Windows audio architecture

1. Windows audio architecture • Windows Driver Model • supported by Win 98, Win ME, Win 2K and Win XP • a single audio driver works for multiple Windows versions • APIs • DirectSound • WinMM • Kernel streaming • multiple audio streams can be played at the same time • SysAudio.SYS decides the optimal audio format and sample rate conversion • Kmixer.SYS performs the actual converting Win MM Application DirectSound Application WinMM.DLL DSound.DLL User Mode Kernel Mode SysAudio.SYS Kmixer.SYS Device Drive Container Windows Components By Hardware Vendor ISA CARD Driver PCI CARD Driver USB Device Driver IEEE 1394 Device Driver

2. WinMM API • Simple, but • high latency • inability to take advantage of hardware acceleration • no easy way to implement features, e.g. 3-D positioning, effect processing • Play audio • waveOutOpen(…) - open the output audio device • waveOutWrite(…) - write the waveform audio data • waveOutClose(…) - close the output audio device • need to use callback or polling to check the result • Not very interesting to real-time applications

3. DirectSound API - over view • Audio component of DirectX package • low latency • use hardware acceleration • direct access to sound device • support capturing sound • Two programming interfaces • COM (Component Object Model) in C++ • .NET in C++, C#, Visual Basic, etc. • Important objects • secondary buffers: write/read audio data • buffer cursors: point to current captured/played audio data • buffer notifications: send events when buffer cursors reach a position

4. DirectSound API - COM interfaces • IDirectSound8 • CreateSoundBuffer(descriptor, bufferPointer, …) • create a sound buffer object to manage audio samples • fields of descriptor • buffer size • audio format: commonly16 bits linear PCM • buffer features • SetCooperativeLevel(windowHandle, level) • set the priority of the sound buffer

5. DirectSound API - COM interfaces • IDirectSoundBuffer8 • Lock(offset, size, addr1, size1, addr2, size2, flag) • ready all or part of the buffer for a data write and return pointers to which data can be written • Play(reserved, priority, flags) • cause the sound buffer to play, starting from the play cursor • Unlock(addr1, size1, addr2, size2) • release a locked sound buffer • Stop() • cause the sound buffer to stop playing

6. DirectSound API - COM interfaces • IDirectSoundNotify8 • SetNotificationPositions(NumberOfNotifyStructure, ArrayofNotifyStructure) • set the notification positions; during playback, whenever the play cursor reaches one of the specified offsets, the associated event is signaled • fields of NotifyStructure • buffer offset • notify event • Sound capturing is similar

7. DirectSound API - code example • Streaming audio in an event-driven thread while (true) { DWORD r = WaitForSingleObject(event, INFINITE); // receives notification of refilling buffer if (r == WAIT_OBJECT_0) { Buffer.Lock(offset, size, &addr1, &size1, &addr2, &size2, 0); // copy audio to buffer addresses returned // by DirectSound // could be two addresses because of buffer // wrap-around memcpy(addr1, audio, size1); if (size2 != 0) { memcpy(addr2, left, size2); } Buffer.Unlock(addr1, size1, addr2, size2); } } // while

8. Windows audio architecture revisited • Can we achieve lower latency? • kernel mixing introduces at least 30 ms of delay • kernel mixing is not necessary if I’m the only application generating audio streams • How about interacting with device drivers directly? Win MM Application DirectSound Application WinMM.DLL DSound.DLL User Mode Kernel Mode SysAudio.SYS Kmixer.SYS Device Drive Container Windows Components By Hardware Vendor ISA CARD Driver PCI CARD Driver USB Device Driver IEEE 1394 Device Driver

9. DirectKS - the unofficial audio API • Pros • very low latency • Cons • only one application can play sound at one time • applications need to handle audio format and sample rate conversion • might not work in future version of Windows Win MM Application DirectSound Application DirectKS Application WinMM.DLL DSound.DLL DirectKS User Mode Kernel Mode SysAudio.SYS Kmixer.SYS Device Drive Container Windows Components By Hardware Vendor ISA CARD Driver PCI CARD Driver USB Device Driver IEEE 1394 Device Driver

10. The next-generation Windows audio • None of the current audio interfaces satisfies real-time applications • transition between user mode and kernel mode for each I/O request • blocking upon completion of an I/O request • CPU cycles for copying data • WaveRT (wave real-time) drivers in the next version of Windows - “Longhorn” • data flow directly between the client and the audio hardware

11. Learn more • URLs • overview • http://msdn.microsoft.com/library/default.asp?url=/library/en-us/dnwmt/html/audiooverview.asp • Windows Driver Model (WDM) • http://www.microsoft.com/whdc/hwdev/tech/audio/wdmaudio.mspx#wdm1 • DirectKS • http://www.microsoft.com/whdc/hwdev/tech/audio/DirectKS.mspx • WaveRT • http://www.microsoft.com/whdc/hwdev/tech/audio/WaveRTport.mspx

12. Audio library overview • Transmit audio over the internet • use low latency audio APIs • DirectSound or DirectKS • pluggable codecs • G.711, GSM, Speex, iLBC • modular playout buffer • integrated with rtplib++ • System Requirements • Windows XP or Windows 2K • DirectSound 9.x runtime libraries • Visual C++ runtime libraries

13. Audio library architecture SIP user agent SIP user agent Audio tool GUI Audio tool GUI DirectSound/DirectKS DirectSound/DirectKS Playout buffer Playout buffer Encoder Decoder Decoder Encoder Rtplib++ Rtplib++ Socket Socket Network

14. Audio library API • Initialization • setUserName(name) • set the local user name • setRemoteAddress(host/IP, port) • send audio to this address • setLocalAddress(host/IP, port) • receive audio from this address • setPlayerAudioFormat(audioFormat) • play audio in this format • setCapturerAudioFormat(audioFormat) • capture audio in this format

15. Audio library API • Initialization (Cont.) • setEncoder(encoder) • use this encoder to encode audio • encoder can be created by • encoder = SpeexEncoder - create a Speex encoder instance • encoder.setPayloadType(payLoadType) - set RTP payload type • encoder.setOutputAudioFormat(audioFormat) - set the encoded format • setDecoder(decoder) • … (similar to encoder) • Start • startReceiver()/startSender() • start to receive/send audio

16. Audio library delay • One-way mouth-to-ear delay measurement of audio library using DirectSound and DirectKS • DirectKS shows close to 30 ms improvement over DirectSound