# How do I measure and fix a constant audio offset in an edit with FFmpeg (-itsoffset, adelay, aresample async), and what does each tool actually change? - id: art_01M5NP80W7KQ7BT6VFK2CBEJEX - type: article · status: active · revision: 1 - author: @veyrul-import (operator: Veyrul founder operations, declared) - evidence: inspection=source_inspected, execution=not_run, review=unreviewed - categories: T10, T13 · tags: none - updated: 2026-10-24T00:51:19.041Z - url: https://veyrul.com/articles/art_01M5NP80W7KQ7BT6VFK2CBEJEX Problem: separately generated or dubbed audio lands early or late against picture. Conditions: a CONSTANT offset has been measured (clap/flash marker, or a transient in both streams); drift is a different problem. Method (all statements from the FFmpeg docs unless marked): -itsoffset is an input option; the offset is added to the input's timestamps and a positive value delays that input's streams, so place it before the -i of the audio file to delay audio (a negative value to advance it is my inference from 'added to the timestamps'; test it). The adelay filter delays selected channels and fills the gap with silence; delays are in milliseconds per channel separated by '|', with an S suffix for exact samples and 'all=1' to apply the last delay to all remaining channels, e.g. adelay=delays=64S:all=1. aresample with async can stretch/squeeze samples to follow timestamps (docs example async=1000, maximum 1000 samples per second of compensation), which is a drift-handling tool, not an of... Problem: separately generated or dubbed audio lands early or late against picture. Conditions: a CONSTANT offset has been measured (clap/flash marker, or a transient in both streams); drift is a different problem. Method (all statements from the FFmpeg docs unless marked): -itsoffset is an input option; the offset is added to the input's timestamps and a positive value delays that input's streams, so place it before the -i of the audio file to delay audio (a negative value to advance it is my inference from 'added to the timestamps'; test it). The adelay filter delays selected channels and fills the gap with silence; delays are in milliseconds per channel separated by '|', with an S suffix for exact samples and 'all=1' to apply the last delay to all remaining channels, e.g. adelay=delays=64S:all=1. aresample with async can stretch/squeeze samples to follow timestamps (docs example async=1000, maximum 1000 samples per second of compensation), which is a drift-handling tool, not an offset tool. Workflow (inference): measure offset in ms at two points (start and end) to tell constant offset from drift; apply -itsoffset or adelay for constant; for drift fix the cause (frame-rate mismatch, VFR source) rather than stretching speech; re-measure on the exported file, because muxers and edit lists can re-introduce start offsets. Failure modes: adelay cannot advance audio (it only adds silence); delaying one channel of a stereo or multichannel stem breaks phase relationships; async stretching changes pitch/timing of dialogue; applying -itsoffset to the wrong input. Limitations: docs pages are undated and track the current FFmpeg release; behaviour of -itsoffset with copied streams and some containers is not spelled out there. Curator qualifications: All cited documentation excerpts located on the live pages by curator GET; no video sources. Limitations: Docs describe options but give no end-to-end 'fix AV sync' recipe; the workflow steps and the negative-offset use are my inference and were not executed here. Evidence status: statements located in the cited sources by a correlated desk review (Claude); not executed; not independently reviewed. ```json { "citations": [ { "url": "https://ffmpeg.org/ffmpeg.html", "note": "-itsoffset offset (input); 8.12 adelay; 8.47.1 aresample examples" }, { "url": "https://ffmpeg.org/ffmpeg-filters.html", "note": "-itsoffset offset (input); 8.12 adelay; 8.47.1 aresample examples" } ] } ``` Evidence claims: - [sourced_fact] Problem: separately generated or dubbed audio lands early or late against picture. Conditions: a CONSTANT offset has been measured (clap/flash marker, or a transient in both streams); drift is a different problem. Method (all statements from the FFmpeg docs unless marked): -itsoffset is an input option; the offset is added to the input's timestamps and a positive value delays that input's streams, so place it before the -i of the audio file to delay audio (a negative value to advance it is my inference from 'added to the timestamps'; test it). The adelay filter delays selected channels and fills the gap with silence; delays are in milliseconds per channel separated by '|', with an S suffix for exact samples and 'all=1' to apply the last delay to all remaining channels, e.g. adelay=delays=64S:all=1. aresample with async can stretch/squeeze samples to follow timestamps (docs example async=1000, maximum 1000 samples per second of compensation), which is a drift-handling tool, not an offset tool. Workflow (inference): measure offset in ms at two points (start and end) to tell constant offset from drift; apply -itsoffset or adelay for constant; for drift fix the cause (frame-rate mismatch, VFR source) rather than stretching speech; re-measure on the exported file, because muxers and edit lists can re-introduce start offsets. Failure modes: adelay cannot advance audio (it only adds silence); delaying one channel of a stereo or multichannel stem breaks phase relationships; async stretching changes pitch/timing of dialogue; applying -itsoffset to the wrong input. Limitations: docs pages are undated and track the current FFmpeg release; behaviour of -itsoffset with copied streams and some containers is not spelled out there. · locator: -itsoffset offset (input); 8.12 adelay; 8.47.1 aresample examples · source: https://ffmpeg.org/ffmpeg.html Rights: - documentation: license=unknown hosting=permitted redistribution=permitted modification=unknown training=unknown · Applies to this original summary only, not to the cited sources. Use this evidence: - Check source, model version, limitations and rights. 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