從 flac 轉換為 alac 時，ffmpeg 僅以位深度 s32p 輸出

我正在嘗試使用ffmpeg. 對於我的環境（Apple & Sonos），最低公分母似乎如下：

44.1kHz 的 ALAC (.m4a) 編碼，& 位深度 = 16 位

我遇到的問題是生成的輸出文件ffmpeg的位深度為 ‘32p’；IE sample_fmt=s32p

我已經用幾組參數完成了轉換，但結果是一樣的。我一直找不到任何可以解釋為什麼會這樣的東西。這是我嘗試過的命令之一：

$ file='01 Jubilee.flac'
$ ffmpeg -i "$file" -acodec alac -ar 44100 -sample_fmt:0 s16 -c:v png "${file/%.flac/.16.m4a}" 

我ffprobe在輸入文件上執行以確定其格式：

$ ffprobe -i "$file" -show_streams  

其中（總而言之）產生：

Stream #0:0: Audio: flac, 176400 Hz, stereo, s32 (24 bit)
Stream #0:1: Video: mjpeg (Progressive), yuvj444p(pc, bt470bg/unknown/unknown), 450x446 [SAR 72:72 DAR 225:223], 90k tbr, 90k tbn, 90k tbc (attached pic)

…並在輸出文件上：

$ ffprobe -i "${file/%.flac/.16.m4a}" -show_streams

其中（總而言之）產生：

Stream #0:0(und): Audio: alac (alac / 0x63616C61), 44100 Hz, stereo, s32p (24 bit), 1564 kb/s (default)
Metadata:
 handler_name    : SoundHandler
Stream #0:1: Video: png, rgb24(pc), 450x446 [SAR 1:1 DAR 225:223], 90k tbr, 90k tbn, 90k tbc (attached pic)  

[STREAM]
index=0
codec_name=alac
codec_long_name=ALAC (Apple Lossless Audio Codec)
profile=unknown
codec_type=audio
codec_time_base=1/44100
codec_tag_string=alac
codec_tag=0x63616c61
sample_fmt=s32p
sample_rate=44100

... 

etc, etc

我只發布了一小部分ffprobe輸出，但如果需要，可以提供所有內容。另外，我的ffmpeg版本：

$ ffmpeg -version
ffmpeg version git-2020-01-13-7225479 Copyright (c) 2000-2020 the FFmpeg developers
built with Apple clang version 11.0.0 (clang-1100.0.33.8)

最後，這是ffmpeg上面轉換的輸出：

$ ffmpeg -v info -hide_banner -i "$file" -acodec alac -ar 44100 -sample_fmt:0 s16 -c:v png "${file/%.flac/.16.m4a}"
Input #0, flac, from '01 Jubilee.flac':
 Metadata:
   track           : 1
   TITLE           : Jubilee
   ARTIST          : Bill Charlap Trio
   album_artist    : Bill Charlap Trio
   ALBUM           : Stardust The Music Of Hoagy Carmichael
   DATE            : 2003
   GENRE           : Jazz
   TRACKTOTAL      : 11
   disc            : 1
   DISCTOTAL       : 1
   ALBUM ARTIST    : Bill Charlap Trio
 Duration: 00:02:23.17, start: 0.000000, bitrate: 6176 kb/s
   Stream #0:0: Audio: flac, 176400 Hz, stereo, s32 (24 bit)
   Stream #0:1: Video: mjpeg (Progressive), yuvj444p(pc, bt470bg/unknown/unknown), 450x446 [SAR 72:72 DAR 225:223], 90k tbr, 90k tbn, 90k tbc (attached pic)
   Metadata:
     comment         : Cover (front)
File '01 Jubilee.16.m4a' already exists. Overwrite? [y/N] y
Stream mapping:
 Stream #0:1 -> #0:0 (mjpeg (native) -> png (native))
 Stream #0:0 -> #0:1 (flac (native) -> alac (native))
Press [q] to stop, [?] for help
[swscaler @ 0x10dc6b000] deprecated pixel format used, make sure you did set range correctly
[ipod @ 0x7fc3a1002200] Frame rate very high for a muxer not efficiently supporting it.
Please consider specifying a lower framerate, a different muxer or -vsync 2
Output #0, ipod, to '01 Jubilee.16.m4a':
 Metadata:
   track           : 1
   TITLE           : Jubilee
   ARTIST          : Bill Charlap Trio
   album_artist    : Bill Charlap Trio
   ALBUM           : Stardust The Music Of Hoagy Carmichael
   DATE            : 2003
   GENRE           : Jazz
   TRACKTOTAL      : 11
   disc            : 1
   DISCTOTAL       : 1
   ALBUM ARTIST    : Bill Charlap Trio
   encoder         : Lavf58.35.102
   Stream #0:0: Video: png, rgb24(progressive), 450x446 [SAR 1:1 DAR 225:223], q=2-31, 200 kb/s, 90k fps, 90k tbn, 90k tbc (attached pic)
   Metadata:
     comment         : Cover (front)
     encoder         : Lavc58.65.103 png
   Stream #0:1: Audio: alac (alac / 0x63616C61), 44100 Hz, stereo, s32p (24 bit), 128 kb/s
   Metadata:
     encoder         : Lavc58.65.103 alac
frame=    1 fps=0.8 q=-0.0 Lsize=   27745kB time=00:02:23.22 bitrate=1587.0kbits/s speed= 113x    
video:396kB audio:27342kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 0.026342%

這在這裡有效。

ffmpeg -i in -c:a alac -ar 44100 -sample_fmt s16p -c:v png -vsync 0 out.m4a

請注意，ALAC 編碼器檢查原始幀是否為 16 位平面

引用自：https://unix.stackexchange.com/questions/563286