Channels, Frequency, and Modulation

Digital TV is carried over the same broadcast and cable TV channels as plain old analog TV. This means that the radio frequency spectrum is divided into 6 MHz units. When the first TV stations came on line, a block of frequencies was assigned to channels 2-12 (VHF). Later a block of higher channel numbers (UHF) became available. UHF channels extend up to 69, but after the Feb. 2009 cutover to digital transmission no TV station will be assigned a channel number above 51. The frequencies previously associated with broadcast channels 52 to 69 are being auctioned off for cell phone or other use.

When cable TV systems appeared, they were not constrained by this historical accident. Channels 2-12 have the same frequencies on cable as in broadcast, but the higher channels were assigned successive 6 MHz blocks up to something around channel 183.

Within the assigned frequency range, a signal is generated by some form of "modulation". The simplest technique is the Amplitude Modulation of the AM radio band. The strength of the signal goes up when the sound gets louder. AM, however, is sensitive to static, interference, and is hard to tune.

When there is no sound, AM generates no signal. The next technique was Frequency Modulation or FM. In FM there is a background carrier wave that is always present. When there is silence, the carrier signal is present but it is doing nothing. The signal is generated by imposing a variation over this base carrier. FM is insensitive to lightning and interference from appliances. The carrier also allows the receiver to automatically tune in to the station once it gets close enough to the base frequency.

AM holds the frequency constant but changes the intensity of the signal. FM holds the intensity constant but changes the frequency. Essentially the two different techniques are completely orthogonal to each other. If you think in AM terms, there is no signal on an FM channel and visa versa.

It is possible to add more signal to a frequency by coming up with new way to modulate the new signal that is independent and therefore invisible to whatever techniques were used to generate the old signal. In the 1950s a radio signal could only be processed by analog components and filters. Today the same digital technology that makes computers and cell phones possible also provides digital signal processing chips that can make fine distinctions that would never be possible with analog components.

In FM radio, there is a big primary signal that provides monaural sound. A second weaker signal is then imposed over it that represents the difference between the left and right signal. By applying this second signal to the first, and then applying the exact opposite of this second signal to the first, the original signal can be converted to stereo. In the first generation of "home theater", an additional signal could be provided to split the left and right signals into front speaker and back speaker components.

When an FM station is far away, the first thing you lose is the stereo. However, if you switch back to "mono" mode, you may still be able to tune in the stronger monaural signal.

A conventional analog TV stations broadcasts separate video and audio signals. The audio part is the same as FM radio. The video part has a large signal component for the black and white (intensity) part of the picture. There is then a secondary component for color. The color part has a third level of signal that splits it to provide enough signals to drive the Red, Green, and Blue electrons of the TV set. The analog video signal has synchronization points to mark the start of a screen refresh (60 times a second), and the break between each of the 240 lines on the screen.

When an analog TV station is far away, the color fades out. Some black and while signal remains, but as the distance increases it is eventually lost in the snow.

Digital TV uses the same 6 MHz range of frequencies, but with different modulation schemes. Actually there are two modulation systems in the US:

• 8VSB is the modulation scheme for over the air digital TV broadcast in North America (ATSC).
• QAM is the modulation scheme used for digital cable TV and direct satellite broadcast.

Terrestrial broadcast TV has to be able to recover when the signal bounces off nearby buildings or hills. With error correction, it squeezes a little less than 20 megabits per second of signal out of the 6 MHz TV channel. Cable TV channels are transmitted in a protected environment, so it doesn't need the same level of error recovery. The high density QAM modulation they use can get twice as much signal, or a bit less than 40 megabits per second out of the same 6 MHz range.