That, among many other things, is the purpose of the Program and System Information Protocol, or PSIP, packets. The PSIP describes eight different tables, of which five are necessary for reception of an ATSC signal. Those tables are:

• STT – System Time Table

• MGT – Master Guide Table

• VCT – Virtual Channel Table

• RRT – Ratings Region Table

• EIT – Event Information Table

• #ETT – Extended Text Table

• #DCCT – Directed Channel Change Table

• #DCCST – Directed Channel Change Selection Code Table

# these tables are optional.

STT – the system time table is simple. It contains the transmitter's opinion of the correct time, sent once every second.

MGT - this table tells the ATSC receiver where to look for the other tables.

VCT – this table tells the receiver where to look for the audio and video streams associated with a given program. It also tells the receiver what to call that program. Some of the fields in the VCT include the:

• Major channel number – generally same as the station's analog channel, this is displayed to the user.

• Minor channel number – this distinguishes between different programs on the same ATSC station. Zero is reserved for the station's analog signal. Generally, 1 is used for the first digital program, and additional minor channel numbers counting up from there.

• Transport Stream Identification - this tells the receivers which audio and video streams go with a given major and minor channel number.

• Short Name – this is the name of the program, something like “KXYZ-HD”. It's the “text ID” commonly mentioned in the VUD.

RRT – The RRT is the implementation of the “V-chip” for digital TV. It defines which program acceptability ratings are possible for a given region or country. The actual rating is in the EIT and/or PMT.

EIT – At least four EITs are (theoretically) required. (EIT-0, EIT-1, EIT-2, EIT-3) Additional EITs through EIT-127 are permitted, and 24 recommended¹³. Each EIT is the guide to 3 hours worth of programming. With a maximum of 128 EITs, 16 days of program guide material can be transmitted. If a station has more than one virtual channel, it may have more than one EIT with the same number. One EIT-0 may apply to WTVF-HD, virtual channel 5-1, while another EIT-0 would apply to WTVF-5+, virtual channel 5-2.

The Extended Text Table (ETT) allows the transmission of more descriptive information about virtual channels or program events. I've never seen it used. Likewise for the DCCT and associated DCCST. These tables are intended to allow the station to cause your receiver to change channels on cue – for example, to carry a special regional commercial, or local weather.

ATSC recommends minimum repetition rates for these tables. The most frequent is the MGT, which is to be transmitted every 150 milliseconds. The VCT is to be sent every 400ms. (this means the “text ID” is sent 2½ times a second, far more frequently than any other identifying information in the DTV signal.) As mentioned above, the STT clock information is to be sent every second. The frequency of the EITs depends on which EIT: EIT-0 is sent twice a second; EIT-1 every three seconds; and EITs 2 and 3 every minute. There appears to be no standard for EITs 4-127.