Channel Scanning and Virtual Channels

After connecting a tuner to cable or an antenna, you run a channel scan so it can find what is available. On cable, the results can be confusing: channels may show up with numbers you have never seen, or with no names at all. The reason is the difference between physical channels and virtual channels.

Physical channels

A physical channel is a 6 MHz slot of frequencies. On cable, the CTA-542 channel plan numbers those slots, with channels running as high as 158 on systems that reach 1 GHz. The plan has three variations, known as Standard (STD), Harmonically Related Carriers (HRC) and Incrementally Related Carriers (IRC). Most tuners detect the right one during a scan, but some let you choose.

Each digital physical channel usually carries several programs. A tuner tells them apart by program number, so a clear QAM channel might be listed as something like 79-3 or 79.3: physical channel 79, program 3.

Virtual channels

A virtual channel is the number viewers see and remember. Broadcasters use the ATSC Program and System Information Protocol (PSIP), which supports two-part numbers such as 7.1, where the major number is often the station's former analog channel and the minor number identifies each program the station sends. The PSIP information travels with the signal, which is why an antenna scan shows familiar numbers and station names.

Cable systems can send similar channel tables in each channel, but many do not, and cable lineups often use one-part numbers delivered to set-top boxes out of band under SCTE 65. A clear QAM tuner usually only sees what is sent in-band. So on cable:

  • A local station may appear under its broadcast number, such as 7.1, if the operator passes that information along.
  • More often it appears under the cable physical channel and program number, with or without a name.
  • The same station can be on a completely different physical channel from one cable system to the next, and can move when the operator rearranges its lineup.

It is not true that every digital channel on cable ends in ".1". The number you see depends on what the operator sends.

Tips for a good scan

  1. Select the right source. Choose "cable" for a cable outlet and "antenna" or "air" for an antenna. Scanning the wrong type finds nothing.
  2. Connect directly. A set-top box, DTA or VCR between the wall and the tuner will block or change what the tuner sees.
  3. Check signal quality first. A weak or noisy signal can make a scan miss channels. See Cable Signal Levels and Troubleshooting.
  4. Rename or reorder channels if your tuner or app allows it. Many network tuner apps can add channel names and guide data.
  5. Scan again when channels go missing. Cable operators move and re-encrypt channels, and broadcast stations sometimes change frequencies. The FCC advises antenna viewers to rescan periodically for this reason.

Sources

Good to know. Cable lineups, encryption policies, equipment fees and device support are set by each provider and manufacturer and can change. Check with your provider and the sources listed before you buy equipment.
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