DTT subtitles instant display while analogue has delay?

mcwhirter

When live programming such as brendan o'connor and rte news is on air, when watching DTT the subtitles are displayed with no delay while on the analogue channel there is a delay. Can anyone explain why?

watty

yes.
The teletext is digital, even old EBU Teletext from 1970s. But it is packed into only a few blank picture lines. All the teletext and the subtitles share this. So speed is limited.

On DVB the subtitles are in a separate stream and unaffected by amount of text to be delivered (The MPEG picture only has picture, no extra blank lines, hence 576 lines digital = 625 lines Analogue). Also on DVB the EBU teletext, MHEG5 text, other MHEG application data, and subtitles can be allocated what ever data rate the broadcaster desires as long as the total bitrate is not used up.

Also the way latency works is different for Analogue and DVB.

zbluebirdz

You will also notice that digital channels are a bit behind compared to the analogue channels ... hence the analogue subtitles sometimes being "on time" or sometimes a tad bit too early on digital channels! :rolleyes:

watty

Takes time to decode digital. You need to buffer the MPEG4 data, hence more delay (latency) on the video.

mossym

watty wrote: »

Takes time to decode digital. You need to buffer the MPEG4 data, hence more delay (latency) on the video.

will be analog delay post tuner as well though , 3D comb and time base correction both delay the video, but it is post tuner

watty

a) Nothing like as much, less than 1ms buffer needed for comb filter. Maybe about 1s buffer for MPEG4.
b)Timebase correction < 10ms delay probably could be less 1ms.

Colour subcarrier is 4.433Mhz approx. possibly a 128us buffer is long enough.