As discussed in the TTWG call, the HRM is parametrized today to fail documents that would refresh the content at every frame of a related video, when the frame rate of that video is around 25fps or above. Since formats such as CEA-608 behave like that, naively converting them to IMSC could create documents that would fail the IRM. I think we should add a note indicating that authoring tools that need to convert such formats to IMSC documents that validate the HRM need to be careful. Maybe we should explain that the IRM was parametrized like that because TTML has more features than those old formats and the potential complexity of these features make it impossible to refresh the screen at video frame rate.
As discussed in the TTWG call, the HRM is parametrized today to fail documents that would refresh the content at every frame of a related video, when the frame rate of that video is around 25fps or above. Since formats such as CEA-608 behave like that, naively converting them to IMSC could create documents that would fail the IRM. I think we should add a note indicating that authoring tools that need to convert such formats to IMSC documents that validate the HRM need to be careful. Maybe we should explain that the IRM was parametrized like that because TTML has more features than those old formats and the potential complexity of these features make it impossible to refresh the screen at video frame rate.