Advanced Functions - Redirecting Channels

The tables we have seen so far provide the basic functions that a network operator needs to make a digital TV service work. It's possible to do some more advanced things, however, that may be useful in some cases. The most interesting of these from an SI perspective is the ability to force a receiver to change channels, either unconditionally or based on a set of criteria that are specified by the network operator. While this may not seem very useful, it can enable a broadcaster to provide different functions based on language settings in the receiver, geographical location, demographics or any one of a number of other features.

The table that makes this possible is called the Directed Channel Change Table (DCCT). This is an optional table, and not every receiver will support it. This table has the following format:

Format of the Directed Channel Change Table. Source: ATSC A/65b (PSIP Specification).
Syntax No. of bits Format
directed_channel_change_table_section () {
table_ID 8 0xD3
section_syntax_indicator 1 '1'
private_indicator 1 '1'
reserved 2 '11'
section_length 12 uimsbf
dcc_subtype 8 0x00
dcc_id 8 uimsbf
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
dcc_test_count 8 uimsbf
for (i = 0; i < dcc_test_count; i++) {
dcc_context 1 uimsbf
reserved 3 '111'
dcc_ from_major_channel_number 10 uimsbf
dcc_from_minor_channel_number 10 uimsbf
reserved 4 '1111'
dcc_to_major_channel_number 10 uimsbf
dcc_to_minor_channel_number 10 uimsbf
dcc_start_time 32 uimsbf
dcc_ end_time 32 uimsbf
dcc_term_count 8 uimsbf
for (j = 0; j < dcc_term_count; j++) {
dcc_selection_type 8 uimsbf
dcc_selection_id 64 uimsbf
reserved 6 '111111'
dcc_term_descriptors_length 10 uimsbf
for (k = 0; k < N; k++) {
dcc_term_descriptor() uimsbf
reserved 6 '111111'
dcc_test_descriptors_length 10 uimsbf
for (j = 0; j < N; j++) {
reserved 6 '111111'
dcc_additional_descriptors_length 10 uimsbf
for (i = 0; i < N; i++) {
CRC_32 32 rpchof

Each entry in the DCCT is a test that decides whether a receiver should change from the source channel to the destination channel.

These tests can be time-limited, so that a channel change is active for a fixed period (e.g. for the length of one event). This allows the network operator to target specific receivers during specific shows, and to show different content in those cases. One example could be a football game - viewers in one area could watch one game featuring their local team, while viewers in another area could be automatically redirected to a different channel showing a game featuring the team local to their area. After the game is over, the redirection is automatically cancelled because it is timed to last the length of the game.

The dcc_context field tells the receiver how it should handle a channel change, should it need to make one. A value of zero means that the redirect is temporary, and that if the DCCT tells it to return to the original channel at a later time (or if the user changes channel) then the redirection is cancelled - in this case, the new channel has the same channel number as the original (redirected) channel. A value of 1 for this field tells the receiver that the redirection is permanent, and the new channel is a completely different channel (with a completely different channel number) from the original channel.

We won't look at the tests that can be performed in any detail here, but this list should give you an idea of the kind of tests that can be performed:

  • Unconditional redirection
  • Unconditional return to original channel
  • Inclusion/exclusion based on postal code
  • Inclusion/exclusion based on location (not based on postal code)
  • Inclusion/exclusion based on genre
  • Inclusion/exclusion based on demographic
  • Rating blocked

More information about how these tests operate, and a detailed look at how they can be specified, is available in section 6.7 of the ATSC PSIP specification (ATSC document number A/65B).

If the standard values for genres and locations defined in the ATSC PSIP standard aren't enough, a broadcaster can extend these by broadcasting the DCC Selection Code Table. This carries additional data that can be used by the receiver in conjunction with the DCCT when checking to see whether a particular test condition is met. We won't look at that table in any detail here - more information can be found in section 6.8 of the ATSC PSIP specification.