Download open caster 2.0 user manual - Avalpa Digital Engineering Srl

Avalpa Broadcast Server user manual
Teletext support
[txt]
This tutorial describes support for digital teletext generation and multiplexing. To describe
how does teletext works and its packets layout is out of this document but we will address
how teletext packets are encapsulated into transport stream.
Once a receiver decoded the packets has two choices not exclusive: modulate the teletext
signal into an analog video output and/or render the teletext as on screen graphics,
however when the video output is HDMI, the only way to support teletext for the decoder is
on screen graphics because up to now HDMI is not supporting in band teletext.
Teletext packets are generated by pythons scripts presented later, let's suppose we have
some teletext packets stored in one file, we will present two utilities that take care of the
forthcoming process: txt2pes and pesdata2ts:
txt2pes pages.txt 15 3600 1800 > txtpage.pes &
Txt2pes will generate PES packets from TXT packets, the first parameter 15 is how many
TXT packets should be inserted in a single PES packet.
This information is related to the PES packet time information and also bit rate. Teletext
has maximum number of packet per frame that is actually split in halft per field.
3600 is the number of PTS clocks for the first PES packet while 1800 is the increment of
the PTS clock every PES packet, 1800 is actually 50 PES packet per second, so the result
is the command line states that every 15 TXT packet a new PES packet will be generated
and every PES packet matches a video field.
pesdata2ts txtpage.pes 1978 > txt.ts &
Pesdata2ts, the second tool we are using, will just encapsulate PES packets in TS packet
with the given PID, in this case is 1978.
To know the bit rate for the teletext transport stream math is easy, after starting to execute
txt2pes it will report about PES packet size, for example:
“pes packet size without 6 byte header is 1144 ”
this means:
1144 + 6 = 1150 bytes per packet
1150 / 184 (TS packet payload) = 7 TS packet every PES packet
7 * 188 * 8 -> 10528 per PES
if PES goes at 3600 means: 10528 * (90000 / 3600) -> 263200 bps
For the previous example we have:
730 + 6 -> 736 / 184 -> 4 * 188 * 8 -> 6016 * (90000 / 1800) -> 300800 bps
So the final processes layout assuming the teletext packets are already generated and
34
http://www.avalpa.com