ASTC (HDTV) : Closed Captioning Datastream Spec (1997)

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ASTC (HDTV) : Closed Captioning Datastream Spec (1997)

Table 6.17 Bit Stream Syntax for the Caption Service Descriptor

Syntax Bits Format

caption_service_descriptor () {

descriptor_tag 8 0x86

descriptor_length 8 uimsbf

reserved 3 ‘111’

number_of_services 5 uimsbf

for (i=0;i<number_of_services;i++) {

language 8*3 uimsbf

cc_type 1 bslbf

reserved 1 ‘1’

if (cc_type==line21) {

reserved 5 ‘11111’

line21_field 1 bslbf

}

else

caption_service_number 6 uimsbf

easy_reader 1 bslbf

wide_aspect_ratio 1 bslbf

reserved 14 ‘11111111111111’

}

}

descriptor_tag — An 8-bit field that identifies the type of descriptor. For
the

caption_service_descriptor() the value is 0x86.

descriptor_length — An 8-bit count of the number of bytes following the
descriptor_length itself.

number_of_services — An unsigned 5-bit integer in the range 1 to 16 that
indicates the number of closed caption services present in the associated
video service. Note that if the video service does not carry television
closed captioning, the caption_service_descriptor() shall not be present
either in the Program Map Table or in the Event Information Table. Each
iteration of the “for” loop defines one closed caption service present as a
sub-stream within the 9600 bit per second closed captioning stream. Each
iteration provides the sub-stream’s language, attributes, and (for advanced
captions) the associated Service Number reference. Refer to Ref. [13] for a
description of the use of the Service Number field within the syntax of the
closed caption stream.

language — A 3-byte language code per ISO 639.2/B (Ref. [7]) defining the
language associated with one closed caption service. The
ISO_639_language_code field contains a three-character code as specified by
ISO 639.2/B. Each character is coded into 8 bits according to ISO 8859-1
(ISO Latin-1) and inserted in order into the 24-bit field.

cc_type — A flag that indicates, when set, that an advanced television
closed caption service is present in accordance with Ref. [13]. When the
flag is clear, a line-21 closed caption service is present. For line 21
closed captions, the line21_field field indicates whether the service is
carried in the even or odd field.

line21_field — A flag that indicates, when set, that the line 21 closed
caption service is associated with the field 2 of the NTSC waveform. When
the flag is clear, the line-21 closed caption service is associated with
field 1 of the NTSC waveform. The line21_field flag is defined only if the
cc_type flag indicates line-21 closed caption service.

caption_service_number — A 6-bit unsigned integer value in the range zero to
63 that identifies the Service Number within the closed captioning stream
that is associated with the language and attributes defined in this
iteration of the “for” loop. See Ref. [13] for a description of the use of
the Service Number. The caption_service_number field is defined only if the
cc_type flag indicates closed captioning in accordance with Ref. [13].

easy_reader — A Boolean flag which indicates, when set, that the closed
caption service contains text tailored to the needs of beginning readers.
Refer to Ref. [13] for a description of “easy reader” television closed
captioning services. When the flag is clear, the closed caption service is
not so tailored.

wide_aspect_ratio — A Boolean flag which indicates, when set, that the
closed caption service is formatted for displays with 16:9 aspect ratio.
When the flag is clear, the closed caption service is formatted for 4:3
display, but may be optionally displayed centered within a 16:9 display.