Using the Media Control Interface

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.

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.

MCI System Commands

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.

Required Commands

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 capabilities `capability cdaudio can eject`
`close`	Closes an open device `close all close cdaudio`
`info`	Gets information about the device driver `info cdaudio product`
`open`	Opens and initializes a device `open cdaudio open waveaudio!c:\win95\media\chimes.wav open c:\win95\media\chimes.wav type waveaudio open waveaudio!chimes.wav alias chimesound open sequencer!canada.mid open canada.mid type sequencer alias anthem`
`status`	Gets device status `status cdaudio mode status sequencer mode`

Basic Commands

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 pause anthem`
`play`	Starts playing the device `play cdaudio play cdaudio from 3 to 4 play waveaudio play chimesound`
`record`	Starts recording data with the device `record waveaudio`
`resume`	Resumes playing or recording after `pause resume cdaudio`
`save`	Save a device file to disk `save waveaudio c:\temp\newsound.wav`
`seek`	Moves to a specified position for the device `seek cdaudio to start seek cdaudio to 3 seek waveaudio to 1000`
`set`	Updates device control settings `set cdaudio door open set cdaudio door closed set waveaudio time format milliseconds set cdaudio time format tmsf`
`status`	Gets device status `status cdaudio current track status anthem length status cdaudio length track 3 status cdaudio number of tracks status waveaudio time format`
`stop`	Stops playing or recording `stop cdaudio stop anthem`

Extended Commands

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.

The MCI Command String Functions

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.

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.