Group PJMEDIA_CODEC¶

group PJMEDIA_CODEC

Media codec framework and management.

Codec Management¶

Codec Manager¶

The codec manager is used to manage all codec capabilities in the endpoint. When used with media endpoint (pjmedia_endpt), application can retrieve the codec manager instance by calling pjmedia_endpt_get_codec_mgr().

Registering New Codec¶

New codec types can be registered to PJMEDIA (or to be precise, to the codec manager) during run-time. To do this, application needs to initialize an instance of codec factory (pjmedia_codec_factory) and registers this codec factory by calling pjmedia_codec_mgr_register_factory().

For codecs implemented/supported by PJMEDIA, this process is normally concealed in an easy to use function such as pjmedia_codec_g711_init().

Codec Factory¶

A codec factory (pjmedia_codec_factory) is registered to codec manager, and it is used to create and release codec instance.

The most important member of the codec factory is the “virtual” function table pjmedia_codec_factory_op, where it contains, among other thing, pointer to functions to allocate and deallocate codec instance.

Codec Instance¶

Application allocates codec instance by calling pjmedia_codec_mgr_alloc_codec(). One codec instance (pjmedia_codec) can be used for simultaneous encoding and decoding.

The most important member of the codec instance is the “virtual” function table pjmedia_codec_op, where it holds pointer to functions to encode/decode media frames.

Codec Identification¶

A particular codec type in PJMEDIA can be uniquely identified by two keys: by pjmedia_codec_info, or by pjmedia_codec_id string. A fully qualified codec ID string consists of codec name, sampling rate, and number of channels. However, application may use only first parts of the tokens as long as it will make to codec ID unique. For example, “gsm” is a fully qualified codec name, since it will always have 8000 clock rate and 1 channel. Other examples of fully qualified codec ID strings are “pcma”, “speex/8000”, “speex/16000”, and “L16/16000/1”. A codec id “speex” (without clock rate) is not fully qualified, since it will match the narrowband, wideband, and ultrawideband Speex codec.

The two keys can be converted to one another, with pjmedia_codec_info_to_id() and pjmedia_codec_mgr_find_codecs_by_id() functions.

Codec ID string is not case sensitive.

Using the Codec Framework¶

Allocating Codec¶

Application needs to allocate one codec instance for encoding and decoding media frames. One codec instance can be used to perform both encoding and decoding.

Application allocates codec by calling pjmedia_codec_mgr_alloc_codec(). This function takes pjmedia_codec_info argument, which is used to locate the particular codec factory to be used to allocate the codec.

Application can build pjmedia_codec_info structure manually for the specific codec, or alternatively it may get the pjmedia_codec_info from the codec ID string, by using pjmedia_codec_mgr_find_codecs_by_id() function.

The following snippet shows an example to allocate a codec:

pj_str_t codec_id;
pjmedia_codec_info *codec_info;
unsigned count = 1;
pjmedia_codec *codec;

codec_id = pj_str("pcma");

// Find codec info for the specified coded ID (i.e. "pcma").
status = pjmedia_codec_mgr_find_codecs_by_id( codec_mgr, &codec_id,
                                       &count, &codec_info, NULL);

// Allocate the codec.
status = pjmedia_codec_mgr_alloc_codec( codec_mgr, codec_info, &codec );

Initializing Codec¶

Once codec is allocated, application needs to initialize the codec by calling member of the codec. This function takes pjmedia_codec_param as the argument, which contains the settings for the codec.

Application shoud use pjmedia_codec_mgr_get_default_param() function to initiaize pjmedia_codec_param. The setting part of pjmedia_codec_param then can be tuned to suit the application’s requirements.

The following snippet shows an example to initialize codec:

pjmedia_codec_param param;

// Retrieve default codec param for the specified codec.
pjmedia_codec_mgr_get_default_param(codec_mgr, codec_info
                             &param);

// Application may change the "settings" part of codec param,
// for example, to disable VAD
param.setting.vad = 0;

// Open the codec using the specified settings.
codec->op->open( codec, &param );

Encoding and Decoding Media Frames¶

Application encodes and decodes media frames by calling encode and decode member of the codec’s “virtual” function table (pjmedia_codec_op).

Concealing Lost Frames¶

All codecs has Packet Lost Concealment (PLC) feature, and application can activate the PLC to conceal lost frames by calling recover member of the codec’s “virtual” function table (pjmedia_codec_op).

