音频基础
- 声波是一种机械波，是一种模拟信号。
- PCM，全称脉冲编码调制，是一种模拟信号的数字化的方法。
- 采样精度（bit pre sample)，每个声音样本的采样位数。
- 采样频率（sample rate）每秒钟采集多少个声音样本。
- 声道（channel）：相互独立的音频信号数，单声道（mono）立体声（Stereo）
- 语音帧（frame），In audio data a frame is one sample across all channels.
iOS 开发中对应的结构
AudioStreamBasicDescription 结构体
struct AudioStreamBasicDescription
{
Float64 mSampleRate;
AudioFormatID mFormatID;
AudioFormatFlags mFormatFlags;
UInt32 mBytesPerPacket;
UInt32 mFramesPerPacket;
UInt32 mBytesPerFrame;
UInt32 mChannelsPerFrame;
UInt32 mBitsPerChannel;
UInt32 mReserved;
};
mSampleRate：The number of frames per second of the data in the stream.
mFormatID:An identifier specifying the general audio data format in the stream. (kAudioFormatLinearPCM, etc.)
mFormatFlags:Format-specific flags to specify details of the format.(kLinearPCMFormatFlagIsSignedInteger, kLinearPCMFormatFlagIsFloat, kLinearPCMFormatFlagIsBigEndian, kLinearPCMFormatFlagIsPacked, kLinearPCMFormatFlagIsNonInterleaved, etc.)
mBytesPerPacket:The number of bytes in a packet of data
mFramesPerPacket:The number of sample frames in each packet of data.
mBytesPerFrame:The number of bytes in a single sample frame of data.
mChannelsPerFrame:The number of channels in each frame of data
mBitsPerChannel:The number of bits of sample data for each channel in a frame of data.
数据样例
(AudioStreamBasicDescription) _format = {
mSampleRate = 44100
mFormatID = kAudioFormatLinearPCM
mFormatFlags = kAudioFormatFlagIsSignedInteger | kAudioFormatFlagIsPacked
mBytesPerPacket = 1
mFramesPerPacket = 1152
mBytesPerFrame = 0
mChannelsPerFrame = 2
mBitsPerChannel = 0
mReserved = 0
}
iOS音频框架
苹果提供的处理音频的系统框架
- 如果需求是播放一段音频（File、Buffer或URL），AVAudioPlayer通常都可以满足需求。
- 如果对性能有要求，或者是播放流式Buffer，那就得用到Audio Queue Services，Audio Queue播放是以Packet形式。
- Extended Audio File Services有一些API，可用帮你计算每个包的大小；
- Audio File Service或Audio File Stream Services更智能，你只管塞数据给它，它自动把包组好给你。
- 在ffmpeg软解播放器中可以使用Auido Queue实现音频播放。
