4.2.1 获取格式内容

获取的方法

属性的键值

kAudioFormatProperty_ASBDFromESDS
kAudioFormatProperty_ChannelLayoutFromESDS
kAudioFormatProperty_OutputFormatList
kAudioFormatProperty_Encoders
kAudioFormatProperty_Decoders
kAudioFormatProperty_FormatIsVBR
kAudioFormatProperty_FormatIsExternallyFramed kAudioFormatProperty_AvailableEncodeBitRates
kAudioFormatProperty_AvailableEncodeSampleRates kAudioFormatProperty_AvailableEncodeChannelLayoutTags kAudioFormatProperty_AvailableEncodeNumberChannels
kAudioFormatProperty_ASBDFromMPEGPacket
||| kAudioFormatProperty_BitmapForLayoutTag
kAudioFormatProperty_MatrixMixMap
kAudioFormatProperty_ChannelMap
kAudioFormatProperty_NumberOfChannelsForLayout
kAudioFormatProperty_ValidateChannelLayout
kAudioFormatProperty_ChannelLayoutForTag
kAudioFormatProperty_TagForChannelLayout
kAudioFormatProperty_ChannelLayoutName
kAudioFormatProperty_ChannelLayoutSimpleName
kAudioFormatProperty_ChannelLayoutForBitmap
kAudioFormatProperty_ChannelName
kAudioFormatProperty_ChannelShortName
kAudioFormatProperty_TagsForNumberOfChannels
kAudioFormatProperty_PanningMatrix
kAudioFormatProperty_BalanceFade
||| kAudioFormatProperty_ID3TagSize
kAudioFormatProperty_ID3TagToDictionary

上面列举了各个属性的意义和使用方法，下面列举几个例子

kAudioFormatProperty_FormatInfo

通过Magic获得文件类型信息，首先要用"AudioFileGetProperty"获得Magic信息：

    stts = AudioFileGetPropertyInfo(musicFD_, kAudioFilePropertyMagicCookieData, &prprtySize, &prprtyWriteable);
VStatus(stts, @"AudioFileGetPropertyInfo: kAudioFilePropertyMagicCookieData");
void * magic = malloc(prprtySize);
if (NULL == magic) {
    NSLog(@"malloc magic is NULL");
    return ;
}
memset(magic, 0, prprtySize);
stts = AudioFileGetProperty(musicFD_, kAudioFilePropertyMagicCookieData, &prprtySize, magic);
VStatus(stts, @"AudioFileGetProperty: kAudioFilePropertyMagicCookieData");
NSLog(@"get magic with length %d", prprtySize);
AudioStreamBasicDescription desc;
UInt32 descSize = sizeof(desc);
stts = AudioFormatGetProperty(kAudioFormatProperty_FormatInfo, prprtySize, magic, &descSize, &desc);
VStatus(stts, @"kAudioFormatProperty_FormatInfo");
NSLog(@"mFormat is %d", desc.mFormatID);

然后传入Magic并从结果AudioStreamBasicDescription获得表示文件格式的AudioFormatID值。

kAudioFormatProperty_FormatName

获得AudioStreamBasicDescription中表示的格式信息:

  AudioStreamBasicDescription desc;
UInt32 descSize = sizeof(desc);
stts = AudioFileGetProperty(musicFD_, kAudioFilePropertyDataFormat, &descSize, &desc);
VStatus(stts, @"AudioFileGetProperty: kAudioFilePropertyDataFormat");
NSLog(@"get data with length %d", descSize);
CFStringRef formatName;
UInt32 formatNameSize = sizeof(formatName);
stts = AudioFormatGetProperty(kAudioFormatProperty_FormatName, descSize, &desc, &formatNameSize, &formatName);
VStatus(stts, @"kAudioFormatProperty_FormatInfo");
NSLog(@"Format is %@", formatName);

这里首先通过AudioFileGetProperty读取文件的kAudioFilePropertyDataFormat然后算得其格式名称，结果如“MPEG Layer 3, 2 channels, 44100 Hz”

kAudioFormatProperty_FormatList

这里首先要介绍两个数据结构：

struct AudioFormatListItem
{
    AudioStreamBasicDescription mASBD;
    AudioChannelLayoutTag          mChannelLayoutTag;
};
typedef struct AudioFormatListItem AudioFormatListItem;

struct AudioFormatInfo
{
    AudioStreamBasicDescription   mASBD;
    const void*                            mMagicCookie;
    UInt32                                    mMagicCookieSize;
};
typedef struct AudioFormatInfo AudioFormatInfo;

一个AudioFormatInfo一个AudioFormatListItem，前者是一个AudioStreamBasicDescription加上Magic信息，后者是AudioStreamBasicDescription加上声道信息。

在使用时，首先要要获得一个Magic Cookie值，比如用通过AudioFileGetProperty，但是大部分音频数据是没有Magic Cookie。这个属性的作用就是从Magic Cookie里面获取各个部分的音频数据，并存放在AudioFormatListItem的list里面。

首先用Magic填充一个AudioFormatInfo，然后获取List的长度：

finfo.mASBD = desc;
finfo.mMagicCookie = magic;
finfo.mMagicCookieSize = prprtySize;

UInt32 finfoSize = sizeof(finfo);
UInt32 finfosSize  = 0;
stts = AudioFormatGetPropertyInfo(kAudioFormatProperty_FormatList, sizeof(finfo), &finfo, &finfosSize);
VStatus(stts, @"AudioFormatGetPropertyInfo:kAudioFormatProperty_FormatList");
size_t itemCount = finfosSize / sizeof(AudioFormatListItem);
AudioFormatListItem *finfos = (AudioFormatListItem *) malloc(finfosSize);

然后在获取每个Item并遍历结果：

stts = AudioFormatGetProperty(kAudioFormatProperty_FormatList, finfoSize, &finfo, &finfosSize, finfos);
VStatus(stts, @"AudioFormatGetProperty: kAudioFormatProperty_FormatList");
for (int i=0; i< itemCount; i++) {
    AudioFormatListItem *item = finfos + i ;
    NSLog(@"channel layout tag is %d", item->mChannelLayoutTag);
}

内存释放需要自己手动处理下。

参考

1. Audio Format Services Reference