ffmpeg（ffmpeg 音频）

Input Seeking (Major Difference)

• -ss BEFORE -i: fast seek, may be inaccurate—starts from nearest keyframe
• -ss AFTER -i: frame-accurate but slow—decodes from start
• Combine both: -ss 00:30:00 -i input.mp4 -ss 00:00:05—fast seek then accurate trim
• For cutting, add -avoid_negative_ts make_zero to fix timestamp issues

Stream Selection

• Default: first video + first audio—may not be what you want
• Explicit selection: -map 0:v:0 -map 0:a:1—first video, second audio
• All streams of type: -map 0:a—all audio streams
• Copy specific: -map 0 -c copy—all streams, no re-encoding
• Exclude: -map 0 -map -0:s—all except subtitles

Encoding Quality

• CRF (Constant Rate Factor): lower = better quality, larger file—18-23 typical for H.264
• -preset: ultrafast to veryslow—slower = smaller file at same quality
• Two-pass for target bitrate: first pass analyzes, second pass encodes
• -crf and -b:v mutually exclusive—use one or the other

Container vs Codec

• Container (MP4, MKV, WebM): wrapper format holding streams
• Codec (H.264, VP9, AAC): compression algorithm for stream
• Not all codecs fit all containers—H.264 in MP4/MKV, not WebM; VP9 in WebM/MKV, not MP4
• Copy codec to new container: -c copy—fast, no quality loss

Filter Syntax

• Simple: -vf "scale=1280:720"—single filter chain
• Complex: -filter_complex "[0:v]scale=1280:720[scaled]"—named outputs for routing
• Chain filters: -vf "scale=1280:720,fps=30"—comma-separated
• Filter order matters—scale before crop gives different result than crop before scale

Common Filters

• Scale: scale=1280:720 or scale=-1:720 for auto-width maintaining aspect
• Crop: crop=640:480:100:50—width:height:x:y from top-left
• FPS: fps=30—change framerate
• Trim: trim=start=10:end=20,setpts=PTS-STARTPTS—setpts resets timestamps
• Overlay: overlay=10:10—position from top-left

Audio Processing

• Sample rate: -ar 48000—standard for video
• Channels: -ac 2—stereo
• Audio codec: -c:a aac -b:a 192k—AAC at 192kbps
• Normalize: -filter:a loudnorm—EBU R128 loudness normalization
• Extract audio: -vn -c:a copy output.m4a—no video, copy audio

Concatenation

• Same codec/params: concat demuxer—-f concat -safe 0 -i list.txt -c copy
• Different formats: concat filter—-filter_complex "[0:v][0:a][1:v][1:a]concat=n=2:v=1:a=1"
• list.txt format: file 'video1.mp4' per line—escape special characters
• Different resolutions: scale/pad to match before concat filter

Subtitles

• Burn-in (hardcode): -vf "subtitles=subs.srt"—cannot be turned off
• Mux as stream: -c:s mov_text (MP4) or -c:s srt (MKV)—user toggleable
• From input: -map 0:s—include subtitle streams
• Extract: -map 0:s:0 subs.srt—first subtitle to file

Hardware Acceleration

• Decode: -hwaccel cuda or -hwaccel videotoolbox (macOS)
• Encode: -c:v h264_nvenc (NVIDIA), -c:v h264_videotoolbox (macOS)
• Not always faster—setup overhead; benefits show on long videos
• Quality may differ—software encoding often produces better quality

Common Mistakes

• Forgetting -c copy when not re-encoding—defaults to re-encode, slow and lossy
• -ss after -i for long videos—takes forever seeking
• Audio desync after cutting—use -async 1 or -af aresample=async=1
• Filter on stream copy—filters require re-encoding; -c copy + -vf = error
• Output extension doesn't set codec—output.mp4 without -c:v uses default, may not be H.264