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; -
-dd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.m4a
File: input.m4a
File type ID: m4af
Num 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 sec
audio bytes: 4088592
audio packets: 14359
bit rate: 98102 bits per second
packet size upper bound: 490
maximum packet size: 490
audio data file offset: 99759
optimized
audio 14700000 valid frames + 2624 priming + 992 remainder = 14703616
format list:
[ 0] format: 2 ch, 44100 Hz, 'aac ' (0x00000000) 0 bits/channel, 0 bytes/packet, 1024 frames/packet, 0 bytes/frame
Channel 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 options
Usage:
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。
- 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；
- 找到音频，点击右键，选择歌曲信息;
歌曲信息
- 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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