Compensating delay in DVB and internet subtitle transmission

Transmission of video over DVB and also in the internet is generally delayed for at least one second, in some case also more than 5 seconds. This means that also subtitles have to be transmitted with this delay by FAB.

Because subtitles which are produced by speech recognition and live transcription are transmitted with a certain delay which is caused by the speech recognition and live transcription, the FAB system can be configured so that this delay is subtracted from the delay which is configured for transmission of subtitles over DVB.

This means that subtitles coming from speech recognition and live transcription will be transmitted by FAB by less delay than other subtitles because speech subtitles are already delayed.

Configuring FAB Subtitler BCAST/XCD

To configure transmission delay for DVB subtitles in FAB Subtitler BCAST/XCD use the setting “MPEG Encoding Chain Delay” for every DVB subtitle output configured in Options / System / Network:

To configure transmission delay for teletext subtitles in FAB Subtitler BCAST/XCD use the setting “Set delay” for the output device configured in Options / System / Network:

To configure the compensation of transmission delay for live transcription in FAB Subtitler BCAST use the setting “Delay of live transcription” in Options / Special / Live transcription:

This last delay values will be subtracted from the transmission delay for DVB and teletext subtitles.

Configuring FAB Subtitler LIVE

Subtitles transmitted from the speech interface can also include a compensation delay. However because FAB Subtitler LIVE generally transmits subtitles to FAB Subtitler BCAST without any delay, the zero delay cannot be reduced to even less.

Therefore FAB Subtitler LIVE must transmit subtitles over network so that such subtitles contain a metadata field which defines the speech recognition delay which shall be compensated by FAB Subtitler BCAST.

To achieve this it necessary to use the ESUB-XF protocol over TCP to transmit subtitles from FAB Subtitler LIVE to FAB Subtitler BCAST:

To configure FAB Subtitler LIVE to compensate delay for subtitles transmitted from the speech interface by speech recognition and live transcription (but not for subtitles transmitted from the prepared subtitle list):