Enable Audio Proxy Server (APS) in PJSIP

Introduction

You can read about APS ​here. The benefits of enabling APS in PJSIP are:

• Low latency. As mentioned somewhere that PCM frame size in some audio layer is about 4KB (4KB/2/8000Hz * 1000ms = 250ms), this seems to be the source of high latency audio in Symbian, while using APS it is possible to record/play encoded audio frames with latency only about 30ms!
• Ability to switch audio-routing (to earpiece or loudspeaker). Using standard API (CMdaAudioOutputStream) the audio is routed to loudspeaker which may not be prefered in a private conversation. While routing audio to earpiece can also avoid echo.

Requirements

To build PJSIP with APS support, you will need:

1. APS release 2.4.3 that is packaged in ​SDK API Plug-in, e.g:
   • for ​MR
   • for ​FP1
   • not sure for FP2, is it included in the FP2 SDK?
2. You may also need ​Extensions plug-in for S60 3rd Edition SDK for Symbian OS, for C++, MR to get sounddevice.h.

Building Steps

Here are a few steps to build PJSIP with APS support:

1. Edit bld.inf and application .mmp files (e.g: symbian_ua.mmp, symbian_ua_gui.mmp, symsndtest.mmp), please make sure SND_USE_APS is set:

   #define SND_USE_NULL	0
   #define SND_USE_APS	1
   

2. If you are using Carbide, please refresh the project first (right click bld.inf from the Symbian Project Navigator pane, and select Refresh). symbian_audio_aps.mmp should be there after the refresh done.
3. Clean and rebuild the whole project, please note that WINSCW target doesn't seem to support APS.
4. Make the installer (.sis file) for the application.

Installation

1. Install APS Server apsserver2.sisx to device, the installer comes with the APS SDK package i.e in folder InstallToDevice/AudioProxyServer243. You will need to do this only once for each device.
2. Sign the application installer with APS support with Developer Certificate using Open Signed Offline/with Publisher ID. (APS requires MultimediaDD capability and this capability is among the restricted capabilities). Please see ​Symbian Signed for more detail.
3. Install the application to the device.

Developing Application with APS Support

There are some notes for developing application with APS support:

• Currently, frame time/PTIME supported by sound device with APS is only 20ms. For example if your application is using pjsua, you will need to specify something like this in pjsua config:

  med_cfg.audio_frame_ptime = 20;
  

• Specify application UID in a global variable named APP_UID whose base type TPtrC, e.g:

  TPtrC APP_UID = _L("2000521C");
  

Limitation

• As mentioned before, sound device will only support 20ms frame time.
• Currently, this APS wrapper can only work in S60 3rd edition device that supports G.711, which perhaps (almost) all S60 3rd edition devices.