Playing Waveform-Audio Files
It's easy to play sounds in your application by using the functions, macros,
and messages discussed in this overview. The techniques and elements documented
here operate only on waveform audio; that is, digitized representations of a
sound's physical shape. If you want to add music to your application, and you do
not care about other kinds of sounds, you might want to use MIDI. For a
discussion of a simple playback MIDI implementation, see the
MCIWnd Window Class. For a discussion of the MIDI interface, see
Musical Instrument Digital Interface (MIDI).
You can use the following functions to play waveform audio in your application
in a single function call:
Function
| Description
|
MessageBeep
| Plays the sound that corresponds to a specified system-alert level.
|
sndPlaySound
| Plays the sound that corresponds to the system sound entered in the registry
or the contents of the specified filename.
|
PlaySound
| Provides all the functionality of sndPlaySound and can directly access resources.
|
The
MessageBeep function is a standard part of the Win32 API; because its capabilities are
very limited and it is documented elsewhere, it is not discussed here.
The functions listed provide the following methods of playing waveform audio:
- Playing waveform-audio files associated with system-alert levels
- Playing waveform-audio files specified by entries in the registry
- Playing in-memory WAVE resources
- Playing waveform-audio files stored on a hard disk or compact disc
read-only memory (CD-ROM)
The
sndPlaySound and
PlaySound functions load an entire waveform-audio file into memory and, in effect,
limit the size of file they can play. Use
sndPlaySound and
PlaySound to play waveform-audio files that are relatively small
up to about 100K. These two functions also require the sound data to be in a
format that is playable by one of the installed waveform-audio drivers,
including the wave mapper.
For larger sound files, use the Media Control Interface (MCI) services. For
more information, see
MCI.
Using Window Messages to Manage Waveform-Audio Playback
The following messages can be sent to a window procedure function for managing
waveform-audio playback.
A
wParam and
lParam parameter is associated with each of these messages. The
wParam parameter always specifies a handle of the open waveform-audio device. For
the
MM_WOM_DONE message,
lParam specifies a pointer to a
WAVEHDR structure that identifies the completed data block. The
lParam parameter is unused for the
MM_WOM_CLOSE and
MM_WOM_OPEN messages.
The most useful message is probably MM_WOM_DONE. When this message signals
that playback of a data block is complete, you can clean up and free the data
block. Unless you need to allocate memory or initialize variables, you probably do
not need to process the MM_WOM_OPEN and MM_WOM_CLOSE messages.
The callback function for waveform-audio output devices is supplied by the
application. For information about this callback function, see the
waveOutProc function.
Retrieving the Current Playback Position
You can monitor the current playback position within the file while waveform
audio is playing by using the
waveOutGetPosition function.
For waveform-audio devices, samples are the preferred time format in which to
represent the current position. Thus, the current position of a waveform-audio
device is specified as the number of samples for one channel from the beginning
of the waveform-audio file. To query the current position of a waveform-audio
device, set the
wType member of the
MMTIME structure to TIME_SAMPLES and pass this structure to
waveOutGetPosition.
The
MMTIME structure can represent time in one or more different formats, including
milliseconds, samples, SMPTE (Society of Motion Picture and Television Engineers),
and MIDI song pointer formats. The
wType member specifies the format used to represent time. Before calling a function
that uses the
MMTIME structure, you must set
wType to indicate your requested time format. Be sure to check
wType after the call to see whether the requested time format is supported. If the
requested time format is not supported, the device driver specifies the time in
an alternate time format and changes the
wType member to the selected time format.
For more information about the
MMTIME structure, see
Multimedia Timers.
Stopping, Pausing, and Restarting Playback
You can stop or pause playback while waveform audio is playing. After playback
has been paused, you can restart it. Windows provides the following functions
for controlling waveform-audio playback.
Function
| Description
|
waveOutPause
| Pauses playback on a waveform-audio output device.
|
waveOutReset
| Stops playback on a waveform-audio output device and marks all pending data
blocks as done.
|
WaveOutRestart
| Resumes playback on a paused waveform-audio output device.
|
Pausing a waveform-audio device by using
waveOutPause might not be instantaneous; the driver may finish playing the current block
before pausing playback.
