Digital TV

Over the air TV, Cable TV and Satellite TV all use different formats to deliver programs.  This article will discuss the television system used to broadcast digital television over the air directly from the TV station to the consumer and why the channels are numbered differently from cable and satellite channels.

Why the Change

In the days of analog television, each TV station only broadcast one program stream and used a very wide chunk of radio spectrum for that single program.  Each TV station occupies 6MHz of radio spectrum.  By contrast, each FM radio station occupies 0.2MHz.  For comparison, only 3 TV stations could broadcast in the same space as 100 FM radio stations.  As communication technology advanced, competition for available RF spectrum grew.  The FCC began to look at ways to reduce the space used by the various services.  Television was the biggest spectrum hog so different methods were explored to compress program information into more efficient channel spaces.

The Advanced Television Systems Committee (ATSC) came up with a way to digitize television signals so that programs could be transmitted in a smaller channel; but rather than rebuild the entire television channel plan from scratch to slice it into smaller channels, the new standard kept the existing 6Mhz channel plan and introduced a method to multiplex or combine several different program streams in the same RF carrier.  This has the net effect of allowing hundreds of TV channels in the same RF spectrum previously used by only fifty.

 Why the channels have a decimal point

During the transition to digital TV, the existing stations kept their analog signals going for a while as they brought up their new digital transmitters.  The new transmitters had to be on a different RF channel than the analog transmitter so they wouldn’t interfere with each other. The existing stations wanted to preserve their brand identity and didn’t want the viewers confused by a new channel number so each digital TV signal was given a virtual channel to match its analog parent.  Analog channel 4 is on RF channel 4 but digital channel 4 may actually be on RF channel 18.  A digital name tag called a packet identifier or PID sent in the program data stream tells the TV to label RF channel 18 as channel 4.1 on the TV.  The reason for the 4.1, 4.2, 4.3 and so on is to identify which program stream the TV is to decode.  All of the streams are sent in the same RF carrier signal and the TV will decode the individual stream indicated by the number after the dot.

The number of sub channels a station broadcasts depends on how much quality the station wants to dedicate to each sub channel.  A large network affiliate may choose to have only 2 HD channels as their primary and secondary program streams; however, many stations send their main programming as an HD stream and lease 2 or 3 SD streams to smaller networks and independent producers.  To maximize profit, many LPTV stations will broadcast only SD sub channels so they can carry as many revenue generating program streams as possible.  The trade-off is quality. The more sub channels a station carries, the less bandwidth is available to each program stream and consequently, the lower the video quality.  Most broadcasters will opt for the balance of one full HD channel and 2 SD subs to maintain higher video quality.

Why digital TVs must scan for channels

In the days of analog TV, channel numbers corresponded to specific frequencies; for example, channel 5 was 77.25MHz.  It was possible to flip through the channels and randomly find TV stations.  After the digital transition, the TV channels displayed no longer correspond to an assigned frequency.  The FCC still licenses a station for a particular RF channel for technical reasons, but the station will encode a virtual channel for the viewer.  These virtual channels are coordinated within a specific viewing area to avoid having duplicated channel numbers.  When a TV scans for channels, it steps through all 50 RF channels and looks for a signal.  When the TV finds a signal, it downloads the identification data to determine the virtual channel to be displayed, how many sub channels and the format of each stream.  Then the TV builds a user-friendly channel list in memory.  When a user selects 48.4, for instance, to watch ION Television, the TV remembers that it had found virtual channel 48 on RF channel 17.  It tunes to RF channel 17, acquires the virtual channel 48 data stream and syncs to the .4 program stream just in time for the commercials.  It’s a good idea to periodically rescan channels because new programs are added all the time.