DTV Primer

Chris Llana, Editor



De-mystifying Digital Channel Numbers

June 4, 2005

[All of the following applies for tuning in TV channels on a digital TV getting programming over-the-air, i.e. with an antenna. If you get network digital channels via cable, then obviously you'll use the numbers the cable company assigns. And if you don't have a digital TV, or have an "HD-Ready" TV that only came with an analog tuner, this information will be for future reference.]

TV stations are now broadcasting on their old analog channels, as well as on another digital channel. Each channel consists of a 6 MHz block of spectrum, i.e. a range of radio wave (RF) frequencies.

There are just so many TV channels, starting at channel 2 and going up to channel 69. At the end of the transition, all TV broadcasts will be moved into channels 2 through 51 (called "in-core" channels) and channels 52 through 69 will be turned back in to the government. The channel 52 - 69 frequencies (698-806 MHz) will then be auctioned off for other wireless services or used for additional radio channels for fire, rescue, or emergency response organizations.

While each TV channel can be used for either an analog or digital signal, digital broadcasts are much more efficient than analog transmissions. Using digital video compression protocols, a single high-definition channel (with five times the picture detail) can fit in the same channel space as a standard-definition analog channel.

Or, that also means you can fit five digital standard-definition channels into the same 6 MHz as one analog channel.

The FCC is now in the process of assigning permanent digital channels to all TV stations, to take effect at the end of the transition. Even if a TV station elects to use its analog channel for its digital broadcasts at the end of the transition, it cannot do so now (obviously that channel is being occupied by its analog broadcasts).

So it has been assigned a temporary channel to use for its digital broadcasts until the transition is completed (expected 12/31/08).

Other stations may want to keep using their temporary digital channel assignment at the end of the transition so they do not have to change any of their digital transmission equipment.

In any case, every station wants to keep its channel number "brand" identity throughout and after the transition, no matter what actual channel their digital broadcast is using. Eg. "Channel 11" wants to keep being "Channel 11," and not "Channel 11" analog, "Channel 37" digital, and "Channel 29" after the transition ends.

Consumers don't want that confusion.

What's a body to do?

Answer: PSIP. Or "Program and System Information Protocol." This is a collection of hidden instructions that the local broadcaster puts in its transmissions, which is read by your digital TV. (It also includes programming information that can be displayed as a program guide by some of the newer digital TVs.)

This protocol will allow the TV station to keep its valuable "branded" analog channel number both during and after the transition, even if the actual digital channel number is something else. Example: after the transition to the new digital TV standard is complete, if a station's old analog channel number was "10" and its new digital channel number is "34," PSIP tells the digital TV receiver to tune to channel 34 when the consumer selects channel "10" with her remote control.

Now during the transition, each station will have an analog channel and a digital channel, and the latter can either be one high-definition channel or divided into as many as five standard-definition channels (or maybe one lower qualtity HD channel and one standard-def channel, but you get the point). So how do you tune in all these different channels that are coming from the same TV station?

PSIP makes it easy. No matter what the frequency the digital channels are actually being broadcast on, you would select the usual analog channel number to tune-in the analog broadcast, and then the same channel number with a ".1" added for the primary digital channel (the HD channel), and then ".2," ".3," and so on for the other standard-definition channels (if there are any).

Another example: A TV station's analog channel number is 17, and it is transmitting digitally on channel 48. You would select "17" for the analog programming, "17.1" for the digital HD programming (primary digital channel), and "17.2", "17.3", "17.4", etc. for any other standard-definition "multi-cast" digital channels the station may be operating.

Obviously, after the transition to DTV is complete, the analog channel will disappear and the digital channel will occupy the main number (which may or may not be the actual broadcast channel number, but it will be the station's "number." Each station will retain its old familiar identity.

Keeping it simple for the consumer.