To add live captions to a YouTube stream with SyncWords, route your existing encoder’s RTMP feed through a SyncWords Service, configure transcription and an Embedded 608 output, then point that output at YouTube using the stream URL and key from YouTube Studio. You must also enable Closed Captions in YouTube’s stream settings and choose Embedded 608/708 as the captions source.
How the caption route works
Your encoder does not send directly to YouTube in this setup. It sends an RTMP PUSH input to SyncWords; SyncWords generates captions and sends an RTMP PUSH output to YouTube. SyncWords’ guide describes the service as needing to be started before it can accept the incoming stream and deliver an output.
- Create a new SyncWords Service or open the Service you already use.
- In Input Media, choose RTMP PUSH. Copy the generated input details into your existing encoder or streaming service. SyncWords names OBS, Wirecast, and StreamYard as examples of sources.
- Configure transcription so captions are generated from the incoming audio.
- If needed, configure real-time translation or audio dubbing. SyncWords warns that this RTMP output supports only languages using Latin character sets; Arabic is an example of a language that will not work on this route.
- Under Output, choose RTMP PUSH and add a destination. In Captions & Subtitles, enable Embedded 608.
- In YouTube Studio, open Stream Settings and copy the Stream URL and Stream Key into the SyncWords destination.
- In YouTube Studio, enable Closed Captions and set Captions Source to Embedded 608/708.
- Start the SyncWords Service before sending the incoming feed. Confirm that the encoder is sending to the SyncWords input and that the output destination is configured for YouTube.
The two caption settings must agree: SyncWords says that if YouTube Closed Captions is off or Embedded 608/708 is not selected as the source, the stream passed through to YouTube will not contain captions. See SyncWords’ YouTube RTMP 608 instructions for its documented workflow.
Check the incoming stream settings
SyncWords’ YouTube guide specifies these incoming parameters. They are the values in its documentation, not settings independently tested here; check that your encoder can meet them before the event.
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| Parameter | SyncWords guide |
|---|---|
| Audio sample rate | 44.1 kHz |
| Frame rate | 29.97 or 30 fps |
| Video bitrate | Minimum 3,500 Kbps |
| Keyframe interval | 2 seconds |
| Aspect ratio | 16:9 or 4:3 |
These values are specific to SyncWords’ documented YouTube RTMP 608 workflow. Do not assume they override requirements for a different destination or a separately configured production chain.
Choose caption channels and buffering
Embedded 608 channels
SyncWords describes 608 captions as traditional closed captions embedded directly into the video signal. You can assign a language to CC1 and, if required, use CC2 through CC4. Its output guide cautions that not all players can read captions from CC2–CC4, so verify compatibility before depending on those additional channels. If CC1 captions are already present in your source and should remain intact, the guide also provides a passthrough choice.
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Buffer according to translation and latency
Buffering is a trade-off, not a universal setting: translated audio needs more time than low-latency same-language captions. SyncWords’ RTMP output guide recommends the following starting points:
| Use case | SyncWords buffer recommendation |
|---|---|
| Translated audio | 60 seconds |
| Translated subtitles only | 30 seconds |
| Same-language captions where low latency matters | 0–5 seconds |
SyncWords also documents frame reordering to support caption embedding across different frame rates and a default late-caption timeout of 30 seconds. Review these advanced settings if captions arrive late or the source and output frame rates differ. The output options are described in the SyncWords RTMP output guide.
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Send to more than one destination
SyncWords documents up to four simultaneous RTMP destinations. Each can have its own connector type, URL, stream key, and dubbing language; all receive the same encoded stream and caption set. This is useful when one event needs multiple RTMP endpoints, but it does not mean each destination can receive a different encoded video or caption set.
Troubleshoot missing or late captions
- YouTube shows no captions: Check that transcription is configured, Embedded 608 is enabled in the SyncWords output, YouTube Closed Captions is enabled, and Captions Source is set to Embedded 608/708.
- SyncWords does not receive the video: Confirm the encoder is sending to the generated SyncWords RTMP PUSH input details, rather than directly to YouTube, and start the SyncWords Service.
- YouTube does not receive the output: Check the YouTube Stream URL and Stream Key copied into the SyncWords destination and verify that the output uses RTMP PUSH.
- A secondary caption language is unavailable: Confirm the player supports the CC channel assigned to it. SyncWords warns that CC2–CC4 are not readable in every player.
- Translated audio or captions fall behind: Review the buffer for the chosen workflow. SyncWords recommends longer buffers for translated audio and subtitles than for low-latency same-language captions.
- Translation or dubbing does not work: Check whether the output language uses a Latin character set; SyncWords specifically warns that its RTMP output does not support non-Latin character sets such as Arabic.
Understand usage billing before a long event
SyncWords’ published Live usage page says billing depends on how long an Event or Service is connected and how many outputs it generates; each language’s captions, subtitles, or audio translation counts as a separate output. Its example counts one hour of English captions, French and Spanish subtitles, and French audio translation as four hours of usage. The page says the default active-service runtime allowance is three times the usage amount, with additional runtime billed at $0.10 per hour. These are the terms published on its help page, not a quote for a particular account or plan; confirm the terms that apply to your contract on the SyncWords Live usage page.
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Or let it run in the cloud
SyncWords adds captions to a stream you already produce. If your separate goal is keeping uploaded video live on a YouTube channel around the clock, StreamNeo is a cloud service for that task: upload a recording or build a playlist, add your YouTube stream key once, and go live. It loops uploaded videos, not a live camera feed.
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