MediaTools: Tone Source Filter to generate reference audio feed, dual Audio Source Filter and Virtual Audio Capture Device

In order to obtain a reference audio source and especially useful for debugging purposes, including:

  • audio input device unrelated to physical device, to avoid conditions when device is already in use by someone else
  • non-zero audio signal which is independent of certain speaker or broadcasting service, including one that makes capture, transmission or rendering issues easily perceptible by human
  • configurable to provide different audio media types, including through well known DirectShow interfaces, such as IAMStreamConfig
  • configurable to present in system as both regular DirectShow Filter, and as an Audio Capture Source which can be enumerated by applications
  • to be able to check how exactly other applications are accessing and configuring audio capture source

I added a new filter to the Media Tools library, a Tone Source Filter. The filter generates PCM Audio data, either infinite sine waveform, or interrupted signal  of requested parameters:

Tone Source Filter General Property Page

The filter accepts connection on PCM media types with flexible sampling rates in range 8 KHz through 200 KHz, 8-bit audio and 1 channel. Media type is also configurable through or, if not configured this way, the first enumerated media type off the Output pin is also configurable through property page and/or persistent setting accessible via private IDispatch derived interface property.

Tone Source Filter Prefered Format Property Page

Additionally, the filter may configured as a system-wide available audio input device, such as for example available to VLC Media Player or Media Player Classic:

VLC Media Player's Option to use Tone Source Filter based device

In order to configure the filter as such device, the library exports function DoToneSourceFilterDevicePropertySheetModal, which can be called using rundll32 utility (“rundll32 Acquisition.dll,DoToneSourceFilterDevicePropertySheetModal” from command line):

Devices Property Page

A partial Visual C++ .NET 2008 source code is available from SVN, release binary included.

File and Class Summary


Acqusition.dll (download) hosts the following classes:

  • DirectShow Filters
    • Tone Source Filter, to generate reference/debug audio data

Class Overview

Tone Source Filter

The filter generates uninterrupted or interrupted reference sine waveform in form of PCM audio data.


The hosting library (DLL) exposes DoToneSourceFilterDevicePropertySheetModal function which provides GUI to configure the filter as one or more audio capture source device, which can be enumerated by System Device Enumerator.

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