iOS音頻開發之`AudioStreamBasicDescription`

這個類提供了對於音頻文件的描述

• An audio stream is a continuous series of data that represents a sound, such as a song.

• A channel is a discrete track of monophonic audio. A monophonic stream has one channel; a stereo stream has two channels.

• A sample is single numerical value for a single audio channel in an audio stream.

• A frame is a collection of time-coincident samples. For instance, a linear PCM stereo sound file has two samples per frame, one for the left channel and one for the right channel.

• A packet is a collection of one or more contiguous frames. A packet defines the smallest meaningful set of frames for a given audio data format, and is the smallest data unit for which time can be measured. In linear PCM audio, a packet holds a single frame. In compressed formats, it typically holds more; in some formats, the number of frames per packet varies.

• The sample rate for a stream is the number of frames per second of uncompressed (or, for compressed formats, the equivalent in decompressed) audio.

首先，音頻文件的產生是模擬信號->PCM以後的數字信號->壓縮、編碼以後的音頻文件。
PCM時採樣頻率叫做sample rate。
每一次採樣可以得到若幹採樣數據，對應多個channel。
每一個採樣點得到的若幹採樣數據組合起來，叫做一個frame。
若幹frame組合起來叫做一個packet。
mSampleRate，就是採用頻率
mBitsPerChannel，就是每個採樣數據的位數
mChannelsPerFrame,可以理解為聲道數，也就是一個採樣時刻產生幾個採樣數據。
mFramesPerPacket，就是每個packet的中frame的個數，等於這個packet中經歷了幾次採樣間隔。
mBytesPerPacket，每個packet中數據的位元組數。
mBytesPerFrame，每個frame中數據的位元組數

計算公式

1. 計算每個packet的持續時間

   duration = (1 / mSampleRate) * mFramesPerPacket
   

2. 計算mBitsPerChannel
   對於kAudioFormatFlagsCanonical的PCM數據，有以下公式。大概每個採樣數據的位元組數用AudioSampleType描述。

   mBitsPerChannel = 8 * sizeof (AudioSampleType);
   

3. 計算mBytesPerFrame

   mBytesPerFrame = n * sizeof (AudioSampleType);
   

   其中n是聲道數目