Generally, as soon as the first waveform-audio data block is sent by using the
waveOutWrite function, the waveform-audio device begins playing. If you do not want the
sound to start playing immediately, call
waveOutPause before calling
waveOutWrite. Then, when you want to begin playing waveform-audio data, call
waveOutRestart.
You cannot use
waveOutRestart to restart a device that has been stopped with
waveOutReset; you must use
waveOutWrite to send the first data block to resume playback on the device.
Looping Playback
Looping a sound is controlled by the
dwLoops and
dwFlags members in the
WAVEHDR structures passed to the device with the
waveOutWrite function. Use the WHDR_BEGINLOOP and WHDR_ENDLOOP flags in the
dwFlags member to specify the beginning and ending data blocks for looping.
To loop a single data block, specify both flags for the same block. To specify
the number of loops, use the
dwLoops member in the
WAVEHDR structure for the first block in the loop.
You can call the
waveOutBreakLoop function to stop a looping sound.
Changing the Volume of Waveform-Audio Playback
Windows provides the following functions to query and set the volume level of
waveform-audio output devices.
Function
| Description
|
waveOutGetVolume
| Retrieves the current volume level of the specified waveform-audio output
device.
|
waveOutSetVolume
| Sets the volume level of the specified waveform-audio output device.
|
Not all waveform-audio devices support volume changes. Some devices support
individual volume control on both the left and right channels. For information
about how to determine the volume-control capabilities of waveform-audio devices,
see
Devices and Data Types.
Some applications allow the user to control the volume for all audio devices
in a system. (Many applications of this type use the audio mixer services; for
more information, see
Audio Mixers.) Unless your application is capable of this kind of master volume control,
you should open an audio device before changing its volume. You should also
query the volume level before changing it and restore the volume level to its
previous level as soon as possible.
Volume is specified in a doubleword value. When the audio format is stereo,
the upper 16 bits specify the relative volume of the right channel and the lower
16 bits specify the relative volume of the left channel. For devices that do
not support left- and right-channel volume control, the lower 16 bits specify the
volume level, and the upper 16 bits are ignored.
Volume-level values range from 0x0 (silence) to 0xFFFF (maximum volume) and
are interpreted logarithmically. The perceived volume increase is the same when
increasing the volume level from 0x5000 to 0x6000 as it is from 0x4000 to 0x5000.
Changing Pitch and Playback Rate
Some waveform-audio output devices can vary the pitch and the playback rate of
waveform-audio data. Not all waveform-audio devices support pitch and
playback-rate changes. For information about how to determine whether a particular
waveform-audio device supports pitch and playback rate changes, see
Devices and Data Types.
The differences between changing pitch and playback rate are as follows:
- Changing the playback rate is performed by the device driver and does not
require specialized hardware. The sample rate is not changed, but the driver
interpolates by skipping or synthesizing samples. For example, if the playback rate
is changed by a factor of two, the driver skips every other sample.
- Changing the pitch requires specialized hardware. The playback rate and sample
rate are not changed.
Windows provides the following functions to query and set waveform-audio pitch
and playback rates.
Function
| Description
|
waveOutGetPitch
| Retrieves the pitch for the specified waveform-audio output device.
|
waveOutGetPlaybackRate
| Retrieves the playback rate for the specified waveform-audio output device.
|
waveOutSetPitch
| Sets the pitch for the specified waveform-audio output device.
|
waveOutSetPlaybackRate
| Sets the playback rate for the specified waveform-audio output device.
|
The pitch and playback rates are changed by a factor specified with a
fixed-point number packed into a doubleword value. The upper 16 bits specify the
integer part of the number; the lower 16 bits specify the fractional part. For
example, the value 1.5 is represented as 0x00018000L. The value 0.75 is represented
as 0x0000C000L. A value of 1.0 (0x00010000) means the pitch or playback rate is
unchanged.
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