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Choose what “different start times” means
- Start at different points in prerecorded media: Use
-ssbefore each relevant-i. This selects a content position in that input. - Delay or adjust timestamps: Use
-itsoffsetbefore the input. It adds an offset to the input timestamps; a positive offset delays the corresponding streams. It does not select a different scene or song. - Start a scheduled broadcast at a particular wall-clock time: The FFmpeg option behavior described here does not establish how to schedule separate YouTube broadcasts. Check the current controls in YouTube Live Control Room; do not assume that one FFmpeg process and stream key can schedule multiple distinct broadcasts.
Seek each input to a different content position
FFmpeg options generally apply to the next input or output file, and options reset between files. Put each input’s seek immediately before that input’s -i, then map that input to its intended output.
ffmpeg
-ss 00:02:00 -i inputA.mp4
-map 0:v? -map 0:a? -c:v libx264 -c:a aac OUTPUT_A
-ss 00:05:00 -i inputB.mp4
-map 1:v? -map 1:a? -c:v libx264 -c:a aac OUTPUT_BHere, the first input begins at 00:02:00 and the second at 00:05:00 in their respective files. The optional map suffix ? makes each video or audio mapping optional if that stream type is absent. The example illustrates option scope, seeks and mappings; it is not a tested, ready-to-publish YouTube command. Replace the output destinations and codec settings to match your actual ingest protocol and production.
Option order and seek behavior
In this example, each -ss appears before its -i, making it an input seek for that particular source. FFmpeg also documents an output-side form of -ss; in that position, it decodes and discards input until the requested position. Do not move the seek casually while copying or adapting a command: the position relative to -i changes its meaning.
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Use -itsoffset for timestamp shifts, not a different scene
-itsoffset is an input option. It adds the specified offset to the timestamps of the corresponding input streams. A positive offset delays those streams in time. Use it when timestamp alignment or timing is the goal; use -ss when the source should begin at a different content position. An offset does not make a video jump to a later point in its playlist or file.
Do not confuse HLS variants with independent stream starts
FFmpeg can create multiple quality renditions of one HLS presentation using stream maps and var_stream_map. For two or more variants, the output filename pattern must contain %v. Those variant playlists describe renditions of the presentation; they do not inherently give separate broadcasts independent content-start schedules.
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If your goal is to create quality variants, configure the mappings and variant map for one presentation. If your goal is distinct broadcasts beginning at separate positions, scope the inputs and outputs separately as in the example, then configure each output for its intended destination. The available FFmpeg behavior does not resolve whether YouTube permits multiple distinct broadcasts to be scheduled through one FFmpeg process and one stream key.
If the input is a live HLS playlist
When FFmpeg reads a live HLS input, its HLS demuxer provides live_start_index to choose a segment index. Negative indices count from the end of the playlist. The demuxer’s prefer_x_start option can prefer an #EXT-X-START tag in the playlist. These are input demuxer behaviors for choosing where to start reading a live playlist, not substitutes for scheduling a YouTube broadcast.
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YouTube HLS ingest requirements
If you send output to YouTube using HLS ingest, follow YouTube’s documented requirements and confirm the current ingest settings in Live Control Room. YouTube specifies:
- TS segments lasting 1–4 seconds.
- A rolling playlist with no more than five outstanding segments.
- HTTPS POST or PUT transport.
- No byte ranges.
YouTube says its Ultra low-latency option is turned off for HLS and that HLS has higher latency because video is sent in segments. These constraints apply to HLS ingestion; they are not a general recommendation that HLS is best for every production.
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Common timing mistakes and fixes
- Both sources begin at the same position: Check that each
-ssis before the intended-i, rather than accidentally applying the seek to an output or another input. - The stream is delayed, but the content has not changed: That is timestamp shifting. Use an input seek with
-ssto select a different source position. - FFmpeg cannot find a mapped stream: Verify the input index in each
-map(the example uses0for the first input and1for the second). The question mark makes mapping optional when a stream type is absent. - Variant output filenames conflict or are invalid: When creating two or more HLS variants, include
%vin the output filename pattern and configure the variant mappings. - An HLS input starts at an unexpected segment: Review the live HLS demuxer’s
live_start_indexand whetherprefer_x_startshould honor the playlist’s#EXT-X-STARTtag. - YouTube rejects or delays HLS output: Check segment duration, rolling playlist length, HTTPS method and byte-range use against YouTube’s HLS requirements; remember that HLS adds latency.
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