此队列属性(iOS音频队列)的含义是什么

Question

我想写一个播放器来播放音乐。我看到的代码如下：

AudioFileGetPropertyInfo(audioFile,
                         kAudioFilePropertyMagicCookieData, &size, nil);
if (size > 0) {
    cookie = malloc(sizeof(char) * size);
    AudioFileGetProperty(audioFile,
                         kAudioFilePropertyMagicCookieData, &size, cookie);
    AudioQueueSetProperty(aduioQueue,
                          kAudioQueueProperty_MagicCookie, cookie, size);
    free(cookie);
}

我不知道为什么要设置AudioQueueProperty，kAudioQueQueueProperty_MagicCookie的含义是什么？我在文档中找不到帮助。谁能为解决这个问题指明方向。

Answer 1

实际上，魔术cookie不仅仅是一个签名，它保存了一些关于编码器的信息，最有用的项目是"最大比特率"one_answers"平均比特率"，特别是对于像AudioFileMPEG4Type这样的压缩格式。对于这种特定类型的魔术cookie与MPEG-4数据文件中的"esds"框相同。您可以在以下位置找到确切的位设置：

http://xhelmboyx.tripod.com/formats/mp4-layout.txt

---

8字节以上。2 ES描述符框=长的无符号偏移量+长的ASCII文本字符串"esds"-如果按照ISO/IEC 14496-10 AVC标准编码，则可选地使用：=长无符号偏移量+长ASCII文本字符串"m4ds"

                -> 4 bytes version/flags = 8-bit hex version + 24-bit hex flags
                    (current = 0)
                -> 1 byte ES descriptor type tag = 8-bit hex value 0x03
                -> 3 bytes extended descriptor type tag string = 3 * 8-bit hex value
                  - types are Start = 0x80 ; End = 0xFE
                  - NOTE: the extended start tags may be left out
                -> 1 byte descriptor type length = 8-bit unsigned length
                  -> 2 bytes ES ID = 16-bit unsigned value
                  -> 1 byte stream priority = 8-bit unsigned value
                    - Defaults to 16 and ranges from 0 through to 31
                    -> 1 byte decoder config descriptor type tag = 8-bit hex value 0x04
                    -> 3 bytes extended descriptor type tag string = 3 * 8-bit hex value
                      - types are Start = 0x80 ; End = 0xFE
                      - NOTE: the extended start tags may be left out
                    -> 1 byte descriptor type length = 8-bit unsigned length
                      -> 1 byte object type ID = 8-bit unsigned value
                        - type IDs are system v1 = 1 ; system v2 = 2
                        - type IDs are MPEG-4 video = 32 ; MPEG-4 AVC SPS = 33
                        - type IDs are MPEG-4 AVC PPS = 34 ; MPEG-4 audio = 64
                        - type IDs are MPEG-2 simple video = 96
                        - type IDs are MPEG-2 main video = 97
                        - type IDs are MPEG-2 SNR video = 98
                        - type IDs are MPEG-2 spatial video = 99
                        - type IDs are MPEG-2 high video = 100
                        - type IDs are MPEG-2 4:2:2 video = 101
                        - type IDs are MPEG-4 ADTS main = 102
                        - type IDs are MPEG-4 ADTS Low Complexity = 103
                        - type IDs are MPEG-4 ADTS Scalable Sampling Rate = 104
                        - type IDs are MPEG-2 ADTS = 105 ; MPEG-1 video = 106
                        - type IDs are MPEG-1 ADTS = 107 ; JPEG video = 108
                        - type IDs are private audio = 192 ; private video = 208
                        - type IDs are 16-bit PCM LE audio = 224 ; vorbis audio = 225
                        - type IDs are dolby v3 (AC3) audio = 226 ; alaw audio = 227
                        - type IDs are mulaw audio = 228 ; G723 ADPCM audio = 229
                        - type IDs are 16-bit PCM Big Endian audio = 230
                        - type IDs are Y'CbCr 4:2:0 (YV12) video = 240 ; H264 video = 241
                        - type IDs are H263 video = 242 ; H261 video = 243
                      -> 6 bits stream type = 3/4 byte hex value
                        - type IDs are object descript. = 1 ; clock ref. = 2
                        - type IDs are scene descript. = 4 ; visual = 4
                        - type IDs are audio = 5 ; MPEG-7 = 6 ; IPMP = 7
                        - type IDs are OCI = 8 ; MPEG Java = 9
                        - type IDs are user private = 32
                      -> 1 bit upstream flag = 1/8 byte hex value
                      -> 1 bit reserved flag = 1/8 byte hex value set to 1
                      -> 3 bytes buffer size = 24-bit unsigned value
                      -> 4 bytes maximum bit rate = 32-bit unsigned value
                      -> 4 bytes average bit rate = 32-bit unsigned value
                        -> 1 byte decoder specific descriptor type tag
                            = 8-bit hex value 0x05
                        -> 3 bytes extended descriptor type tag string
                            = 3 * 8-bit hex value
                          - types are Start = 0x80 ; End = 0xFE
                          - NOTE: the extended start tags may be left out
                        -> 1 byte descriptor type length
                            = 8-bit unsigned length
                          -> ES header start codes = hex dump
                    -> 1 byte SL config descriptor type tag = 8-bit hex value 0x06
                    -> 3 bytes extended descriptor type tag string = 3 * 8-bit hex value
                      - types are Start = 0x80 ; End = 0xFE
                      - NOTE: the extended start tags may be left out
                    -> 1 byte descriptor type length = 8-bit unsigned length
                      -> 1 byte SL value = 8-bit hex value set to 0x02

"

来自kAudioFilePropertyMagicCookieData的MagicCookie从ES描述符开始（只需忽略映射中描述的前4个字节，其余字节将与magick Cookie完全匹配）。

一个神奇饼干的例子是这样的：

03 80 80 80 22 00 00 00 04 80 80 80 14 40 15 00 18 00 00 00 FA 00 00 00 FA 00 05 80 80 80 02 12 08 06 80 80 80 01 02

最大比特率为偏移量18->0XFA00（或64000）平均比特率在偏移22->0XFA00（或64000）处

尽管根据苹果的文档，magic cookie是读/写的，但在创建或转换文件之前，我没有机会更改比特率。

希望这能帮助到别人。

Answer 2

"magic cookie"是一种文件类型签名，由文件开头的唯一字节序列组成，表示文件格式。音频队列框架使用该信息来确定如何从文件流中解码或提取音频信息（而不是使用或信任文件扩展名）。您发布的代码从文件中读取这组字节，并将其作为cookie传递到音频队列。（例如，让它们被解释为PCM样本是错误的）。