Scan for DTV Channels Using a Digital TV Converter Box

Some time[1] on June 12, 2009 most US TV stations stopped broadcasting analog TV and also changed their DTV broadcast frequencies, requiring all converter boxes and digital televisions which use an antenna to be rescanned. Digital TV (DTV) transmissions use the same VHF and UHF frequency channels as analog TV did, but a DTV-capable receiver is required in order to receive them. Existing TVs, VCRs, TiVos, DVRs that can only receive traditional analog signals require a converter box in order to convert the new digitally modulated signals into a form that traditional analog tuners can understand and receive.

Background

Prior to June 12, 2009 most TV stations broadcasted in both analog and digital (DTV) formats. For the DTV broadcasts the FCC allocated each station a temporary additional TV channel frequency on which to broadcast digitally (usually some unused UHF channel in the area). The new DTV format allows stations to identify themselves by transmitting their "virtual" channel number, as well as other program information, along with the picture and sound. This allows a station to identify its DTV broadcasts as being a "virtual" channel number that is different from the actual RF (radio frequency) TV channel frequency it is using for its digital broadcasts.

For example, a station in an area is channel 11. Prior to June 12, 2009 channel 11 broadcasted its analog TV signal on the frequency designated as "RF channel 11" which is centered at 201 MHz (megahertz). Prior to June 12, 2009 the FCC also allowed this station to broadcast digital TV on a second frequency, which happened to be "RF channel 19" centered at 503 MHz. On this frequency the station broadcasted DTV programming as well as information identifying it by its call letters and identifying itself as "virtual digital channel 11-1", even though its broadcast was physically occupying the RF channel 19 frequency. It seems confusing to have the station identifying itself as "channel 11" broadcasting at the same time on 2 different RF TV channel frequencies, RF channel 11 and RF channel 19, but this was necessary prior to June 12, 2009 because stations needed a second frequency to allow them to broadcast using the new DTV format while continuing to broadcast analog TV on their original frequencies.

Why Rescanning is Necessary After June 12, 2009

At some time on June 12, 2009 most US TV stations did the following:

1. Stations stopped broadcasting traditional analog TV signals. From the example above, channel 11 turned off their analog TV transmitter on RF channel 11. There was no signal on that frequency (at least for a brief time).

2. Many stations re-tuned their DTV transmitters from the temporary second channel allocated to them by the FCC for initial broadcasting of DTV to the channel frequency that their analog broadcasts had used. The channel 11 example station turned off its DTV transmissions on RF channel 19, re-tuned its DTV transmitter to RF channel 11, and then began broadcasting DTV on the channel 11 frequency. It no longer uses the temporarily allocated pre-transition channel 19 frequency.

What does this mean to you? It means that you must "re-scan" your DTV converter box if you have not done so since June 12, 2009, because many of the channel frequencies that it originally memorized when you set it up and scanned prior to June 12, 2009 have changed.

Note that not all DTV transmissions changed channel frequencies for their DTV transmissions[2]. Some continued to broadcast their DTV signals on the alternate channel granted to them by the FCC. Others changed their digital transmissions to their old analog channel frequencies. Yet others changed their DTV transmissions to a new channel frequency that is different from both the analog and temporary pre-transition digital channel frequencies. Now that the transition to digital TV has occurred, many stations still use a DTV channel frequency that is different from the "virtual" channel number that they identify themselves as when scanned. For example, a station might identify itself as "DTV virtual channel 68-1" yet actually be transmitting their DTV RF signal on TV channel 42, not 68. In this example this table would show the "68-1" under the column "Channel Numbers/Virtual" and would show "42" as the actual RF TV channel used for broadcast under the column "Channel Numbers/Digital". The old analog "Channel 68" continues to advertise itself as virtual digital "Channel 68-1" even though it actually transmits its DTV signal on RF TV channel 42.

You do not need a different antenna for DTV, since DTV broadcasts will use the same VHF and UHF frequencies as analog TV did.

Steps

  1. Connect your DTV converter box to a good antenna. If you are using rabbit ears that you have to adjust differently for each channel, you might wish to ask a neighbor who has a good rooftop TV antenna if you can temporarily connect your DTV converter box to his antenna to do the scan (most good DTV converter boxes will "remember" the channel scan information even if you unplug the power to take it back to your house).
  2. Use the remote control for your DTV converter box to enter the "channel scan" mode. Some remote control units have a button for "CH SCAN", while for others you must enter the "MENU" mode and navigate to the "AUTO CHANNEL SCAN" or "AUTO TUNING" function. The first scan after June 12, 2009 should be a "full" auto scan, not a "channel add" scan. A "full" scan completely re-scans, removing previously memorized channel frequencies (since many channels used for DTV broadcast changed on June 12, 2009).
  3. Start the "Scan" function. The DTV converter box will examine all the TV channel frequencies (2 through 69) and if it finds a DTV broadcast, it will memorize the that frequency and the call letters and DTV virtual channel number associated with it.
  4. After the full autoscan has completed, use the "channel up/down" buttons on your DTV converter box remote control to verify that all the channels you expect to see are there. The virtual channel number appears first, then a dash (-), then subchannel numbers starting with 1. If you expect to see a channel 11, for example, be sure that "11-1" is one of the channels that appears as you traverse the channels.
  5. If not all the channels appear, follow the "tip" below regarding adding of channels using a "channel add scan" or "manual tuning" function to add channels to the list of memorized DTV channels that your converter box stores.