If the codec’s algorithm supports PLC, the recover function will use the codec’s PLC. Otherwise for codecs that don’t have intrinsic PLC, PJMEDIA will suply the PLC implementation from the Packet Lost Concealment (PLC) implementation.

Closing and Releasing the Codec¶

The codec must be closed by calling close member of the codec’s operation. Then it must be released by calling pjmedia_codec_mgr_dealloc_codec().

Defines

PJMEDIA_CODEC_MAX_FMTP_CNT¶

PJMEDIA_CODEC_MGR_MAX_CODECS¶: Declare maximum codecs

Typedefs

typedef char pjmedia_codec_id[32]¶: Codec identification (e.g. “pcmu/8000/1”). See Codec Identification for more info.

typedef struct pjmedia_codec_default_param pjmedia_codec_default_param¶: Opaque declaration of default codecs parameters.

Enums

enum pjmedia_rtp_pt¶

Standard RTP static payload types, as defined by RFC 3551. The header file <pjmedia-codec/types.h> also declares dynamic payload type numbers that are used by PJMEDIA when advertising the capability for example in SDP message.

Values:

enumerator PJMEDIA_RTP_PT_PCMU¶: audio PCMU

enumerator PJMEDIA_RTP_PT_G721¶: audio G721 (old def for G726-32)

enumerator PJMEDIA_RTP_PT_GSM¶: audio GSM

enumerator PJMEDIA_RTP_PT_G723¶: audio G723

enumerator PJMEDIA_RTP_PT_DVI4_8K¶: audio DVI4 8KHz

enumerator PJMEDIA_RTP_PT_DVI4_16K¶: audio DVI4 16Khz

enumerator PJMEDIA_RTP_PT_LPC¶: audio LPC

enumerator PJMEDIA_RTP_PT_PCMA¶: audio PCMA

enumerator PJMEDIA_RTP_PT_G722¶: audio G722

enumerator PJMEDIA_RTP_PT_L16_2¶: audio 16bit linear 44.1KHz stereo

enumerator PJMEDIA_RTP_PT_L16_1¶: audio 16bit linear 44.1KHz mono

enumerator PJMEDIA_RTP_PT_QCELP¶: audio QCELP

enumerator PJMEDIA_RTP_PT_CN¶: audio Comfort Noise

enumerator PJMEDIA_RTP_PT_MPA¶: audio MPEG1/MPEG2 elemetr. streams

enumerator PJMEDIA_RTP_PT_G728¶: audio G728

enumerator PJMEDIA_RTP_PT_DVI4_11K¶: audio DVI4 11.025KHz mono

enumerator PJMEDIA_RTP_PT_DVI4_22K¶: audio DVI4 22.050KHz mono

enumerator PJMEDIA_RTP_PT_G729¶: audio G729

enumerator PJMEDIA_RTP_PT_CELB¶: video/comb Cell-B by Sun (RFC2029)

enumerator PJMEDIA_RTP_PT_JPEG¶: video JPEG

enumerator PJMEDIA_RTP_PT_NV¶: video NV by nv program by Xerox

enumerator PJMEDIA_RTP_PT_H261¶: video H261

enumerator PJMEDIA_RTP_PT_MPV¶: video MPEG1 or MPEG2 elementary

enumerator PJMEDIA_RTP_PT_MP2T¶: video MPEG2 transport

enumerator PJMEDIA_RTP_PT_H263¶: video H263

enumerator PJMEDIA_RTP_PT_DYNAMIC¶: start of dynamic RTP payload

enum pjmedia_codec_priority¶

Specify these values to set the codec priority, by calling pjmedia_codec_mgr_set_codec_priority().

Values:

enumerator PJMEDIA_CODEC_PRIO_HIGHEST¶: This priority makes the codec the highest in the order. The last codec specified with this priority will get the highest place in the order, and will change the priority of previously highest priority codec to NEXT_HIGHER.

enumerator PJMEDIA_CODEC_PRIO_NEXT_HIGHER¶: This priority will put the codec as the next codec after codecs with this same priority.

enumerator PJMEDIA_CODEC_PRIO_NORMAL¶: This is the initial codec priority when it is registered to codec manager by codec factory.

enumerator PJMEDIA_CODEC_PRIO_LOWEST¶: This priority makes the codec the lowest in the order. The last codec specified with this priority will be put in the last place in the order.

enumerator PJMEDIA_CODEC_PRIO_DISABLED¶: This priority will prevent the codec from being listed in the SDP created by media endpoint, thus should prevent the codec from being used in the sessions. However, the codec will still be listed by pjmedia_codec_mgr_enum_codecs() and other codec query functions.

