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Media Server

[English|简体中文]

Overview

The Media Server provides a comprehensive set of functions. Based on an event loop model, it handles various media events and client interactions, offering functions such as audio and video playback, audio and video recording, focus management, policy execution, and session control.

Project Directory

.
├── focus_stack.c
├── focus_stack.h
├── media_daemon.c
├── media_focus.c
├── media_graph.c
├── media_policy.c
├── media_server.c
├── media_server.h
├── media_session.c
├── media_stub.c
└── README.md

Module Introduction

Media Daemon

Media Daemon is the core of Media Server, responsible for creating and managing Media modules, such as Media Focus, Media Graph, Media Session, Media Policy, etc. The core principle of the Media Daemon is to use the poll function to listen to the RPC socket fdand the message queue fd registered by the audio/video device driver, to process the RPC commands and trigger the work of FFmpeg.

Media Daemon架构图

The main work of Media Daemon is carried out in a loop, the general steps are as follows:

  • Initialize the creation of each module instance, such as Media Focus, etc.
  • Get event fd through the media_get_pollfds interface to obtain the event fd of each module.
  • Use poll to block and wait for the event. When an event is detected, poll returns.
  • Handle the event by calling media_poll_available when poll returns.
  • Execute run once (mainly ffmpeg) to ensure effective resource management and scheduling.

Media Focus

The Media Focus module is an important part of the Media Server, designed to provide playback policies for scenarios where multiple audio streams are mixed together. It helps to ensure that only one audio stream is played as the primary audio content at a time, while other audio streams are either downgraded to secondary streams or paused. The media focus mechanism is a cooperative preemption type, allowing applications to still play music without using media focus, but they cannot access the audio focus management system, which may impact user experience.

  • The configuration file for the default sound event type interaction is located in /etc/media.
  • The input of sound event types is based on the different MEDIA_SCENARIO_XXX macros in the media wrapper. Currently, it contains 11 types of sound events.
  • Supports application-initiated focus request, dropping focus request, focus change notification, etc.

Media Garph

The principle of the Media Graph is to link the inputs and outputs of audio and video related filters together to form a playback and recording chain. The main strategies are as follows:

  • Load the graph configuration file to create and configure the media graph and corresponding filters.
  • Provide a series of functions to handle the commands and events of filters, including opening, closing, playing, pausing, stopping, setting event callbacks, handling command queues, and other operations.
  • Encapsulate the operation interfaces of Media Player and Media Recorder and call the FFmpeg library to realize playback and recording functions.

Media Policy

The Media Policy module on the Server side provides a series of functions to handle operations such as setting, obtaining and notification of media policies, and provides a unified interface to APP in different projects to transform the user's routing and volume control information into control commands for the device driver; in different projects, Policy will use different configuration files to handle the Policy interfaces for mapping control commands. Media Policy's policies are implemented through configuration files:

  • Modify ffmpeg filter graph configuration file to control the filter of graph through policy, for volume and link control.
  • Write pfw configuration file to realize plugin extension and so on.

Media Server

The Media server module listens to different types of sockets, receives connection requests from clients, and uses callback functions to process the received data. The following features are supported:

  • Create server instances, destroys server instances.
  • The media_server_get_pollfds interface gets a list of file descriptors for polling.
  • The media_server_poll_available handles file descriptor events.
  • The media_server_notify sends notifications to specific connections and manages connection data, etc.

Media Session

The media session module on the server side plays a crucial role in the media framework. Through the designed controller and controlled architecture, it realizes efficient control of media playback and accurate status notifications.

  • Controllers: Only wants to control other streams, receive status change notifications, or query information, and is not responsible for the creation and destruction of streams.
  • Controlled entity: Holds the playback status of certain streams and needs to be responsible for the creation, destruction, and playback functions of these streams. At the same time, it needs to update its own status information in a timely manner.
  • For example, when a speaker plays music from a mobile phone:
    • Controller: The UI interface is the controller.
    • Controlled entity: The Bluetooth module that establishes an audio channel with the mobile phone is the controlled entity.