Instruction manual

HDE-3000 - 80 -
Contents of the Caption Data Packets
To be fully compliant with all of the latest specifications, VANC caption encoders must
encode three types of data: EIA-708 captions, EIA-608 captions, and caption service
descriptors. All of this data is encapsulated within the caption data packet, so the VANC
caption encoder only has to encode one ancillary data packet per field/frame. Note that the
caption data packet can also contain time code, but this is not required because LTC time
code is preferred.
The VANC caption encoder must encode EIA-708 captions for HDTV’s and DTV
receivers. HDTV’s and DTV receivers are required to be capable of decoding and
displaying EIA-708 captions. Some can also decode and display EIA-608 captions, but this
is not required.
The VANC caption encoder must also encode EIA-608 captions for down converters.
When a down converter (either a professional down converter or a consumer set top box)
down converts HD video to NTSC video, the EIA-608 data must exist in the HD video so that
it can be encoded into line 21 of the NTSC video. This is why the EIA-608 data encoded in
the HD video is referred to as “compatibility data”. Many set top boxes are available that
can receive DTV transmissions and down convert to NTSC. These boxes allow the
reception and viewing of DTV transmissions on a standard TV. These boxes encode the
EIA-608 captions into line 21 of their NTSC video output so that a standard TV can display
the EIA-608 captions, block programs with certain vchip ratings, etc.
The VANC caption encoder must also encode caption service descriptors (defined in
EIA-708-B). The caption service descriptors provide a list of the caption services (EIA-608
and EIA-708) that are encoded into the HD video.
Caption Data Bandwidth Requirements
Sections 4.1 and 4.2 of EIA-708-B require the caption data to maintain a data rate of
9600 bits per second (1200 bytes per second). Note that 960 bits per second is used by the
EIA-608 caption data which leaves 8640 bits per second available for the EIA-708 caption
data. To achieve this required data rate, the size of the encoded caption data packets must
be fixed for each video field/frame rate. EIA-708-B provides a method of filling up the
unused portion of the caption data packets with “null” data. The following table lists the
sizes of the caption data packets for each field/frame rate. Note that the CDP length is the
size of the VANC packet payload, and it is also the value of the VANC packet DC (data
count) word. Also note that the total VANC packet length includes the ancillary data flag
words (000 3FF 3FF).
Field/Frame CDP total VANC
Rate cc_count length packet length
60/59.94 10 43 50
50 12 49 56
30/29.97 20 73 80
25 24 85 92
24/23.98 25 88 95