Visual Basic 6 Programming Blue Book: The Most Complete, Hands-On Resource for Writing Programs with Microsoft Visual Basic 6!:Multimedia Magic
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Visual Basic 6 Programming Blue Book: The Most Complete, Hands-On Resource for Writing Programs with Microsoft Visual Basic 6!
(Publisher: The Coriolis Group)
Author(s): Peter G. Aitken
ISBN: 1576102815
Publication Date: 08/01/98
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MCI Commands A program controls multimedia devices by means of MCI commands, a remarkably simple set of commands applicable to all multimedia devices. Although some commands are specific for certain devices, the basic sequence of steps for using a multimedia device usually goes like this:
1. Open the device. For some devices, this step includes specifying the file where the data is stored—for example, a WAV file on disk. For other devices, such as playing audio from a CD or video from a videodisc, the data is part of the device, so there is no file to open. 2. Play the device (or, in some cases, record). This step may include pausing, seeking to a new location in the data, and so on. 3. Close the device.
Let’s take a look at some simple MCI commands. To play a WAV file—perhaps CHIMES.WAV, which comes with Windows—here is the first command:
Open c:\media\chimes.wav type waveaudio alias chime
The Open command opens the desired WAV file, specifies its type (from Table 16.1), so Windows knows which driver to use, and assigns an alias— a name you can use in subsequent commands to refer to this device. Opening the device does not actually play the sound. The command for that is:
Play chime
You use the Play command with the alias assigned to the device. The sound will start playing, and control will pass back to the program. Although CHIMES.WAV is a short sound, you can play longer sounds that continue to play to the end while the program performs other tasks. When you finish with the device, you close it as follows:
Close chime
The Close command terminates play, so you have to delay its execution until the sound has completed playing. I’ll show you how to do this later. The basic MCI commands are listed in Table 16.2, although you will not be exploring them all here. My goal is to show you how to perform the most common multimedia tasks in your Visual Basic program. For more details about multimedia, refer to one of the books published on the topic. Table 16.2 The basic MCI commands.
Command Action
Capability Requests information about a device’s capabilities
Close Closes a device
Info Requests specific information about a device (such as its driver name)
Open Opens and initializes a device
Pause Suspends playback or recording
Play Starts playback
Record Begins recording
Resume Resumes play or recording on a paused device
Save Saves recorded data
Seek Moves to a specific position on the media
Set Changes a device’s control settings
Status Obtains device status information
Stop Stops playback or recording
Note that you cannot simply type these commands into Basic code; you have to send them to the MCI. How? Examining the two available methods is the topic for the remainder of this chapter.
Sending MCI Commands With mciSendString One way to send commands to the MCI is to use the function mciSendString. Not a part of Visual Basic, this function is part of the Windows Applications Programming Interface (API), the huge collection of functions that provides most of Windows’ capabilities. When a Visual Basic program—or one written in any other language, for that matter—performs any operating system-related task, it is actually calling Windows API functions to do the job. When you open a file by writing code in Basic, Visual Basic translates it into the necessary Windows API call to perform the task. Not all API capabilities are included in Visual Basic; but fortunately, when you run into this situation, you can call the API function directly—which is exactly what you will do here. Before a program can use a Windows API function, you must declare it. The declaration provides the program with certain information about the function, such as its name, the library it is in, and the number and types of its arguments. You can save time by copying API function declarations from the API Text Viewer application (typically installed along with Visual Basic) and pasting them into our code.
TIP: Using The API Text Viewer The API Text Viewer is part of the default Visual Basic installation. To run it, find the entry “API Text Viewer” on your Windows Start menu (its exact location on the menu depends on your Visual Basic installation. Select Open Text File from the File menu, and load WIN32API.TXT. Then in the API Type box select Constants (to view predefined Windows constants), Declares (to view Windows API function declarations), or Types (to view predefined Windows data types). Select an item in the Available Items list and click on Add to put it in the Selected Items list. Once you have all the desired items selected, click on the Copy button to copy the contents of the Selected Items box to the Clipboard. Switch to Visual Basic and paste the items into your code.
Here’s the declaration for the mciSendString function:
Declare Function mciSendString Lib “winmm” Alias “mciSendStringA” _ (ByVal lpstrCommand As String, ByVal lpstrReturnString As String, _ ByVal uReturnLength As Long, ByVal hwndCallback As Long) As Long
This declaration tells you the following:
• The function is named mciSendString. • It is located in the library named WINMM (a library is a disk file where Windows keeps its functions). • Its alias (the name it goes under in the library) is mciSendStringA. While this alias is essential for the function declaration, it has no other relevance to your program. • It takes four arguments—two Strings and two Longs. • It returns a type Long value.
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