在上一篇《Mac OS 下使用 you-get 下载音悦台 MV 、网易云音乐等》给大家介绍了如何下载MV、音乐等资源，但是出现了一些不常见的音频格式，导致在有些播放器上无法解码播放，所以在这篇文章中教大家一个利用 Apple 自带的工具 afconvert 做音频格式转换，当然如果有格式工厂等格式转换软件的话就没必要使用这种方法了，不过学习倒还是可以的，毕竟自己动手，丰衣足食嘛，哈哈哈?….

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afconvert 介绍

afconvert 是 Apple 提供的一个音频格式转换工具。

• 语法

      afconvert [option...] input_file [output_file]      Options may appear before or after the direct arguments. If output_file      is not specified, a name is generated programmatically and the file      is written into the same directory as input_file.      afconvert input_file [-o output_file [option...]]...      Output file options apply to the previous output_file.      Other options may appear anywhere.

• 一般选项

    { -d | --data } data_format[@sample_rate][/format_flags][#frames_per_packet]        [-][BE|LE]{F|[U]I}{8|16|24|32|64}          (PCM)            e.g.   BEI16   F32@44100        or a data format appropriate to file format (see -hf)        format_flags: hex digits, e.g. '80'        Frames per packet can be specified for some encoders, e.g.: samr#12        A format of "0" specifies the same format as the source file,            with packets copied exactly.        A format of "N" specifies the destination format should be the            native format of the lossless encoded source file (alac, FLAC only)    { -c | --channels } number_of_channels        add/remove channels without regard to order.    { -l | --channellayout } layout_tag        layout_tag: name of a constant from CoreAudioTypes.h          (prefix "kAudioChannelLayoutTag_" may be omitted)        if specified once, applies to output file; if twice, the first          applies to the input file, the second to the output file    { -b | --bitrate } total_bit_rate_bps         e.g. 256000 will give you roughly:             for stereo source: 128000 bits per channel             for 5.1 source: 51000 bits per channel                 (the .1 channel consumes few bits and can be discounted in the                 total bit rate calculation)    { -q | --quality } codec_quality        codec_quality: 0-127    { -r | --src-quality } src_quality        src_quality (sample rate converter quality): 0-127 (default is 127)    { --src-complexity } src_complexity        src_complexity (sample rate converter complexity): line, norm, bats    { -s | --strategy } strategy        bitrate allocation strategy for encoding an audio track        0 for CBR, 1 for ABR, 2 for VBR_constrained, 3 for VBR    --prime-method method        decode priming method (see AudioConverter.h)    --prime-override samples_prime samples_remain        can be used to override the priming information stored in the source        file to the specified values. If -1 is specified for either, the value        in the file is used.    --no-filler        don't page-align audio data in the output file    --soundcheck-generate        analyze audio, add SoundCheck data to the output file    --media-kind "media kind string"        media kinds are: "Audio Ad", "Video Ad"    --anchor-loudness        set a single precision floating point value to        indicate the anchor loudness of the content in dB    --generate-hash        generate an SHA-1 hash of the input audio data and add it to the output file.    --codec-manuf codec_manuf        specify the codec with the specified 4-character component manufacturer code    --dither algorithm        algorithm: 1-2    --mix        enable channel downmixing    { -u | --userproperty } property value        set an arbitrary AudioConverter property to a given value        property is a four-character code; value can be a signed        32-bit integer or a single precision floating point value.        e.g. '-u vbrq ' sets the sound quality level             (: 0-127)        May not be used in a transcoding situation.    -ud property value        identical to -u except only applies to a decoder. Fails if there is no        decoder.    -ue property value        identical to -u except only applies to an encoder. Fails if there is no        encoder.

• 输入文件选项

    --read-track track_index        For input files containing multiple tracks, the index (0..n-1)        of the track to read and convert.    --offset number_of_frames        the starting offset in the input file in frames. (The first frame is        frame zero.)    --soundcheck-read         read SoundCheck data from source file and set it on any destination         file(s) of appropriate filetype (.m4a, .caf).    --copy-hash         copy an SHA-1 hash chunk, if present, from the source file to the output file.    --gapless-before filename        file coming before the current input file of a gapless album    --gapless-after filename        file coming after the current input file of a gapless album

• 输出文件选项

    -o filename        specify an (additional) output file.    { -f | --file } file_format        use -hf for a complete list of supported file/data formats    --condensed-framing field_size_in_bits        specify storage size in bits for externally framed packet sizes.        Supported value is 16 for aac in m4a file format.

