iPhone/iPad开发札记2012/03/12 Audio Unit

1. MixerHost 

* AudioBufferList 

Declaration: struct AudioBufferList {
   UInt32      mNumberBuffers;
   AudioBuffer mBuffers[1];
};

Q:WHY AudioBuffer[1]? WHY NOT AudioBuffer *? seems the same ?

注：参见此文对指针与数据区别的分析：http://wenku.baidu.com/view/93fc57202f60ddccda38a0b5.html

mBuffers只是标明一个buffer array的起始地址，具体空间大小由mNumberBuffers决定。

另外，在本程序里，mNumberBuffers为1/2，分别表示mono/stereo，而每个buffer本身只有一个channel

* AudioBuffer 

struct AudioBuffer {
   UInt32 mNumberChannels;
   UInt32 mDataByteSize;
   void   *mData;
};
typedef struct AudioBuffer AudioBuffer;

Q:  跟踪发现，mNumberChannels总是1，什么原因？是否因为right channel的data在另一个buffer中？和interleaved及non-interleaved之间有什么关系？

   A: 此struct用于多种场景，某些情况下在一个buffer中会有多个channel的数据，有些情况下只有一个channel的数据。与AudioBufferList配合使用。

* InputRenderCallback()

//    Audio unit input render callbacks are invoked on a realtime priority thread (the highest 

//    priority on the system). To work well, to not make the system unresponsive, and to avoid 

//    audio artifacts, a render callback must not:

//

//        * allocate memory

//        * access the file system or a network connection

//        * take locks

//        * waste time

//

//    In addition, it's usually best to avoid sending Objective-C messages in a render callback.

//

2. My own Audio Unit code in conference project 

* inside InputRenderCallback()

// get audio data from audio engine 

readAudioData(buffer, length)           // length should be 1280=320*4 bytes 

// fill the buffer handed to me 

    // Fill the buffer or buffers pointed at by *ioData with the requested number of samples 

   //    of audio from the sound stored in memory.

    for (UInt32 frameNumber = 0; frameNumber < inNumberFrames; ++frameNumber) {

        outSamplesChannelLeft[frameNumber]                 = dataInLeft[sampleNumber];

    }

3. 调试Conference程序使用G711

* playback 

errors as follows: 

(gdb) continue

2012-03-12 18:03:26.217 test555[1369:207] AQBufferCallback...

2012-03-12 18:03:27.395 test555[1369:207] audio data ready

2012-03-12 18:03:27.395 test555[1369:207] AQBufferCallback...

[Switching to process 1369]

Program received signal:  “EXC_BAD_ACCESS”.

other erros: 

2012-03-12 18:16:03.056 test555[1475:207] AQBufferCallback...

mi_cmd_exec_interrupt: Inferior not executing.

mi_cmd_exec_interrupt: Inferior not executing.

(gdb) continue

(gdb) continue

2012-03-12 18:16:03.976 test555[1475:207] audio data ready

test555(1475,0xb024f000) malloc: *** error for object 0x551bde4: incorrect checksum for freed object - object was probably modified after being freed.

*** set a breakpoint in malloc_error_break to debug

Program received signal:  “SIGABRT”.

(gdb) continue

Program received signal:  “SIGABRT”.