HOWTO: Monitor Audio Volume Levels

ID: Q181550

The information in this article applies to:

• Microsoft Win32 Application Programming Interface (API)

SUMMARY

This article describes how to monitor audio volume levels using the mixerXXX APIs.

MORE INFORMATION

You can use the mixerXXX APIs to monitor the volume levels of the input and output lines of your sound card. This involves the following steps:

1. Open the mixer device associated with the sound card.

2. Obtain the peak meter mixer control associated with the line you want to

   monitor.

The following code implements these steps. Link with "winmm.lib".

Sample Code

    #include <windows.h>
    #include <mmsystem.h>

    MMRESULT rc;              // Return code.
    HMIXER hMixer;            // Mixer handle used in mixer API calls.
    MIXERCONTROL mxc;         // Holds the mixer control data.
    MIXERLINE mxl;            // Holds the mixer line data.
    MIXERLINECONTROLS mxlc;   // Obtains the mixer control.

    // Open the mixer. This opens the mixer with a deviceID of 0. If you
    // have a single sound card/mixer, then this will open it. If you have
    // multiple sound cards/mixers, the deviceIDs will be 0, 1, 2, and
    // so on.
    rc = mixerOpen(&hMixer, 0,0,0,0);
    if (MMSYSERR_NOERROR == rc) {
        // Couldn't open the mixer.
    }

    // Initialize MIXERLINE structure.
    ZeroMemory(&mxl,sizeof(mxl));
    mxl.cbStruct = sizeof(mxl);

    // Specify the line you want to get. You are getting the input line
    // here. If you want to get the output line, you need to use
    // MIXERLINE_COMPONENTTYPE_SRC_WAVEOUT.
    mxl.dwComponentType = MIXERLINE_COMPONENTTYPE_DST_WAVEIN;

    rc = mixerGetLineInfo((HMIXEROBJ)hMixer, &mxl,
                           MIXER_GETLINEINFOF_COMPONENTTYPE);
    if (MMSYSERR_NOERROR == rc) {
        // Couldn't get the mixer line.
    }

    // Get the control.
    ZeroMemory(&mxlc, sizeof(mxlc));
    mxlc.cbStruct = sizeof(mxlc);
    mxlc.dwLineID = mxl.dwLineID;
    mxlc.dwControlType = MIXERCONTROL_CONTROLTYPE_PEAKMETER;
    mxlc.cControls = 1;
    mxlc.cbmxctrl = sizeof(mxc);
    mxlc.pamxctrl = &mxc;
    ZeroMemory(&mxc, sizeof(mxc));
    mxc.cbStruct = sizeof(mxc);
    rc = mixerGetLineControls((HMIXEROBJ)hMixer,&mxlc,
                               MIXER_GETLINECONTROLSF_ONEBYTYPE);
    if (MMSYSERR_NOERROR == rc) {
        // Couldn't get the control.
    }

    // After successfully getting the peakmeter control, the volume range
    // will be specified by mxc.Bounds.lMinimum to mxc.Bounds.lMaximum.

    MIXERCONTROLDETAILS mxcd;             // Gets the control values.
    MIXERCONTROLDETAILS_SIGNED volStruct; // Gets the control values.
    long volume;                          // Holds the final volume value.

    // Initialize the MIXERCONTROLDETAILS structure
    ZeroMemory(&mxcd, sizeof(mxcd));
    mxcd.cbStruct = sizeof(mxcd);
    mxcd.cbDetails = sizeof(volStruct);
    mxcd.dwControlID = mxc.dwControlID;
    mxcd.paDetails = &volStruct;
    mxcd.cChannels = 1;

    // Get the current value of the peakmeter control. Typically, you
    // would set a timer in your program to query the volume every 10th
    // of a second or so.
    rc = mixerGetControlDetails((HMIXEROBJ)hMixer, &mxcd,
                                 MIXER_GETCONTROLDETAILSF_VALUE);
    if (MMSYSERR_NOERROR == rc) {
        // Couldn't get the current volume.
    }
    volume = volStruct.lValue;

    // Get the absolute value of the volume.
    if (volume < 0)
        volume = -volume;

REFERENCES

Win32 Software Development Kit for Windows 95 and Windows NT: mixer

MSDN Library, SDK Samples, Multimedia Samples, Mixapp: Sample Mixer Application

Additional query words: peak meter, audio volume

Keywords          : MMMixer 
Version           : WIN95 WINNT:4.0
Platform          : Win95 winnt
Issue type        : kbhowto

Last Reviewed: April 3, 1998