Advantages:
l Subtitle format conversion available:
- Teletext to DVB
- Imitext to DVB
Disadvantages:
l Viewers must have suitable decoders.
Advantages:
l No separate subtitle inserter required.
l Suitable for situations where the video
encoding standard needs to be changed.
l No special subtitle decoder required by
viewers.
Disadvantages:
l The video must be decoded and recoded:
- Cost of equipment
- Loss of video quality
- Delay in the video path
Open Subtitles - using a subtitle
inserter at a headend.
Closed subtitles for a range of languages
are supplied by the original broadcaster
in a suitable format such as Teletext, DVB
bitmap or Imitext. At each headend a
professional receiver/decoder (IRD) decodes
the video and a separate subtitle inserter
unit (Polistream) adds open subtitles into
the video. The resulting combined video
with open subtitles is then re-encoded
for broadcast. The subtitle language is
selected from those supplied by the original
broadcaster.
Advantages:
l High quality subtitle text.
l Animated or static logos.
l Other localisation components can also be
added such as tickers and clocks.
l Suitable for situations where the video
encoding standard needs to be changed.
Disadvantages:
l The video must be decoded and recoded:
- Cost of equipment;
- Loss of video quality
- Delay in the video path
Open Subtitles - using an MPEG2
subtitle inserter at a headend.
Closed subtitles for a range of languages are
supplied by the original broadcaster in a
suitable format such as Teletext, DVB bitmap
or Imitext. At each headend a professional
receiver (IRD) generates a decrypted
compressed transport stream (ASI) and
a separate MPEG-2 subtitle inserter unit
(Polistream) adds open subtitles into the
compressed video. The subtitle language is
selected from those supplied by the original
broadcaster.
Advantages:
l The video signal is not decoded and stays
in MPEG-2 format.
l Multiple video channels can be modified in
a single unit.
l Static logos can be added.
l Minimal through delay.
l Cost saving on decoders, encoders and
multiplexes.
l The original broadcaster maintains control
of the final video image.
Disadvantages:
l Short through delay (approx. 800mS).
l Currently only available for MPEG-2
streams.
l Small increase in video bit rate.
Closed Subtitles - with video format
conversion
Closed subtitles for a range of languages
are supplied by the original broadcaster
in a suitable format such as Teletext, DVB
bitmap, Imitext or SCTE 27 (EIA 608 &708)
closed caption.
In this case the subtitles are delivered
as closed subtitles and the viewer, with a
suitable receiver or STB, can select which
language subtitles they wish to see. This
configuration applies when the video format
needs to be converted (e.g. NTSC to PAL, HD
to SD, MPEG-2 to MPEG-4) at the headend.
At each headend a professional receiver/
decoder (IRD) decodes the video. Subtitle
data is removed from the broadcast signal
and then re-inserted into the recoded output
(Polistream). A delay is added to the subtitle
stream to match the delay in the video path.
Closed Subtitles - with re-
multiplexing and subtitle
transcoding
Closed subtitles for a range of languages
are supplied by the original broadcaster
in a suitable format such as Teletext,
DVB bitmap, Imitext or SCTE 27 (EIA
608) closed captions.
In this case the subtitles are delivered
as closed subtitles and the viewer, with a
suitable receiver or STB, can select which
language subtitles they wish to see.
No video format conversion is
required. At each headend a professional
receiver generates a transport stream
(ASI) which is then re-multiplexed with
other services to produce the required
output multiplex(es).
If required, subtitle format conversion
can be performed at this point
(Polistream): For example to convert from
Teletext to DVB Bitmapped subtitles.
Advantages:
l Cost saving on decoders, encoders and
multiplexes.
l The video signal is not decoded and stays
in MPEG-2 format.
l Subtitle format conversion available:
- Teletext to DVB
- Imitext to DVB
• Lower cost.
Delays and Content Control
Where:
l channels are rebroadcast there is often
a requirement to delay the service or to
exercise some degree of control over the
content of the channel.
l the video signal stays in the
compressed domain as in points 3 and 5
(above) a delay can be added to the ASI
transmission path.
l the video signal is decoded to
uncompressed video as in points 1, 2 and
4 (above) then any delay needs to be in
the SDI (or HD-SDI) path, normally in a
video server.
Once uncompressed video is available then
content control can be added. This allows
video editing, splices and insertions with
relatively short pass through delays of only
30-60 minutes. Screen can offer solutions
for both delays and content control complete
with subtitle support.
LATAM Briefing 13