, MMCSS drastically lowers its priority to allow background tasks to run. This means that when using lower buffer sizes, if another task runs in the background, MMCSS might reduce the priority of DAW and driver threads, letting the background task preempt them — causing audible crackling.

Different DAWs behave differently: some register with MMCSS, while others manually manage scheduling priority. We provide a checkbox in the ASIO Settings Control Panel to select whether the driver thread itself should register with MMCSS. This setting should match the behavior of your DAW.

While ideally no background tasks should run during your session, this is not always possible. If you experience crackling caused by background tasks taking over CPU time, try toggling the MMCSS registration checkbox in the ASIO Settings Control Panel, or try disabling MMCSS entirely and manually setting your DAW to real-time priority.

To disable MMCSS completely:

1. Run regedit

2. Navigate to Computer\HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\MMCSS

3. Change the value of Start to 4

4. Restart your computer

This will permanently disable the service.

We are communicating the negative effects of MMCSS to the maintainers of the ASIO standard (Steinberg) and hope more drivers and DAWs will make MMCSS registration optional.