Functions

pjmedia_codec_param *pjmedia_codec_param_clone(pj_pool_t *pool, const pjmedia_codec_param *src)¶

Duplicate codec parameter.

Parameters

pool – The pool.
src – The codec parameter to be duplicated.

Returns

Duplicated codec parameter.

pj_status_t pjmedia_codec_mgr_init(pjmedia_codec_mgr *mgr, pj_pool_factory *pf)¶

Initialize codec manager. Normally this function is called by pjmedia endpoint’s initialization code.

Parameters

mgr – Codec manager instance.
pf – Pool factory instance.

Returns

PJ_SUCCESS on success.

pj_status_t pjmedia_codec_mgr_destroy(pjmedia_codec_mgr *mgr)¶

Destroy codec manager. Normally this function is called by pjmedia endpoint’s deinitialization code.

Parameters: mgr – Codec manager instance.
Returns: PJ_SUCCESS on success.

pj_status_t pjmedia_codec_mgr_register_factory(pjmedia_codec_mgr *mgr, pjmedia_codec_factory *factory)¶

Register codec factory to codec manager. This will also register all supported codecs in the factory to the codec manager.

Parameters

mgr – The codec manager instance. Application can get the instance by calling pjmedia_endpt_get_codec_mgr().
factory – The codec factory to be registered.

Returns

PJ_SUCCESS on success.

pj_status_t pjmedia_codec_mgr_unregister_factory(pjmedia_codec_mgr *mgr, pjmedia_codec_factory *factory)¶

Unregister codec factory from the codec manager. This will also remove all the codecs registered by the codec factory from the codec manager’s list of supported codecs.

Parameters

mgr – The codec manager instance. Application can get the instance by calling pjmedia_endpt_get_codec_mgr().
factory – The codec factory to be unregistered.

Returns

PJ_SUCCESS on success.

pj_status_t pjmedia_codec_mgr_enum_codecs(pjmedia_codec_mgr *mgr, unsigned *count, pjmedia_codec_info info[], unsigned *prio)¶

Enumerate all supported codecs that have been registered to the codec manager by codec factories.

Parameters

mgr – The codec manager instance. Application can get the instance by calling pjmedia_endpt_get_codec_mgr().
count – On input, specifies the number of elements in the array. On output, the value will be set to the number of elements that have been initialized by this function.
info – The codec info array, which contents will be initialized upon return.
prio – Optional pointer to receive array of codec priorities.

Returns

PJ_SUCCESS on success.

pj_status_t pjmedia_codec_mgr_get_codec_info(pjmedia_codec_mgr *mgr, unsigned pt, const pjmedia_codec_info **inf)¶

Get codec info for the specified static payload type. Note that this can only find codec with static payload types. This function can be used to find codec info for a payload type inside SDP which doesn’t have the corresponding rtpmap attribute.

Parameters

mgr – The codec manager instance. Application can get the instance by calling pjmedia_endpt_get_codec_mgr().
pt – Static payload type/number.
inf – Pointer to receive codec info.

Returns

PJ_SUCCESS on success.

char *pjmedia_codec_info_to_id(const pjmedia_codec_info *info, char *id, unsigned max_len)¶

Convert codec info struct into a unique codec identifier. A codec identifier looks something like “L16/44100/2”.

Parameters

info – The codec info
id – Buffer to put the codec info string.
max_len – The length of the buffer.

Returns

The null terminated codec info string, or NULL if the buffer is not long enough.

pj_status_t pjmedia_codec_mgr_find_codecs_by_id(pjmedia_codec_mgr *mgr, const pj_str_t *codec_id, unsigned *count, const pjmedia_codec_info *p_info[], unsigned prio[])¶

Find codecs by the unique codec identifier. This function will find all codecs that match the codec identifier prefix. For example, if “L16” is specified, then it will find “L16/8000/1”, “L16/16000/1”, and so on, up to the maximum count specified in the argument.

Parameters

mgr – The codec manager instance. Application can get the instance by calling pjmedia_endpt_get_codec_mgr().
codec_id – The full codec ID or codec ID prefix. If an empty string is given, it will match all codecs.
count – Maximum number of codecs to find. On return, it contains the actual number of codecs found.
p_info – Array of pointer to codec info to be filled. This argument may be NULL, which in this case, only codec count will be returned.
prio – Optional array of codec priorities.

