MediaConvert
User Guide

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SCC, SRT, STL, TTML (Sidecar)

SCC, SRT, STL, and TTML are sidecar captions formats. With these formats, you provide input captions as a separate file. AWS Elemental MediaConvert handles all sidecar formats the same way. The service can pass them through to the output in the same format or convert them into another sidecar format. In all cases, you must create one captions selector for the entire set of captions tracks.

Provide the following values for the captions selector fields:

  • External captions file: The URI to the captions file. MediaConvert accepts captions files from Amazon S3.

  • Time delta: (Optional) Use this setting if you need to adjust the sync between the captions and the video:

    • Type a positive number to add to the times in the captions file. For example, type 15 to add 15 seconds to all the times in the captions file.

    • Type a negative number to subtract from the times in the captions file. For example, type -5 to subtract 5 seconds from the times in the captions file.

      Enter the time delta in seconds, regardless of the format used in your captions file to specify start and end times. The number that you enter for Time delta simply delays the captions or makes the captions play earlier, regardless of the timecode formats.

Note

To make sure that your captions are properly synchronized with your video, check that the value for Timecode source in the Video selector section matches the timecodes in your captions file. For example, if the timecodes in your captions file start at zero but your video has embedded timecodes starting at 01:00:00:00, change the default value for Timecode source from Embedded to Start at 0. If other aspects of your job prevent that, use the Time delta setting to adjust your captions, as described in Use Cases for Time Delta.

If you use the API or an SDK, you can find this setting in the JSON file of your job, called TimecodeSource, located in Settings, Inputs. When you use SCC, you must provide a value for TimecodeSource. Otherwise, MediaConvert will not insert the captions.

Converting Dual SCC Input Files to Embedded Captions

If you want to use two SCC files as your captions input and embed the captions as two output captions channels embedded in your output video stream, set up your captions according to this procedure.

To convert dual SCC to embedded captions

  1. Set up two input captions selectors. Follow the procedure in Creating Input Captions Selectors. Specify values as follows:

    • In each captions selector, choose SCC for Source.

    • For Source file, choose one of your input SCC files in each selector.

    • If you want both 608 and 708 captions embedded in your outputs, choose Upconvert for Force 608 to 708 upconvert in both captions selectors.

  2. Set up captions in your outputs. Follow the procedure in Setting Up Captions in Outputs. Follow these specific choices:

    • Specify the captions in the same output as the video that you want the captions embedded in.

    • Choose Add captions twice, to create Captions 1 and Captions 2 tabs in the Encoding settings section.

    • For Captions source, in each of the captions tabs, choose one of the captions selectors that you created in the preceding step of this procedure.

    • For CC channel number, choose a number for each of the captions tabs that don't share a field. For example, in Captions 1, choose 1 for CC channel number and in Captions 2, choose 3 for CC channel number.

      Don't choose the combinations 1 and 2 or 3 and 4, because those pairs of channels share the same field.

    • If you chose Upconvert in the preceding step of this procedure, optionally specify a service number for 708 service number. Within an output, each captions tab must specify a different service number.

      If you upconvert and don't specify a value for 708 service number, the service uses the value that you specify for CC channel number as your 708 service number.

Use Cases for Time Delta

How you use Time delta depends on the problem you're trying to solve and the captions format you're working with.

Adjusting for Different Timecodes Between Video and Captions Files

With timecode-based captions formats, such as SCC and STL, the timecodes in the captions might be based on the assumption that the video timecodes start at a different timecode than they actually do. You use Time delta to adjust for the difference.

Example problem: For example, your video file might have embedded timecodes that start at 00:05:00:00 and the first instance of dialogue that requires captions might be one minute into the video, at timecode 00:06:00:00. Your captions file might be written on the assumption that your video timecodes start at 00:00:00:00, with the first caption starting at 00:01:00:00. If you don't use Time delta, MediaConvert would not include this first caption, because it occurs before the start of the video.

Solution: Add five minutes to the captions. Enter 300 for Time delta.

Similarly, your captions file might be aligned with the captions that are embedded in your video, but you might need to use the input Timecode source setting to change the input timecodes, which creates a difference between the video and captions.

Example problem: For example, your video file might have embedded timecodes that start at 00:05:00:00 and the first instance of dialogue that requires captions might be one minute into the video, at timecode 00:06:00:00. Your captions file is written to sync correctly, with the first caption starting at 00:06:00:00. But you need to change your embedded captions at the input to sync correctly with your audio file. So you set the input Timecode source to Start at Zero. If you don't use Time delta, MediaConvert would put the first caption in your output at six minutes into the video.

Solution: Subtract five minutes from the captions. Enter -300 for Time delta.

Correcting Slight Errors in Captions Sync

With any type of sidecar format, there might be a small error in your input captions file, so that the captions are consistently a little late or a little early.

Example problem: Your video has embedded captions that start at zero. The first instance of dialogue that requires captions is at 00:06:15:00, but the captions appear on the screen three seconds late, at 00:06:18:00.

Solution: Subtract three seconds from the captions. Enter -3 for Time delta.