Maintaining ATSC compliance
Nov 1, 2008 12:00 PM, By John Wilkie
Keeping up with the A/53, A/52 and A/65 standards will ensure proper signal reception.
AC-3 descriptors
Sometimes, consistency even means technically inaccurate. The best example is the AC-3 descriptor, which contains a field that indicates the number of audio channels in an AC-3 stream. So, the thinking goes, that means the AC-3 descriptor must change when an interstitial message with only stereo audio airs within a TV program with 5.1 audio. This thinking is incorrect.
The AC-3 descriptor is bound to the concept of a TV program (event), and a message or spot within a program is still part of the program. A/65 provides for the AC-3 descriptor to also appear in the PMT, but that descriptor is still bound to the televised event, because it must have the same values as the AC-3 descriptor in the event information table (EIT).
TVCT descriptors
With some PMT section overlap, the TVCT lists all virtual channels that broadcasters want to inform viewers about. However, the TVCT entry for a channel includes a short name, such as “KGET-DT,” and can include information on analog channels. In the receiver, the program_number in a TVCT entry is used to match the entry with the appropriate PMT section, and the source_id is used to link to announced events. TVCT entries cannot have any duplicate source_ids, and the sections must be transmitted at least once every 400ms.
On the redundancy side, the TVCT entry for a digital channel must include the service location descriptor, which duplicates much of the functionality of the PMT for the same channel.
When transmitting a channel description, the channel extended text table (CETT) and its unique PID must be properly listed in the master guide table (MGT), and the ETM_location field must be set to one or two. Otherwise, the CETT won't be associated with the virtual channel in most receivers. Likewise, descriptions of televised events won't be matched up with the appropriate EIT entry unless the ETM_location for the EIT entry is set correctly.
All PSIP tables (except the system time table) must have their summary data listed in the MGT, which should be transmitted at least once every 150ms. And now with an ATSC-compliant transport stream, proudly assert the “GA94” (Grand Alliance '94 or ATSC) registration descriptor, located in PMT sections and the TVCT. Sadly, this registration descriptor is seen more often than true ATSC compliance.
John Willkie is the founder of EtherGuide Systems and is a member of the ATSC S1 and SMPTE S22-WG10 subcommittees.
| Want to use this article? Click here for options! |  |