Returns

PJ_SUCCESS if at least one codec info is found.

pj_status_t pjmedia_codec_mgr_set_codec_priority(pjmedia_codec_mgr *mgr, const pj_str_t *codec_id, pj_uint8_t prio)¶

Set codec priority. The codec priority determines the order of the codec in the SDP created by the endpoint. If more than one codecs are found with the same codec_id prefix, then the function sets the priorities of all those codecs.

Parameters

mgr – The codec manager instance. Application can get the instance by calling pjmedia_endpt_get_codec_mgr().
codec_id – The full codec ID or codec ID prefix. If an empty string is given, it will match all codecs.
prio – Priority to be set. The priority can have any value between 1 to 255. When the priority is set to zero, the codec will be disabled.

Returns

PJ_SUCCESS if at least one codec info is found.

pj_status_t pjmedia_codec_mgr_get_default_param(pjmedia_codec_mgr *mgr, const pjmedia_codec_info *info, pjmedia_codec_param *param)¶

Get default codec param for the specified codec info.

Parameters

mgr – The codec manager instance. Application can get the instance by calling pjmedia_endpt_get_codec_mgr().
info – The codec info, which default parameter’s is being queried.
param – On return, will be filled with the default codec parameter.

Returns

PJ_SUCCESS on success.

pj_status_t pjmedia_codec_mgr_set_default_param(pjmedia_codec_mgr *mgr, const pjmedia_codec_info *info, const pjmedia_codec_param *param)¶

Set default codec param for the specified codec info.

Parameters

mgr – The codec manager instance. Application can get the instance by calling pjmedia_endpt_get_codec_mgr().
info – The codec info, which default parameter’s is being updated.
param – The new default codec parameter. Set to NULL to reset codec parameter to library default settings.

Returns

PJ_SUCCESS on success.

pj_status_t pjmedia_codec_mgr_alloc_codec(pjmedia_codec_mgr *mgr, const pjmedia_codec_info *info, pjmedia_codec **p_codec)¶

Request the codec manager to allocate one instance of codec with the specified codec info. The codec will enumerate all codec factories until it finds factory that is able to create the specified codec.

Parameters

mgr – The codec manager instance. Application can get the instance by calling pjmedia_endpt_get_codec_mgr().
info – The information about the codec to be created.
p_codec – Pointer to receive the codec instance.

Returns

PJ_SUCCESS on success.

pj_status_t pjmedia_codec_mgr_dealloc_codec(pjmedia_codec_mgr *mgr, pjmedia_codec *codec)¶

Deallocate the specified codec instance. The codec manager will return the instance of the codec back to its factory.

Parameters

mgr – The codec manager instance. Application can get the instance by calling pjmedia_endpt_get_codec_mgr().
codec – The codec instance.

Returns

PJ_SUCESS on success.

struct pjmedia_codec_info¶: #include <codec.h>
Identification used to search for codec factory that supports specific codec specification.

struct pjmedia_codec_fmtp¶: #include <codec.h>
Structure of codec specific parameters which contains name=value pairs. The codec specific parameters are to be used with SDP according to the standards (e.g: RFC 3555) in SDP ‘a=fmtp’ attribute.

struct pjmedia_codec_param¶

#include <codec.h>

Detailed codec attributes used in configuring a codec and in querying the capability of codec factories. Default attributes of any codecs could be queried using pjmedia_codec_mgr_get_default_param() and modified using pjmedia_codec_mgr_set_default_param().

Please note that codec parameter also contains SDP specific setting, dec_fmtp and enc_fmtp, which may need to be set appropriately based on the effective setting. See each codec documentation for more detail.

struct pjmedia_codec_op¶: #include <codec.h>
This structure describes codec operations. Each codec MUST implement all of these functions.

struct pjmedia_codec¶: #include <codec.h>
This structure describes a codec instance.

struct pjmedia_codec_factory_op¶: #include <codec.h>
This structure describes operations that must be supported by codec factories.

struct pjmedia_codec_factory¶: #include <codec.h>
Codec factory describes a module that is able to create codec with specific capabilities. These capabilities can be queried by codec manager to create instances of codec.

struct pjmedia_codec_desc¶: #include <codec.h>
Codec manager maintains array of these structs for each supported codec.

struct pjmedia_codec_mgr¶: #include <codec.h>
The declaration for codec manager. Application doesn’t normally need to see this declaration, but nevertheless this declaration is needed by media endpoint to instantiate the codec manager.