extract-audio

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Original

English

🇨🇳

Translation

Chinese

Format Decision Tree

格式决策树

User wants	Format	Flags	Why
Music, archive quality	FLAC	`-c:a flac`	Lossless, no quality loss
Music, small + transparent	MP3 VBR	`-c:a libmp3lame -q:a 0`	~200kbps avg, perceptually lossless
Podcast / voice	MP3 128k CBR	`-c:a libmp3lame -b:a 128k`	Sufficient for speech, universally compatible
Mobile / streaming	AAC 192k	`-c:a aac -b:a 192k`	Better than MP3 at equivalent bitrate
DAW / editing	WAV	`-c:a pcm_s16le -ar 44100`	No encoding loss, widest DAW support
Source already target format	Copy	`-c:a copy`	No re-encode, instant, lossless

用户需求	格式	参数	原因
音乐、归档级质量	FLAC	`-c:a flac`	无损，无质量损失
音乐、体积小且音质接近无损	MP3 VBR	`-c:a libmp3lame -q:a 0`	平均约200kbps，感知无损
播客/语音内容	MP3 128k CBR	`-c:a libmp3lame -b:a 128k`	满足语音需求，兼容性广泛
移动设备/流媒体	AAC 192k	`-c:a aac -b:a 192k`	相同比特率下表现优于MP3
数字音频工作站（DAW）/编辑	WAV	`-c:a pcm_s16le -ar 44100`	无编码损失，支持绝大多数DAW
源文件已是目标格式	复制	`-c:a copy`	无需重新编码，即时完成，无损

Process

流程

1. Probe audio streams

1. 探测音频流

bash

ffprobe -v quiet -print_format json -show_streams "$INPUT" | \
  python3 -c "
import json, sys
streams = [s for s in json.load(sys.stdin)['streams'] if s['codec_type']=='audio']
for i, s in enumerate(streams):
    print(f'Stream {i}: {s[\"codec_name\"]} {s.get(\"bit_rate\",\"?\")} bps {s.get(\"channel_layout\",\"?\")}')
"

bash

ffprobe -v quiet -print_format json -show_streams "$INPUT" | \
  python3 -c "
import json, sys
streams = [s for s in json.load(sys.stdin)['streams'] if s['codec_type']=='audio']
for i, s in enumerate(streams):
    print(f'Stream {i}: {s[\"codec_name\"]} {s.get(\"bit_rate\",\"?\")} bps {s.get(\"channel_layout\",\"?\")}')
"

2. Determine format

2. 确定格式

Apply the decision tree above if the user didn't specify. If the source audio codec already matches the target, use

-c:a copy

to avoid transcoding.

If multiple audio streams exist, ask the user which to extract — or use

-map 0:a

to extract all. Once the user responds, apply

-map 0:a:N

(where N is the zero-based stream index they chose) or

-map 0:a

for all streams in the Phase 3 command.

若用户未指定格式，应用上述决策树。若源音频编码已匹配目标格式，使用

-c:a copy

避免转码。

若存在多个音频流，询问用户要提取哪一个——或使用

-map 0:a

提取所有流。用户回复后，在第3阶段的命令中应用

-map 0:a:N

（其中N为用户选择的从零开始的流索引）或

-map 0:a

提取所有流。

3. Construct command

3. 构建命令

bash

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bash

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General pattern (-vn drops the video stream entirely):

ffmpeg -i "$INPUT" -vn [FORMAT_FLAGS] "$OUTPUT"

Examples:

ffmpeg -i video.mp4 -vn -c:a libmp3lame -q:a 0 audio.mp3 # MP3 VBR best quality ffmpeg -i video.mp4 -vn -c:a libmp3lame -b:a 128k podcast.mp3 # MP3 128k CBR ffmpeg -i video.mp4 -vn -c:a flac archive.flac # FLAC lossless ffmpeg -i video.mp4 -vn -c:a aac -b:a 192k mobile.aac # AAC ffmpeg -i video.mp4 -vn -c:a pcm_s16le -ar 44100 edit.wav # WAV for DAW ffmpeg -i video.mp4 -vn -c:a copy original.m4a # Copy audio stream

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4. Confirm and run

4. 确认并执行

Show: detected source codec and bitrate, chosen output format, output path. Wait for approval, then run.

Report output file size and duration:

ffprobe -v quiet -show_format "$OUTPUT" | grep -E "duration|size"

展示：检测到的源编码和比特率、选定的输出格式、输出路径。等待用户确认后执行。

报告输出文件大小和时长：

ffprobe -v quiet -show_format "$OUTPUT" | grep -E "duration|size"

Key Decisions

关键决策

Preserve generation quality: avoid transcoding chains that degrade source fidelity. Each decision below is an application of this principle.

Lossy-to-lossy warning: if the source is already lossy (MP3, AAC, OGG) and the user wants a different lossy format, warn them that re-encoding degrades quality. Recommend keeping the source format or using
```
-c:a copy
```
where container compatibility allows.
For files >1 hour, ask whether the user wants the full file or a specific range — trimming can be added with
```
-ss
```
and
```
-to
```
before
```
-vn
```
.
M4A vs AAC: AAC is the codec, M4A is the container. Use
```
.m4a
```
extension for Apple device compatibility; use
```
.aac
```
for a raw stream.

保留生成质量：避免会降低源文件保真度的转码链。以下每项决策均遵循此原则。

有损转有损警告：若源文件已是有损格式（MP3、AAC、OGG），且用户想要转换为另一种有损格式，需警告用户重新编码会降低质量。建议保留源格式，或在容器兼容的情况下使用
```
-c:a copy
```
。
对于时长超过1小时的文件，询问用户是要完整文件还是特定片段——可在
```
-vn
```
前添加
```
-ss
```
和
```
-to
```
参数进行修剪。
M4A与AAC：AAC是编码格式，M4A是容器。为兼容苹果设备，使用
```
.m4a
```
扩展名；若需原始流，使用
```
.aac
```
扩展名。