To write a Multimedia Framework (MMF) codec plugin, the following needs to be done:

• Derive from the CMMFCodec base class

• Write a resource file defining the codecs available in the plugin DLL

• Define codec-specific UIDs

• Write the appropriate build file

• Consider implementation issues

Codecs derived from CMMFCodec can be audio, or video, codecs. Codecs that use the Image Conversion Library (ICL) must conform to the ICL codec API.

Codecs derived from CMMFCodec are synchronous as it is expected that the codec will be operating in its own thread of execution, usually via a CMMFDataPath or CMMFDataPathProxy.

The functions in the CMMFCodec base class, from which a specific CMMFCodec must derive, are: CMMFCodec::NewL() and CMMFCodec::ProcessL(). Optional additions are CMMFCodec::ConfigureL() and CMMFCodec::ResetL().

One resource file (.RSS) is required for each codec plugin DLL. This enables ECom to identify and instantiate the codec, or codecs, present in the DLL.

An example resource file follows:

// nnnnnnnn.RSS
//
#include <acmeCodecUIDs.hrh>
#include <mmfPluginInterfaceUIDs.hrh>
#include "RegistryInfo.rh"

RESOURCE REGISTRY_INFO theInfo
    {
    dll_uid = KAcmeCodecPluginDLLUid;
    interfaces =
        {
        INTERFACE_INFO
            {
            interface_uid = KMmfUidPluginInterfaceCodec ;  // CMMFCodec
            implementations =
                {
                IMPLEMENTATION_INFO
                    {
                    implementation_uid = KAcmeCodec1Uid;
                    version_no = 1;
                    display_name = "ABCD to WXYZ Acme codec";
                    default_data = "ABCD, WXYZ" ; // FourCC codes
                    },
                IMPLEMENTATION_INFO
                    {
                    implementation_uid = KAcmeCodec2Uid;
                    version_no = 1;
                    display_name = "mp3 to PCM16 Acme codec";
                    default_data = " MP3,  P16" ; // FourCC codes.
                    }
                };
            }
        };
    }

Where:

• nnnnnnnn is the UID of the ECom plugin DLL.

• AcmeCodecUids.hrh is a header file containing the codec-specific UIDs.

• Dll Uid = KAcmePluginCodecDLLUid is the same as nnnnnnnn.

• version_no is the version number. Note that although this field can be read by ECom, the codec instantiation does not make use of it.

• display_name contains display name information and, where used, the preferred supplier name.

• default_data is a pair of 4 character codes that indicate the assumed source data type the codec expects and the data type into which the codec will decode the source data.

A number of audio codecs are included as part of the MMF which provide a number of standard codec conversions between common audio data types such as uLaw, aLaw, pcm8, pcm16 and gsm610. These can be found in the multimedia component directory under \Mmf\Src\Plugin\Codec\Audio.

Since most of these codecs are relatively simple, they provide a good source of example code for writing a codec plugin. There are also a number of test codecs in the UnitTest directory in the \acod and \basecl sub-directories which could be used as a template for writing a new CMMFCodec plugin.

Codec-specific UIDs are defined in a header file. An example is shown below, this contains one plugin only, KExCodecUID which has a UID of 0x101F81D7:

#ifndef _MmfExCodecUIDS_H_
#define _MmfExCodecUIDS_H_
#define KExCodecUID 0x101F81D7

#endif

There are no special requirements for an MMF codec plugin mmp file over and above a conventional ECom plugin.

The main points to note are:

• TARGETTYPE is ECOMIIC

• TARGETPATH is \System\Libs\Plugins

• UID should be the ECOM DLL

• LIBRARY needs to include ecom.lib

Note that it is not necessary to include mmfserverbaseclasses.lib.

When implementing a codec plugin, consider the following:

• Memory usage. Codec implementations should avoid reallocating the destination buffer as the sink may require the buffer to be a certain size.

• Error handling. If an error occurs during the codec’s data processing then the codec should leave if the error is non-recoverable. If the error may only impair the data rendering then the codec should continue.

• Performance considerations. The codec's derived ProcessL() function should attempt to fill the destination buffer as fully as possible to minimise the number of writes to the data sink.

• UIDs. In addition to the codec plugin DLL having its own UID, each codec within the codec DLL must be allocated its own UID. If more than one codec performs the same data type conversion then they will each require their own UID.