• 其他选项

    { -v | --verbose }        print progress verbosely    { -t | --tag }        If encoding to CAF, store the source file's format and name in a user        chunk. If decoding from CAF, use the destination format and filename        found in a user chunk.    { --leaks }        run leaks at the end of the conversion    { --profile }        collect and print performance information

• 帮助选项

    { -hf | --help-formats }        print a list of supported file/data formats, as below.    { -h | --help }        print help

查看所支持的文件和数据格式列表

Audio file (-f) and the data formats (-d) which each supports:

    '3gpp' = 3GP Audio (.3gp)               data_formats: 'Qclp' 'aac ' 'aace' 'aacf' 'aacg' 'aach'                              'aacl' 'aacp' 'samr'     '3gp2' = 3GPP-2 Audio (.3g2)               data_formats: 'Qclp' 'aac ' 'aace' 'aacf' 'aacg' 'aach'                              'aacl' 'aacp' 'samr'     'adts' = AAC ADTS (.aac, .adts)               data_formats: 'aac ' 'aach' 'aacp'     'ac-3' = AC3 (.ac3)               data_formats: 'ac-3'     'AIFC' = AIFC (.aifc, .aiff, .aif)               data_formats: I8 BEI16 BEI24 BEI32 BEF32 BEF64 UI8 'ulaw'                              'alaw' 'MAC3' 'MAC6' 'ima4' 'QDMC' 'QDM2'                              'Qclp' 'agsm'     'AIFF' = AIFF (.aiff, .aif)               data_formats: I8 BEI16 BEI24 BEI32     'amrf' = AMR (.amr)               data_formats: 'samr' 'sawb'     'm4af' = Apple MPEG-4 Audio (.m4a, .m4r)               data_formats: 'aac ' 'aace' 'aacf' 'aacg' 'aach' 'aacl'                              'aacp' 'ac-3' 'alac' 'ec-3' 'paac' 'pac3'                              'pec3'     'm4bf' = Apple MPEG-4 AudioBooks (.m4b)               data_formats: 'aac ' 'aace' 'aacf' 'aacg' 'aach' 'aacl'                              'aacp' 'paac'     'caff' = CAF (.caf)               data_formats: '.mp1' '.mp2' '.mp3' 'QDM2' 'QDMC' 'Qclp'                              'Qclq' 'aac ' 'aace' 'aacf' 'aacg' 'aach'                              'aacl' 'aacp' 'ac-3' 'alac' 'alaw' 'dvi8'                              'ec-3' 'flac' 'ilbc' 'ima4' I8 BEI16 BEI24                              BEI32 BEF32 BEF64 LEI16 LEI24 LEI32 LEF32                              LEF64 'ms\x00\x02' 'ms\x00\x11' 'ms\x001'                              'opus' 'paac' 'pac3' 'pec3' 'qaac' 'qac3'                              'qach' 'qacp' 'qec3' 'samr' 'ulaw' 'zaac'                              'zac3' 'zach' 'zacp' 'zec3'     'ec-3' = EC3 (.ec3)               data_formats: 'ec-3'     'flac' = FLAC (.flac)               data_formats: 'flac'     'MPG1' = MPEG Layer 1 (.mp1, .mpeg, .mpa)               data_formats: '.mp1'     'MPG2' = MPEG Layer 2 (.mp2, .mpeg, .mpa)               data_formats: '.mp2'     'MPG3' = MPEG Layer 3 (.mp3, .mpeg, .mpa)               data_formats: '.mp3'     'mp4f' = MPEG-4 Audio (.mp4)               data_formats: 'aac ' 'aace' 'aacf' 'aacg' 'aach' 'aacl'                              'aacp' 'ac-3' 'ec-3'     'NeXT' = NeXT/Sun (.snd, .au)               data_formats: I8 BEI16 BEI24 BEI32 BEF32 BEF64 'ulaw'     'Sd2f' = Sound Designer II (.sd2)               data_formats: I8 BEI16 BEI24 BEI32     'WAVE' = WAVE (.wav)               data_formats: UI8 LEI16 LEI24 LEI32 LEF32 LEF64 'ulaw'                              'alaw'     'RF64' = WAVE (.wav)               data_formats: UI8 LEI16 LEI24 LEI32 LEF32 LEF64 'ulaw'                              'alaw' 

对于以上这么多参数，是不是一脸懵逼，通常情况下，我们只需要转换音乐文件格式，其他的比如比特率、音轨、采样速率等我们不需要关心，那么我们只要用到 -f 和 -d 两个参数即可。

• -f 指的是 file format，也就是需要转换的目标格式;
• -d d指的是 data format，是数据格式的意思，理解这些参数的含义需要一定的音乐知识背景

举个栗子：

//将格式为 .m4a 的 input.m4a 文件转换成格式为 .m4b 的文件格式afconvert -f m4bf -d 'aac ' "input.m4a"

在终端执行完以上命令，如果不出异常，会在与 input.m4a 文件的同级目录有一个 input.m4b的文件。

如果需要指定输出文件的名字可以这样写：

afconvert -f m4bf -d 'aac ' "input.m4a" "output.m4b"

经测试，.m4a格式的音频文件可以转换成.flac、.wav、.m4b、.mp4、.caf、.snd、.au等很多的格式。

But 不能转换成.mp3的格式，悲催???

