Now you are familiar with a standard composite video signal. Note that we have not mentioned sound. If your VCR has a yellow composite-video jack, you've probably noticed that there are separate sound jacks right next to it. Sound and video are completely separate in an analog TV.
You are probably familiar with five different ways to get a signal into your TV set:
- Broadcast programming received through an antenna
- VCR or DVD player that connects to the antenna terminals
- Cable TV arriving in a set-top box that connects to the antenna terminals
- Large (6 to 12 feet) satellite-dish antenna arriving in a set-top box that connects to the antenna terminals
- Small (1 to 2 feet) satellite-dish antenna arriving in a set-top box that connects to the antenna terminals
The first four signals use standard NTSC analog waveforms as described in the previous sections. As a starting point, let's look at how normal broadcast signals arrive at your house.
A typical TV signal as described above requires 4 MHz of bandwidth. By the time you add in sound, something called a vestigial sideband and a little buffer space, a TV signal requires 6 MHz of bandwidth. Therefore, the FCC allocated three bands of frequencies in the radio spectrum, chopped into 6-MHz slices, to accommodate TV channels:
- 54 to 88 MHz for channels 2 to 6
- 174 to 216 MHz for channels 7 through 13
- 470 to 890 MHz for UHF channels 14 through 83
The composite TV signal described in the previous sections can be broadcast to your house on any available channel. The composite video signal is amplitude-modulated into the appropriate frequency, and then the sound is frequency-modulated (+/- 25 KHz) as a separate signal.
To the left of the video carrier is the vestigial lower sideband (0.75 MHz), and to the right is the full upper sideband (4 MHz). The sound signal is centered on 5.75 MHz. As an example, a program transmitted on channel 2 has its video carrier at 55.25 MHz and its sound carrier at 59.75 MHz. The tuner in your TV, when tuned to channel 2, extracts the composite video signal and the sound signal from the radio waves that transmitted them to the antenna.