The Windows API provides two methods for controlling the multimedia capabilities of the system. The first is a series of low-level functions. The second is the Windows Media Control Interface (MCI). The media control interface provides a high-level, device-independent interface for controlling multimedia devices on the system and is the recommended method for controlling multimedia devices in Windows.
There are two methods for issuing MCI commands: command messages and command strings. Command messages are sent by filling a memory structure (GROUP) and passing it to the MCI system. Command strings are English-like strings that are sent to the MCI system and parsed to determine the action to be taken. Command strings provide the same functionality as command messages with a simpler format. This tip examines how to use the media control interface using MCI command strings.
Using MCI, a program has the ability to play and record (as appropriate) on
any supported multimedia device. This includes the usual multimedia devices
(wave audio, MIDI sequencer, CD audio) as well as some more unusual devices
(VCR, videodisc, digital audio tape). The configuration information for MCI
is stored in the WIN.INI and SYSTEM.INI files. The [mci] section
of SYSTEM.INI maps the system device names to the device drivers controlling
those devices. The [mci extensions] section of WIN.INI maps file
extensions to the appropriate multimedia device.
The MCI configuration information for my system is shown below. There are five multimedia devices registered on the system: CD audio, MIDI sequencer, wave audio, AVI video and MPEG video.
[mci] cdaudio=mcicda.drv sequencer=mciseq.drv waveaudio=mciwave.drv avivideo=mciavi.drv MPEGVideo=mciqtz.drv
This configuration method makes MCI extensible. New multimedia devices with the proper drivers can be integrated with the MCI system even though the devices weren't invented when MCI was designed. The system can also use multiple devices of the same type by configuring them correctly in SYSTEM.INI. I recently completed a project using a “CD jukebox” - a five-platter CD-ROM unit. After modifying the MCI configuration information, I was able to address each of the CD-ROM platters as an independent device.
The MCI system commands are divided into four categories: system, required, basic and extended commands. The commands are described below along with some commonly used options.
System commands are handled by the MCI system instead of by an MCI device.
These commands provides provide system-level information. The system commands
are break and sysinfo.
| Command | Description |
break |
Set up key to break MCI action |
sysinfo |
Gets MCI system information
sysinfo all quantity
|
Required commands must be implemented for every device. Each device must
also support a defined set of options for each command. The required commands
are capability, close, info, open
and status.
| Command | Description |
capability |
Gets information about device capabilitiescapability cdaudio can eject
|
close |
Closes an open device
close all
|
info |
Gets information about the device driverinfo cdaudio product
|
open |
Opens and initializes a device
open cdaudio
|
status |
Gets device status
status cdaudio mode
|
Basic commands are device control commands that are may or may not be
implemented depending on the device. The commands that are implemented must
support a standard set of options for that command. The basic commands are load,
pause, play, record, resume,
save, seek, set, status
and stop.
| Command | Description |
load |
Loads a device file from disk |
pause |
Pauses playing or recording
pause cdaudio
|
play |
Starts playing the device
play cdaudio
|
record |
Starts recording data with the devicerecord waveaudio
|
resume |
Resumes playing or recording after pause
|
save |
Save a device file to disksave waveaudio c:\temp\newsound.wav
|
seek |
Moves to a specified position for the device
seek cdaudio to start
|
set |
Updates device control settings
set cdaudio door open
|
status |
Gets device status
status cdaudio current track
|
stop |
Stops playing or recording
stop cdaudio
|
Extended commands are additional commands that may be implemented for
a specific device or that extended that options on a required or basic command.
Examples of extended commands are delete and spin.
Delete is used to delete a section of a wave audio file. Spin is used to start
or stop spinning on a videodisc device. These commands are implemented only
for the devices they apply to.
Two API functions are involved when using MCI command strings: mciSendString and mciGetErrorString. The functions are prototyped and used in the Media Control Interface Tester sample program. This program allows the user to enter and execute an MCI command string. The program provides a way to test the result of various MCI command strings.
The program includes some sample MIDI files for testing. You can use these along with other multimedia files on your system to test some of the following command strings and see their results.
To test the wave audio device on your system, try:
open c:\win95\media\chimes.wav type waveaudio play c:\win95\media\chimes.wav close c:\win95\media\chimes.wav open c:\win95\media\chimes.wav type waveaudio alias chimes play chimes close chimes open waveaudio!c:\win95\media\chimes.wav alias chimes play chimes close chimes
To test the MIDI sequencer on your system, try:
open sequencer!canada.mid alias anthem play anthem pause anthem resume anthem stop anthem seek anthem to start play anthem stop anthem close anthem
To test the CD drive on your system, try:
open cdaudio play cdaudio set cdaudio time format tmsf play cdaudio from 3 to 4 close cdaudio
If you do much work with the MCI system, you will probably want to obtain a Windows API reference so that you have full documentation for the MCI command structure. For basic MCI work, you now have to tools needed to integrate the media control interface and its capabilities into your Clarion programs.
Download the sample code