Symbian OS audio and architecture

The Audio Subsystem in the Symbian OS phone mainly contains two types of independent audio data streams. One is telephone sound data, and the other is multimedia data. These two crucial use cases require sound quality and the ability to talk for a long time. A Digital Audio bus dedicated to audio data is used to meet these requirements. The original hardware audio format actually used on the Symbian OS phone is 16-bit pulse encoding modulation (PCM) data. The sound quality range is between 8 kHz for single-channel calls and 48 khz for audio playback. PCM audio hardware can be very simple and requires few settings to ensure the volume and select the correct output path. Then all the work is done to fill the data into the hardware at the required rate-the DMA hardware is easy to complete this task. If there is a delay in data transmission, the audio hardware will immediately produce jitters, non-consecutive overlaps, and other phenomena. 2.9.1 telephone audio Telephone sound data is the most basic element in a call. There are still strict restrictions on sound latency on the basis of solving the lag problem caused by long-distance satellite transmission that gives users high-quality calls. To ensure this, the system designer optimized the control software and hardware path for low latency response and low power consumption during calls. BP contains a DSP that does not process audio in the audio band through Symbian OS. During a call, Symbian OS will run in low-power mode and will only be awakened when the monitor needs to be updated. A normal call ends in a analog audio circuit. This circuit contains a converter from analog to digital and from digital to analog, which is connected to the microphone and speaker. When a Bluetooth (BT) headset is used, PCM data is directly transmitted to the BT module through its own special interface. Symbian OS requires an additional audio path to merge the system sound into the ongoing call sound. This is to deal with such things as text message prompts, low power, and second call. Transmit the original audio data sent by connecting to the BP through IPC to complete the Sound fusion. In the BP, the DSP blends the system sound into the audio stream. 2.9.2 multimedia audio Multimedia audio is a common part of the sound generated in addition to voice data in the system. The main multimedia voices include: • Ringtones in multiple formats • SMS warning prompt • Alarm alarms and calendar prompts • Video Phone Number • MP3 playback • Sound recorded by the recorder • Video collection and Playback At the system level, the Symbian Multimedia Framework (MMF) controls media players, file formats, and plug-ins. The multimedia device framework (MDF) will contain multimedia digital signal codecs, which provide transmission services between PCM Data and devsound on the device driver layer. Video telephony (VT) is a special example, because in this case, real-time audio data is not transmitted through Symbian OS. The audio part of the phone will form a 64 KB/s data stream along with the video. The VT telephone system must separate the incoming data streams, decode the audio and video sections, and then play them synchronously. This work is done by dedicated hardware or DSP. It requires all 200 MHz arm CPUs to run multimedia digital signal codecs. The main complexity of audio systems is the increasing number of audio sources and receivers and the way they may be connected. For example, the current phone number includes multimedia headphones, speakers, Bluetooth and FM radio. If the hardware cannot mix and route all possible audio sources in the combination, problems may occur. Currently, some audio use cases are not compatible with each other, but require mutual interruption. ========================================================== ========================================================== ===== Note: This article is translated by bequan from Symbian press's Symbian OS internals: Real-Time Kernal programming book. Anyone who needs to repost or reference, please first contact bequan via email (a-pangu@163.com or symbianchn@gmail.com), can be reposted or referenced after obtaining permission, otherwise it is deemed as infringement. When reprinting or referencing, please indicate the above translation information in the article.

 

Commented by Vincent:

The calling process is roughly as follows: Client thread: the client app calls client utility. Controller thread: controller Plugin-> devsound, devvideo, devasr, devmidi

 

The Controller framekwork aims to provide multimedia in support for Symbian.
These plugins are called controller plugin, which can transmit multimedia data from the source to one or more sinks.
For example, the audio controller can output a file source to a speaker (speak sink) or save the microphone data source to a file.
Controller plugin generally supports one or more multimedia formats, such as WAV or MPEG4.
Plugin is managed through the ECOM framework, so users are allowed to install extensions.