ios - 这个队列属性(iOS 音频队列)是什么意思？

Question

我想写一个播放器来播放音乐。我看到如下代码:

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);
}

不知道为什么要设置AudioQueueProperty，kAudioQueueProperty_MagicCookie是什么意思？我无法从文档中找到帮助。 谁能给出解决问题的方向。

Answer 1

实际上magic cookie不仅仅是一个签名，它包含一些关于编码器的信息，最有用的项目是“最大比特率”和“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 magic 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+ 字节版本。 2 ES描述符框 = 长无符号偏移量 + 长 ASCII 文本字符串 'esds' - 如果编码为 ISO/IEC 14496-10 AVC 标准，则可选择使用: = 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 Magic Cookie 从 ES Descriptor 开始(只需忽略映射中描述的前 4 个字节，其余部分将与 magick cookie 完全匹配)。

示例魔术 cookies 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

最大比特率位于偏移量 18 -> 0XFA00(或 64,000) 平均比特率位于偏移量 22 -> 0XFA00(或 64,000)

虽然根据 Apple 文档，magic cookie 是可读/写的，但我没有机会在创建或转换文件之前更改比特率。

希望对某人有所帮助。