Group PJMED_SND

group PJMED_SND

PJMEDIA abstraction for sound device hardware.

Warning: this sound device API has been deprecated and replaced by PJMEDIA Audio Device API. Please see for more information.

This section describes lower level abstraction for sound device hardware. Application normally uses the higher layer Sound Device Port abstraction since it works seamlessly with Media Ports Framework.

The sound hardware abstraction basically runs asychronously, and application must register callbacks to be called to receive/ supply audio frames from/to the sound hardware.

A full duplex sound stream (created with pjmedia_snd_open()) requires application to supply two callbacks:

callback to be called when it has finished capturing one media frame, and
callback to be called when it needs media frame to be played to the sound playback hardware.

Half duplex sound stream (created with pjmedia_snd_open_rec() or pjmedia_snd_open_player()) will only need one of the callback to be specified.

After sound stream is created, application need to call pjmedia_snd_stream_start() to start capturing/playing back media frames from/to the sound device.

Typedefs

typedef struct pjmedia_snd_stream pjmedia_snd_stream: Opaque declaration for pjmedia_snd_stream.

typedef pj_status_t (*pjmedia_snd_play_cb)(void *user_data, pj_uint32_t timestamp, void *output, unsigned size)

This callback is called by player stream when it needs additional data to be played by the device. Application must fill in the whole of output buffer with sound samples.

Param user_data:: User data associated with the stream.
Param timestamp:: Timestamp, in samples.
Param output:: Buffer to be filled out by application.
Param size:: The size requested in bytes, which will be equal to the size of one whole packet.
Return:: Non-zero to stop the stream.

typedef pj_status_t (*pjmedia_snd_rec_cb)(void *user_data, pj_uint32_t timestamp, void *input, unsigned size)

This callback is called by recorder stream when it has captured the whole packet worth of audio samples.

Param user_data:: User data associated with the stream.
Param timestamp:: Timestamp, in samples.
Param input:: Buffer containing the captured audio samples.
Param size:: The size of the data in the buffer, in bytes.
Return:: Non-zero to stop the stream.

Functions

pj_status_t pjmedia_snd_init(pj_pool_factory *factory)

Init the sound library.

Parameters:: factory – The sound factory.
Returns:: Zero on success.

int pjmedia_snd_get_dev_count(void)

Get the number of devices detected by the library.

Returns:: Number of devices.

const pjmedia_snd_dev_info *pjmedia_snd_get_dev_info(unsigned index)

Get device info.

Parameters:: index – The index of the device, which should be in the range from zero to pjmedia_snd_get_dev_count - 1.

pj_status_t pjmedia_snd_set_latency(unsigned input_latency, unsigned output_latency)

Set sound device latency, this function must be called before sound device opened, or otherwise default latency setting will be used,

Choosing latency value is not straightforward, it should accomodate both minimum latency and stability. Lower latency tends to cause sound device less reliable (producing audio dropouts) on CPU load disturbance. Moreover, the best latency setting may vary based on many aspects, e.g: sound card, CPU, OS, kernel, etc.