User guide

MS-DMT Guide v1.04

Released 05-30-2014

TESTING FOR SYSTEM LATENCY

Windows latency must be tested and any excessive latency must be mitigated to within

tolerances for real-time audio streaming applications for best results using MS-DMT. MS-

Windows versions are not true real-time operating systems and do not provide low-latency

response to events, predictable time-based scheduling or explicit resource allocation. MS-

Windows was designed to maximize aggregate throughput and to achieve approximately fair

sharing of resources, which for the most part, does improve with each new version of Windows.

When a Windows device driver cannot process data immediately in its Interrupt Service Routine

(ISR) it schedules a Deferred Procedure Call (DCP), both of which operate at the highest priority

code that runs in the system. Neither an ISR or DCP can be pre-empted by the operating system,

they each run to process completion. ISRs and DPCs that run too often or too long degrade

system performance by using too much CPU time and can cause audio timing issues, serial port

timing issues, video pauses, erratic mouse behavior and numerous other system problems.

The recommended tool for audio latency testing of DPCs which can be used through Windows 7

can be downloaded from: http://www.thesycon.de/deu/latency_check.shtml Another tool for

Vista or later can be found at: http://www.resplendence.com/download/LatencyMon.exe

These tools will identify both DPC and ISR issues down to the offending process, thus it should

be a better tool for those with Vista or greater to track down the processes that require

disabling.

Here is the Latency Check tool is running on an XP Home SP3 system with excessive latency,

which requires mitigation prior to use of the software.