Introduction
Audio Mixer Functions are a set of functions that control the
routing of audio lines to a destination device for playing or
recording. They can also control volume and other effects.
Although there are only ten functions and two messages in this
API, some people feel a little difficult to use them because of
the flexibility. Here I am going to present four simple programs
to demonstrate how to use this powerful API. I kept these four
programs as simple as possible and one program does one thing
only so that you can understand them easily. I also tried to
separate the actual operations from the user interface as much as
possible. All the functions related to Audio Mixer Functions have
the prefix "amd". I did not write C++ classes for those
functions as the main purpose here is to give you a tutorial. In
order to master this API, you may read the Platform SDK documentation
on Audio Mixers and play around with these four programs. You may
also wish to check out the Platform SDK Audio Sample "Mixapp:
Sample Mixer Application", which is not so easy to read.
To better understand Audio Mixer Functions, keep in mind the
following key facts.
- There are zero, one or more audio mixer devices in a system.
- Each audio mixer device has one or more destination audio lines.
- Each destination audio line has zero, one or more source audio
lines associated with it.
- Each (destination or source) audio line has zero, one or more
mixer controls associated with it. A mixer control can perform any
number of functions (such as control volume), depending on the
characteristics of the associated audio line.
All the four programs are MFC dialog based. For the sake of
brevity, they always use the first mixer device if there are more
than one mixer device present in the system. The mixerOpen() function
is called with the CALLBACK_WINDOW flag in the fdwOpen parameter
and a window handle in the dwCallback parameter in the Program 1,
2 and 4 so that the MM_MIXM_CONTROL_CHANGE message can be received
in order to refresh the state of the controls.
Program 1. Master Mute
This program is equivalent to the "Mute all" check
box of Windows Volume Control for Playback. The Master Mute
control is the MIXERCONTROL_CONTROLTYPE_MUTE control of the
MIXERLINE_COMPONENTTYPE_DST_SPEAKERS destination audio line. It
belongs to the switch controls (MIXERCONTROL_CT_CLASS_SWITCH),
which are two-state switches. It uses the
MIXERCONTROLDETAILS_BOOLEAN structure to retrieve and set control
properties. Whenever the check box is clicked,
CMuteDlg::amdSetMasterMuteValue() is called to set the Master Mute
value accordingly. The MM_MIXM_CONTROL_CHANGE message is received
once the state of the Master Mute control has changed. In this
case, CMuteDlg::amdGetMasterMuteValue() is called to retrieve the
current value of the Master Mute control and update the state of
the check box accordingly.
Program 2. Master Volume
This program is equivalent to the "Volume Control"
trackbar of Windows Volume Control for Playback. The Master Volume
control is the MIXERCONTROL_CONTROLTYPE_VOLUME control of the
MIXERLINE_COMPONENTTYPE_DST_SPEAKERS destination audio line. It
belongs to the fader controls (MIXERCONTROL_CT_CLASS_FADER),
which have a linear scale. It uses the
MIXERCONTROLDETAILS_UNSIGNED structure to retrieve and set
control properties. Whenever the trackbar is clicked,
CVolumeDlg::amdSetMasterVolumeValue() is called to set the Master
Volume value accordingly. The MM_MIXM_CONTROL_CHANGE message is
received once the state of the Master Volume control has changed.
In this case, CVolumeDlg::amdGetMasterVolumeValue() is called to
retrieve the current value of the Master Volume control and
update the state of the trackbar accordingly.
Program 3. Wave Meter
This program is equivalent to the "Wave Meter"
control of Windows Volume Control for Playback. The Wave Meter
control is the MIXERCONTROL_CONTROLTYPE_PEAKMETER control of the
MIXERLINE_COMPONENTTYPE_SRC_WAVEOUT source audio line associated
with the MIXERLINE_COMPONENTTYPE_DST_SPEAKERS destination audio line.
It belongs to the meter controls (MIXERCONTROL_CT_CLASS_METER),
which measure data passing through an audio line. It uses the
MIXERCONTROLDETAILS_SIGNED structure to retrieve and set control
properties. A timer is created in order to monitor the value of
the Wave Meter control. CMeterDlg::amdGetWaveMeterValue() is called
to retrieve the current value of the Wave Meter control and
update the state of the progress bar accordingly once a WM_TIMER
message is received.
Run it and play a sound file using other applications, you can see
it dancing. Note that the sound from MIDI or CD Audio is not
reflected here since it passes through the
MIXERLINE_COMPONENTTYPE_SRC_SYNTHESIZER or
MIXERLINE_COMPONENTTYPE_SRC_COMPACTDISC source audio lines
respectively.
Program 4. Microphone Select
This program is equivalent to the "Microphone Select"
check box of Windows Volume Control for Recording. The Microphone Select
control is one of the items of the MIXERCONTROL_CONTROLTYPE_MIXER or
MIXERCONTROL_CONTROLTYPE_MUX controls of the
MIXERLINE_COMPONENTTYPE_DST_WAVEIN destination audio line.
It belongs to the list controls (MIXERCONTROL_CT_CLASS_LIST),
which provide single-select or multiple-select states for complex
audio lines. It uses the MIXERCONTROLDETAILS_BOOLEAN structure to
retrieve and set control properties. Whenever the check box is clicked,
CSelectDlg::amdSetMicSelectValue() is called to set the Microphone Select
value accordingly. The MM_MIXM_CONTROL_CHANGE message is received
once the state of the Microphone Select control has changed. In this
case, CSelectDlg::amdGetMicSelectValue() is called to retrieve the
current value of the Microphone Select control and update the state of
the check box accordingly.
Happy programming and good luck!