报如下错误信息：Error: ExtAudioFileSetProperty ('cfmt') failed ('fmt?')

通过查阅文档，发现了一条：

MacOS and afconvert do not include an MP3 encoder, though there is a MP4 (lossy) encoder. If you need to create MP3, use Lame or iTunes.
大致意思是：MacOS和afconvert不包括MP3编码器，但有一个MP4(有损)编码器。如果你需要创建MP3，请使用Lame或iTunes。

不过不用太悲观，转不了.mp3的格式，我们还可以转其它格式，毕竟现在的很多音乐播放器支持很多种格式的???，如果非得转成.mp3不可的话，可以到在这里cloudconvert在线转换。

网易云音乐的.ncm 格式无法转换，任何格式都转不了，因为 afconvert压根就不支持 .ncm 格式，直接报未知错误：An unknown error occurred.

引用 官方?

• Convert an Audio MP3 File to an iPhone Ringtone (m4af file):
  $ afconvert input.mp3 ringtone.m4r --file m4af

• Convert an AAC audio file to the iPhone ‘Core Audio File Format’ (CAFF) at a low 32 kbs bitrate:
  $ afconvert --data aac --bitrate 32000 input.aac output.caf --file 'caff'

• Convert a WAV file to an MP4 file with lossy aac format at a high bitrate of 256 kbs:
  $ afconvert -d aac -b 256000 input.wav output.mp4 -f mp4f

题外话?
“If I were going to convert to any religion I would probably choose Catholicism because it at least has female saints and the Virgin Mary” ~ Margaret Atwood

有道翻译?‍♀️：“如果我要皈依任何宗教，我可能会选择天主教，因为它至少有女圣人和圣母玛利亚”~玛格丽特·阿特伍德

说到这了，顺便再分享一个查看音频详细信息的命令 afinfo ，举个栗子：

➜  我的MV afinfo input.m4aFile:           input.m4aFile type ID:   m4afNum Tracks:     1----Data format:     2 ch,  44100 Hz, 'aac ' (0x00000000) 0 bits/channel, 0 bytes/packet, 1024 frames/packet, 0 bytes/frame                no channel layout.estimated duration: 333.333333 secaudio bytes: 4088592audio packets: 14359bit rate: 98102 bits per secondpacket size upper bound: 490maximum packet size: 490audio data file offset: 99759optimizedaudio 14700000 valid frames + 2624 priming + 992 remainder = 14703616format list:[ 0] format:      2 ch,  44100 Hz, 'aac ' (0x00000000) 0 bits/channel, 0 bytes/packet, 1024 frames/packet, 0 bytes/frameChannel layout: Stereo (L R)----

由此可以看出音频详细数据（能看懂的就下面这些数据，其他的也不懂???）
音频文件类型：m4af
音频数据格式：'aac '
采样速率：44100 Hz = 44.100 kHz
比特率：98102 bit/s ≈ 98 kbps
时长：333.333333秒 ≈ 5分33秒

afinfo 其它选项可通过 afinfo -h 查看

➜  我的MV afinfo -h            Audio File Info     Version: 2.0     Copyright 2003-2016, Apple Inc. All Rights Reserved.     Specify -h (-help) for command optionsUsage:afinfo [option...] audio_file(s)Options: (may appear before or after arguments)  {-h --help}    print help  {-b --brief}    print a brief (one line) description of the audio file  {-r --real}    get the estimated duration after obtaining the real packet count  { --leaks }        run leaks at the end  { -i --info }      print contents of the InfoDictionary  { -u --userprop } 4-cc      find and print a property or user data property (as string or bytes) [does not print to xml]  { -x --xml }      print output in xml format  { --warnings }      print warnings if any (by default warnings are not printed in non-xml output mode)

编辑和更改音频文件的封面图片

通过上面格式转换操作后，大家会发现格式转换后，音频文件的封面图没了，如果想要给音频文件添加文件可按如下操作。

1. 双击要添加封面图的音频文件，此时会被自动添加到 iTunes 的资料库中，开始播放；

2. 找到音频，点击右键，选择歌曲信息;

   

   歌曲信息
3. 选择插图一栏，点击下面的添加图片，把你想要设置的封面图添加进去就可以了，大功告成~_~
   

   添加图片

参考资料：An A-Z Index of the Apple macOS command line (OS X)

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