LEARN ABOUT IT: DIGITAL TV RECEPTION
The ATSC signal goes round and round…and it (usually) comes out
Ever wonder why you have trouble with off-air digital TV reception? It might be the weather, or it might be your antenna. It might even have nothing to do with RF at all, but rather with the digital video, audio, and program data sent by the TV station.
Receiving analog (NTSC) TV is pretty simple. The intensity of the analog RF signal determines how clean your picture is – stronger signals mean less snow and noise on the audio. The amplitude of the signal determines the luminance of the picture, and the 3.58 MHz chrominance subcarrier provides color and sync information.
Digital is completely different. Once your DTV receiver acquires a digital TV signal, it is demodulated into a stream of MPEG data packets. These packets contain video, audio, clock, and MPEG program data. Ancillary data known as PSIP (Program and System Information Protocol) provides additional information on the TV stations virtual channel, time and date, program rating, program format (SD or HD), program guide information, and audio formats.
The most important information is contained in the MPEG Program Map Table (PMT) and Program Association Table (PAT). A six-digit string of characters and numbers (example: 0x00B3) defines an MPEG ‘program’ and it is made up of video packets, audio packets, and clock (synchronization) information.
The individual packets have specific identifiers (IDs) and in industry shorthand are referred to as “VPIDs” and “APIDS”. The PMT contains the specific PIDs that make up an MPEG-2 program, while the PAT identifies each program with that previously mentioned unique six-character identifier.
As your DTV receiver analyzes the MPEG-2 data stream, it scans PAT data to determine how many ‘programs’ are present and then uses the PMT to pick off and group together the necessary PIDs to create each program. All of this happens at very high speeds with sufficient buffering so you see and hear a smooth low of video and audio.
At least, that’s the way it’s supposed to happen. If the station’s encoder is not programmed correctly, there may be conflicts between the PAT and PMT. One good example is mislabeling an audio PID as a video PID, or vice-versa. Another glitch is when an MPEG program is identified with a specific PID in the Program Association table, but the six-character bits for that program don’t match the one in the PMT.
Of course, none of these problems have anything to do with the weather, your antenna, or the level of RF signal making its way into your antenna jack. But the average viewer who is new to DTV can’t possibly know any of this and will assume it must be a problem with the set-top receiver or integrated TV, and subsequently blames the manufacturer(s).
As for channel and program data: Today’s set-top receivers and integrated ATSC TV receivers are designed to follow the ATSC specification for PSIP. If that information is incorrect – well, there’s not much the receiver can do about it. So, you may see unusual or nonsensical virtual channel numbers, incorrect program descriptions, or missing or incorrect time and date information.
If the PAT and PMT information is correct, then you’ll still be able to watch your favorite SD and HD programs, although you won’t know much about them. But if the PAT or PMT information is not correct, you won’t see and/or hear anything at all, even if the PSIP data is correct.
How does your receiver acquire that data? When you do a channel scan, the tuner or TV sequentially tubes all available TV channels looking for ATSC signals. Once the tuner or TV locks on them, the 8VSB signal is demodulated and the Transport Stream ID (unique to every TV station), Virtual Channel Table (VCT) and PAT/PMT information are read and stored into a buffer.
This process allows your DTV tuner to lock up on a station more quickly each time to select it. Additional tables within PSIP provide time and date (System Timetable), program descriptions and schedules (Extended Text Table and Event Interval Table), and program ratings (Region Rating Table).
When the FCC established its mandate for digital TV broadcasting in 1997, it inexplicably left out a requirement for PSIP. That’s like traveling on a new highway without any road signs or a map! Fortunately, the FCC has come to its senses and PSIP is now mandatory for all off-air digital broadcasts, beginning this month.
Look for more broadcasters to pay better attention to PAT and PMT information as they set up their encoders to transmit full, dynamic PSIP along with video and audio. Combined with the introduction of 4th-gen and 5th-gen 8VSB receiver chipsets into set-top receivers and TVs, this transition to digital TV might just get off the ground!
For more information on PSIP and how it works, go to http://www.psip.org/