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Streaming .WAV File Player
Play .WAV files of any length using the Multimedia API without problems


NOTE: this code has been superceded by the version at the new site.



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Wave Streaming Demo Application

Download the WaveStream Sample application and class (47kb)

&nbsp Before you Begin &nbsp
&nbsp This project requires the SSubTmr.DLL component. Make sure you have loaded and registered this before trying the project. &nbsp

Playing Waves
Playing digital audio can be achieved at a number of different levels under windows. At the simplest level, there is the Multimedia Control Interface (MCI) control provided with VB. This is turn is a very thin wrapper around the MCI API provided with Win32, which offers about the same level of control for playing waves as the APIPlaySound and sndPlaySound functions.

At a deeper level, your application can take charge of reading chunks of audio from disk and streaming it directly to the sound card. The advantage of these methods is you get more control over how the audio is played. With the MCI and PlaySound methods, the system has to allocate memory for the entire sound before it starts. With a streaming approach, you can read the sound in in small chunks (say 0.1s of audio) allowing you to play audio samples of any length (and remember that a CD quality stereo WAV file occupies around 10Mb/minute!). Another advantage of using the multimedia methods is you can affect the sound before you play it. You could filter it by averaging the bytes before putting them into the sound card's buffer. Or you could create an echo effect by preparing an additional audio buffer which you save attenuated samples of the main sound to.

Using the multimedia API at this level gives you more control, but the downside is additional complexity. This sample demonstrates how you can play back standard .WAV files, however, if you need to be able to play back sounds which have are not directly compatible with your soundcard (for example, .WAV files with ADPCM compression or .MP3 files) then this sample alone will not work because it would need to invoke the Audio Compression Manager (ACM) functions to do that (which is the subject of a future article, if I can figure it out!).

The cWavPlayer class
The cWavPlayer class supplied with the download wraps up access to stream digital audio to the sound card. Fundamentally, it works like this:

  • OpenFile
    Opens a Multimedia IO device handle, reads the header of the WAV file to check the format of the WAV file and then creates a number of buffers to hold 0.1s each of the wave data read in from the file.
  • Play
    Creates a WaveOut handle for passing data to the soundcard from the WAV file and specifies that when data is needed by the soundcard it should post a message to the window. It then prepares each of the buffers to send data to the wave out handle and initiates the play callback loop by posting the message to the window.
  • Play callback loop
    Reads data from the file into the wave buffer pointed to by the wave header, and then writes it out to the sound card's driver. When the data is exhausted, the buffer is unprepared and finally once all the buffers have been unprepared the WaveOut handle is closed.
To use the class is straightforward. Add the class to your project, and ensure the project references SSubTmr.DLL. Then open a .WAV file with the OpenFile method. At this stage you can read various information about the wave file, such as the sampling frequency (SamplesPerSecond), the bit depth (BitsPerSample), whether the file is stereo or mono (Channels).

To play the file, use the Play method. You can pause it at any time by choosing the Stop method, and change the position in the file using the FileSeek method.

You can get the position of the playback at any time using the Position method. Remember that the audio is buffered into 0.1s chunks for playback, so that is the minimum resolution. To get a better resolution, you can reduce the buffer time in the cWavPlay class by changing the BUFFER_SECONDS constant value, but this can lead to problems on slower machines.



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Copyright 2000, Steve McMahon ( steve@vbaccelerator.com). All Rights Reserved.
Last updated: 21 March 2000