CMMFFormatDecode Class Reference

Creates a sink buffer for the specified media ID. The default version returns NULL.

If EFalse, then the caller owns the buffer.

Creates a source buffer.

This is a virtual function that each derived class must implement.

This function should create a buffer of length 0, the maximum length should be equal to the maximum

possible frame size. In the case of non framed data it should be set to an arbitrary size, which is

effectively a trade off between being too small which will affect performance as more source reads

are required, and being too large which will give very coarse positioning granularity. For pcm data,

a buffer size of 4K is a good compromise. The same compromise also applies when deciding to put multiple

audio frames into one format frame. The sink buffer size may also effect the format buffer size and

the controller may use this to suggest a buffer size to the format by calling the format's SuggestSourceBufferSize()

method. Alternatively the MDataSource::CreateSourceBufferL() function that takes the additional aSinkBuffer

parameter may also be used when deciding the source buffer maximum size of the created source buffer.

should normally be set to EFalse as format plugins do not create the reference buffer.

Returns the duration of the clip for the specified media type.

This is a virtual function that each derived class must implement.

Adds a buffer to a clip.

Request from CMMFDataPath to fill the specified buffer.

The CMMFFormat needs to break this down into one or more reads from the clip

(from MDataSource - CMMFFormatDecode is a MDataSource to a CMMFDataPath consumer).

This is a virtual function that each derived class must implement.

This method is the means by which data is obtained from the data source. aBuffer is

the buffer that needs filling from the source data stream as identified by aMediaId. The

format should read the frame number of the buffer via the buffer's CMMFBuffer::FrameNumber()

method. From this the format should be able to determine the actual position of the data on

the data source. The technique here depends on the nature of the format. For a linear audio

format, the position can be obtained by a simple calculation of the frame size, the header size

and where appropriate the datatype.

For non-linear formats either an index table of frame number and location will need to be

created in the NewL() or some form of approximating algorithm will be required. Some formats have

an index table as part of the format e.g. AVI. If no random access is required then no index table

is required, the format can keep track of the current read position and increment it on each access,

and reset it if the frame number is reset to 0 or 1. Given that for non-linear i.e. variable bit rate

formats, the size of the data read may vary from frame to frame, then the format should either set

the request size of the buffer for the required frame or call the source's ReadBufferL() (either

CMMFClip::ReadBufferL(), CMMFDescriptor ::ReadBufferL() or CMMFFile::ReadBufferL()) function that

takes the aLength parameter.

It is the responsibility of the format decode to determine the size and position of the source data

for each frame. For linear audio formats, the format decode should fill the buffer up to its maximum

length. For multimedia formats e.g. mp4, AVI etc, the format decode is effectively acting as a demultiplexor,

and must obtain the appropriate data from the source depending on the aMediaId.

Returns the time interval for one frame for the specified media type.

This is a virtual function that each derived class must implement.

In the case of video, a frame would usually be one frame of video. In the case of

audio, a frame may correspond to a frame of audio, if the particular audio data type

is framed eg mp3. There are two definitions of a frame. A format frame, which may

consist of several frames of a particular framed audio data type. This may be the case,

for example for GSM610 (a low quality audio data type used in telephony) where a frame

is only 65 bytes large. In this case a format frame consists of several GSM610 data type

frames as passing the data out 65 bytes at a time would be inefficient. In the case of

non-framed data such as pcm, a frame can be an arbitrary size determined by the format plugin.

Gets the number of meta data entries.

Meta Data support. The decode format is only capable of reading meta data entries from the clip.

This is an optional method, used to return the number of meta data entries present in the format.

A meta data entry is a format-specific field such as authorship, copyright, security details etc.

The function is optional as many formats do not provide support for such additional meta data fields.

The default leaves with KErrNotSupported.

the number of meta data entries.

Gets the supported bit rates.

The default leaves with KErrNotSupported.

This should return an array of bit rates supported by the format where applicable. Note that this

refers to the possible bit rates supported by the format, not the actual bit rate, which will be

one of the supported bit rates. The implementation of this function is optional.

Gets the supported data types for the given media type id.

The default leaves with KErrNotSupported.

The implementation of this procedure should return an array of data types supported by the format where

applicable. Note that this refers to the possible data types that may be supported by the format, not the

actual data type of the format, which will be one of the supported data types. For example, for a format

decode plugin capable of supporting pcm16 and GSM610 the procedure would return an array of length two the

first array member containing the fourCC code ' P16' and the second containing the value GSM6. The

implementation of this function is optional.

Gets the supported number of channels.

The default leaves with KErrNotSupported.

The implementation of this procedure should return an array of channels supported by the format

where applicable. Note that this refers to the possible number of channels supported by the format,

not the actual number of channels, which will be one of the supported channels. For example, for a

format decode plugin capable of supporting mono and stereo the procedure would return an array of

length two the first array member containing the value 1 and the second containing the value 2. The

implementation of this function is optional.

Gets the supported sample rates.

The default implementation leaves with KErrNotSupported.

This should return an array of sample rates supported by the format where applicable. Note

that this refers to the possible sample rates supported by the format, not the actual sample rate,

which will be one of the supported sample rates. The implementation of this function is optional.

Returns the ECom plugin UID of this format.

Returns the specified meta data entry.

This method is optional as many formats do not provide support for such additional meta data fields.

The default leaves with KErrNotSupported.

Allocates and constructs an ECom format decode object.

This is used to explicitly instantiate a format decoder where the UID is known. This method might

be used by controllers that only support one type of format such that the UID of the format is

already known. For example, if an mp3 controller has been instantiated, there is no need for any

further format recognition as this has already been performed in the controller instantiation, thus

the controller can instantiate an mp3 format directly from its UID.

The implementation UID. This is used by Com to create the plugin.

The controller MDataSource and is the source of the data for the format decode plugin. The

format decode plugin is the sink of data for the MDataSource.

leaves with KErrNotFound.