Extended Text Descriptions

As we mentioned earlier, a broadcaster can transmit a longer description of a channel in a VCT, or of an event listed in an EIT or AEIT. These descriptions are broadcast as part of the Extended Text Table (ETT). This table contains a number of entries called Extended Text Messages, or ETMs. Each ETM consists of a pair of values: an ETM identifier and a multi-lingual text message.

An ETM identifier is a 32-bit value, which takes a slightly different format depending on what the ETM describes. The format of ETM IDs for channels and eventds is shown below.

Format of the ETM Identifier. Source: ATSC A/65b (PSIP Specification)
MSB LSB
bit
31 16
15 2
1 0
channel ETM_id source_id 0...0 0 0
event ETM_id source_id event_id 1 0

Not every event may have a description, and searching the ETT for a given ETM identifier may not be very efficient. For this reason, each entry in the VCT and EIT includes a flag (the ETM location field) to say whether an ETM for that event is available. This means that the receiver only has to look up extended descriptions for those events where it is actually available.

Format of the Extended Text Table. Source: ATSC A/65b (PSIP Specification)
Syntax No. of bits Format
extended_text_table_section () {
table_id 8 0xCC
section_syntax_indicator 1 '1'
private_indicator 1 '1'
reserved 2 '11'
section_length 12 uimsbf
ETT_table_id_extension 16 0x0000
reserved 2 '11'
version_number 5 uimsbf
current_next_indicator 1 '1'
section_number 8 0x00
last_section_number 8 0x00
protocol_version 8 uimsbf
ETM_id 32 uimsbf
extended_text_message()
CRC_32 32 rpchof
}

Extended descriptions in a satellite network

As with the EIT and AEIT, in satellite systems a number of different sources will share a single table for their extended descriptions. This table is called the Aggregate Extended Text Table, or AETT, and it has the following structure:

Format of the Aggregate Extended Text Table. Source: ATSC A/81 (Direct-To-Home Satellite Broadcast Standard)
Syntax No. of bits Format
aggregate_extended_text_table_section () {
table_id 8 0xD7
section_syntax_indicator 1 '1'
private_indicator 1 '1'
reserved 2 '11'
section_length 12 uimsbf
AETT_subtype 8 uimsbf
MGT_tag 8 uimsbf
reserved 2 '11'
version_number 5 uimsbf
current_next_indicator 1 '1'
section_ number 8 uimsbf
last_section_number 8 uimsbf
if (AETT_subtype == 0) {
num_blocks_in_section 8 uimsbf
for (i = 0; i < num_blocks_in_section; i++) {
ETM_id 32 uimsbf
reserved 4 '1111'
extended_text_length 12 uimsbf
extended_text_message()
}
}
else
reserved n*8  
CRC_32 32 rpchof
}

One difference that you will notice is that each ETT section only carries a single extended text message.

Extended text messages

In both the ETT and AETT, the structure of the text messages that are contained in the table are the same. An extended text message is carried as a multiple string structure, as follows:

Format of the multiple string structure used for carrying extended text descriptions. Source: ATSC A/65b (PSIP Specification)
Syntax No. of bits Format
multiple_string_structure () {
number_strings 8 uimsbf
for (i = 0; i < number_strings; i++) {
ISO_639_language_code 24 uimsbf
number_segments 8 uimsbf
for (j = 0; j < number_segments; j++) {
compression_type 8 uimsbf
mode 8 uimsbf
number_bytes 8 uimsbf
for (k = 0; k < number_bytes; k++) {
compressed_string_byte[k] 8 bslbf
}
}
}
}

This is a structure for carrying multilinugal strings, and so text descriptions can be carried in more than one language simultaneously. If a given text description is too long for a receiver to handle, that receiver may truncate the message that it displays - a receiver doesn't have to display the entire description.