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CEI encoder

The Maestro streamer contains an element adapting an extensible set of audio encoders in the form of functions conforming to the CEI (Common Encoder Interface). This element enables the user to choose and configure a suitable encoder at runtime.

Header files

CEI itself and the CEI encoders are using following header files, in which you may be interested:

cei.h - contains types used by the element itself and an encoder implementing the CEI
cei_enctypes.h - contains a list of possible encoders and types used for interfacing with a CEI encoder
cei_table.h - contains a table of functions implementing integrated CEI encoders

Instantiating the element

This element’s index is ELEMENT_ENCODER_INDEX and its type is TYPE_ELEMENT_ENCODER, as defined in streamer_api.h. It has one source pad (data input) and one sink pad (data output). It is initialized like any other element, meaning that it is instantiated and inserted into the pipeline using the create_element, add_element_pipeline and link_elements functions. Inversely, for destroying the element, the unlink_elements, remove_element_pipeline and destroy_element are used. This element alone does not depend on any additional software layers other than these required by the Maestro streamer itself, so no pre-initialization before this element instantiation is necessary.

Element properties

Use Maestro streamer property API (streamer_set_property and streamer_get_property) for setting or getting these. The constants are defined in streamer_element_properties.h.

PROP_ENCODER_CHUNK_SIZE
- Synopsis: Determines the length of a chunk pulled from the sibling of the source pad and essentially influences the size of allocated buffers. If the actual amount of data pulled is smaller, the rest is zero-filled.
- Type: unsigned 32-bit integer
- Default value: 1920
- Constraints:
  - Must be bigger than zero, otherwise STREAM_ERR_INVALID_ARGS is returned.
  - Cannot be changed if the actual encoder has been created. If done so, STREAM_ERR_ELEMENT_BAD_STATUS is returned.
PROP_ENCODER_TYPE
- Synopsis: Determines the exact encoder (CEI implementation) to be used.
- Type: CeiEncoderType (cei_enctypes.h)
- Default value: CEIENC_LAST
- Constraints:
  - Must not be equal to CEIENC_LAST, otherwise STREAM_ERR_INVALID_ARGS will be returned.
  - Selected encoder must be implemented, otherwise STREAM_ERR_INVALID_ARGS will be returned.
  - Cannot be changed if the actual encoder has been created. If done so, STREAM_ERR_ELEMENT_BAD_STATUS will be returned.
- Behaviour influenced: The encoder element process function will return FLOW_ERROR if this property isn’t set.
PROP_ENCODER_CONFIG
- Synopsis: Determines encoder-specific configuration (application, bitrate, …).
- Type: Pointer to the encoder-specific configuration structure.
- Default value: Determined by the encoder.
- Constraints:
  - The encoder has to be configurable. If it is not, STREAM_ERR_ERR_GENERAL will be returned on any access.
  - The structure has to conform to the encoder requirements. If the encoder returns an error code, STREAM_ERR_GENERAL will be returned.
PROP_ENCODER_BITSTREAMINFO
- Synopsis: Specifies information about the incoming bitstream (sample rate, sample depth, …).
- Type: Pointer to CeiBitstreamInfo (cei_enctypes.h).
- Default value:
```
    (CeiBitstreamInfo) {
        .sample_rate = 0,
        .num_channels = 0,
        .endian = AF_LITTLE_ENDIAN,
        .sign = TRUE,
        .sample_size = 0,
        .interleaved = TRUE
    }
```
- Constraints:
  - Cannot be changed if the actual encoder has been created. If done so, STREAM_ERR_ELEMENT_BAD_STATUS will be returned.
  - As of now, only bitstreams containing 16-bit interleaved (if 2 or more channels will be encoded) samples are supported. If anything else was set to the sample_size and interleaved members, STREAM_ERR_INVALID_ARGS will be returned.
- Behaviour influenced:
  - Given the characteristics of some elements available, different packets of data (header and payload, referred to as “chunk” above) may be pulled by this element. Each packet can contain a different header, which may or may not contain useful information about the bitstream. If a packet with the AudioPacketHeader (todofile.h) is pulled at first and any other iteration of the streamer pipeline, the bitstream parameters configured by this property are implicitly available and are not expected to be specified by the user. Other packet header types (such as RawPacketHeader) don’t contain any bitstream parameters and require the user to specify the parameters manually using this property. Failure to do so will result in the element’s process function returning FLOW_ERROR. Same situation will occur if a packet with the AudioPacketHeader is received and its contents differ from the already acquired bitstream parameters.
  - As of now, CEI is defined to work with 16-bit signed little-endian (s16le) samples, which are interleaved if the bitstream contains more than one channels. This element handles endianness and unsigned to signed conversion.

CEI definition - implementing your own encoder

The CEI defines following function pointer types:

CeiFnGetMemorySize: Returns number of bytes required for encoder state for a given number of channels.
CeiFnEncoderInit: Initialize an encoder for a given sample rate and channel count.
CeiFnEncoderGetConfig: Copy current or default configuration to a given structure pointer.
CeiFnEncoderSetConfig: Configure the encoder from a given structure pointer.
CeiFnEncode: Encode a given buffer to a given output buffer.

Detailed descriptions of function behaviour, parameters and expected return values are available as docblocks in the cei.h file.

Each encoder is implemented as a set of pointers pointing to functions conforming to these types, grouped in the CeiEncoderFunctions structure. Specifying the CeiEncoderGetConfig fnGetConfig and CeiFnEncoderSetConfig fnSetConfig members is optional, as an encoder does not have to be configurable. If so desired, specify NULL. Implementation of the remaining functions is mandatory, however. If at least one of these functions isn’t implemented and NULL is specified instead, the encoder will be considered as not implemented.

To register an implemented encoder with the element, add a new entry to the CeiEncoderType enum and add the CeiEncoderFunctions struct value to the table CeiEncoderFunctions ceiEncTable[] located in the cei_table.h header file. Note and match the order of items in that table, as a CeiEncoderType value is used as an index. Same goes for the size_t ceiEncConfigSizeTable[]. If configuration is not applicable, specify 0 at the appropriate index. If configuration is applicable, describe the configuration structure in the cei_enctypes.h header file and add its size to that table.