Trouble shooting

Dropouts or static are a common problem. This is often a matter of a system with a high latency.

The DPC latency problem

Windows is not a real time operating system. All requests to the operating system are delivered on a best effort basis. There are no guarantees whatsoever that requests are delivered within a certain time frame, which are the characteristics of a real-time operating system. That is not a problem for most devices and tasks but this is bad news for audio applications (which are considered soft real-time) because they need to deliver data to the subsystem and the hardware in buffers several times per second. If one or more buffers miss their deadlines and are not delivered in time it has audible consequences which are recognized as dropouts, clicks and pops.

Source: Resplendence

DPC=Deferred Procedure Call

Windows Resource Monitor

Win 7 Resource monitor is an excellent tool to watch system activity.

Look for processes peaking, excessive I/O on the HD, network traffic, etc.

Thesycon DPC Latency Checker

This tool is no longer available.

DPC Latency Checker is a Windows tool that analyses the capabilities of a computer system to handle real-time data streams properly. It may help to find the cause for interruptions in real-time audio and video streams, also known as drop-outs.


Thesycon USB Descriptor Dumper

Thesycon’s descriptor dumper is a Windows utility that displays the USB descriptors of any USB device. The dump is in plain text format and can be saved to a file or copy-pasted into an email. This is most useful for developers and technical support personnel.

This piece of freeware can be used to analyze your USB DAC.

Supported bit depth and sample rate, transfer mode, synchronization, buffer size, etc.


AS Format Type 1 Descriptor:

0x18 bBitResolution (24 Bits/sample)
0x04 bSamFreqType (Discrete sampling frequencies)
0x00AC44 tSamFreq(1) (44100 Hz)
0x00BB80 tSamFreq(2) (48000 Hz)
0x015888 tSamFreq(3) (88200 Hz)
0x017700 tSamFreq(4) (96000 Hz)


Endpoint Descriptor (Audio):

Transfer Type: Isochronous-Transfer
Synchronization Type: Adaptive
Usage Type: Data
0x0240 wMaxPacketSize (576 Bytes)

Resplendence LatencyMon

LatencyMon checks if a system running Windows is suitable for processing real time audio by measuring DPC and ISR excecution times as well as hard pagefaults. It will provide a comprehensible report and find the kernel modules and processes responsible for causing audio latencies which result in drop outs. It also provides the functionality of an ISR monitor, DPC monitor and a hard pagefault monitor.

You get a nice overview.


DPC and ISR count

Overview per CPU

There is a good online help.

RightMark Audio Analyzer

RMAA suite is designed for testing quality of analog and digital paths of any audio devices, be it a sound card, an MP3 player, a consumer CD/DVD player or an acoustic set. The results are obtained by playing and recording test signals passed through the tested audio path by means of frequency analysis algorithms.


RMMA options


A critical evaluation and good guidelines on how to use this software can be found at NwAvGuy.

  1. RightMark Audio Analyzer - NwAvGuy, 2011
  2. Glitch Free - Brad Robinson