Tips

  • Some DTV converter boxes have a "signal strength" bar that helps you adjust your rabbit ears (or other antenna) for the strongest DTV signal on the channel you are watching. This function works slowly, so pause a few seconds after each adjustment to allow the indicator to update.
  • If your antenna is not very good, your DTV converter box might miss some channels that are really there but just not received because the antenna isn't adjusted well enough for that station during the scan. In that case try to temporarily connect to a better antenna during the scan, or if your converter box allows it, force the converter box to manually tune to the RF channel frequency of the station you are trying to add while adjusting the antenna. See this table to find the actual post-transition RF TV channel number that the station will be using for its digital broadcasts.
  • Once you get a good scan of all the DTV channels after June 12, 2009, if your antenna is not very good (such as rabbit ears) and you need to add channels in the future, it may be better to try to add additional channels (using a "channel add" scan or manual entry of RF TV channel number if your converter box has either of these capabilities) rather than doing a complete automatic rescan (which erases previous scan results) to avoid the risk of missing some channels that you scanned last time (due to your antenna being in a different position).
  • Some DTV converter boxes allow you to add channels without doing a complete re-scan, by forcing it to look for a DTV signal on a particular RF channel frequency, or doing a "channel add" scan (without deleting previous scan results). This is useful if your antenna must be adjusted to different positions to receive different channels (you can do multiple scans with the antenna in different positions to add channels to the memorized list without deleting any previous scan results). This document[3] lists various converter box models their features. Scroll down to "ADDITIONAL/ADVANCED FEATURES" to determine channel scanning/adding capabilities. Under "Adding Channels" the word "None" indicates the box can only do a full scan, replacing previous scan results, not add to the previously memorized channel list. The "channel add" feature (called "EZ Add" on Zenith models), denoted by the word "Scan" under the "Adding Channels" column, indicates the capability to do a scan which adds DTV channels found to the existing channel list without erasing the results of previous scans. The word "Direct" under the "Adding Channels" column indicates the capability to manually add a channel to the list of available channels by entering a known RF TV channel frequency (which can be found here for a station in your area under the column "Channel Numbers/Digital").
  • If you have any questions or suggestions please feel free to post them on the "Discussion" page for this article (press "Discuss" tab near top).

Further Information on DTV Channel Numbering

In traditional analog television there was a 1-to-1 correspondence between the channel number which the station advertised and the RF channel frequency it occupied. For example, an analog station advertising itself as "Channel 11" occupied the frequency allocated to channel 11 by the FCC. The frequency occupied by channel 11 covers 198 to 204 MHz and is centered at the mid-point between those frequencies (at 201 MHz). This meant that if you knew the channel number of a TV station you could force your analog TV to that station frequency by entering the channel number, no scanning necessary, and the TV would go to the correct frequency.

In DTV, however, stations broadcast information such as their call letters and their "virtual" channel number continuously as background data, and the RF channel they use might be different from the virtual channel number. A channel might advertise itself as "DTV 68" or "68-1" but actually use RF channel 42. Your DTV converter box (or digital TV) can't know this without scanning all of the channel frequencies to find it. When doing a scan, your DTV tuner is checking each of the possible RF channel frequencies for a signal, and if one is found, memorizing the advertised channel number.

The advantage to this is that stations can continue to advertise themselves as the same "virtual" channel number as their previous analog channel number, yet in reality transmit their RF signal on a TV channel number which is different from the "virtual" number.

After the transition some DTV stations began to transmit their digital signals on the same RF TV channel as their advertised virtual channel number (DTV virtual channel 11-1, in the example above, moved to the RF TV channel 11 on June 12, 2009, replacing its original analog transmission on channel 11 with a digital transmission on channel 11). Others did not (DTV virtual channel 68-1 transmits on RF TV channel 42).

A disadvantage to virtual channels is that you must be able to receive all the channels in your area in one scan. If your antenna has to be oriented in one position or direction to receive some channels and in another direction to receive others, it may be almost impossible to pick up and memorize all the channels in a single scan. In that case you must rely on the ability of your converter box or DTV set to either perform a "channel add" scan or manually tune to the RF channel of a station (not all have this capability). For the latter, find the RF channel number of the station from this table under the column "Channel Numbers/Digital" enter this number into the RF channel box in the "manual channel add" or "manual tuning" function of your converter box, then adjust your antenna until the signal can be received and added to the memorized channel list.

Due to higher bandwidth efficiency, DTV broadcasts can fit more than one channel of video/audio information into the same 6 MHz wide RF channel, but the additional channels are usually at lower video resolution than the primary or contain images which do not change quickly (such as weather maps). For example, if the primary channel is virtual "5-1", there may be a virtual "5-2" and "5-3" as well, all occupying the same RF channel (which could be any RF channel, not necessarily RF channel 5). Usually the suffix "-1" (such as "5-1") indicates the primary channel and has the highest resolution. The higher numbered virtual channels ("5-2", "5-3" in this example) usually must have lower resolution or be slowly-changing images in order to fit within the boundaries of one RF channel.

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Sources and Citations

  1. ↑ http://fjallfoss.fcc.gov/edocs_public/attachmatch/FCC-09-47A1.pdf specifies that stations may transition any time of day on June 12, 2009.
  2. ↑ http://www.dtv.gov/stationlist.htm lists all US TV stations, channels used, and transition dates.
  3. ↑ http://www.fcc.gov/cgb/consumerfacts/converterboxfeatures.html lists capabilities of various converter boxes; scroll down to "ADDITIONAL/ADVANCED FEATURES" for channel scanning/adding.
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