Analog, Digital and HDTV

For years, watching TV has involved analog signals and cathode ray tube (CRT) sets. The signal is made of continually varying radio waves that the TV translates into a picture and sound. An analog signal can reach a person’s TV over the air, through a cable or via satellite. Digital signals, like the ones from DVD players, are converted to analog when played on traditional TVs. (You can read about how the TV interprets the signal in How Television Works.)

This system has worked pretty well for a long time, but it has some limitations:

• Conventional CRT sets display around 480 visible lines of pixels. Broadcasters have been sending signals that work well with this resolution for years, and they can’t fit enough resolution to fill a huge television into the analog signal.
• Analog pictures are interlaced — a CRT’s electron gun paints only half the lines for each pass down the screen. On some TVs, interlacing makes the picture flicker.
• Converting video to analog format lowers its quality.

Photo courtesy Consumer Guide Products Analog TVs like this one can’t use digital signals without a set-top converter.

United States broadcasting is currently changing to digital television (DTV). A digital signal transmits the information for video and sound as ones and zeros instead of as a wave. For over-the-air broadcasting, DTV will generally use the UHF portion of the radio spectrum with a 6 MHz bandwidth, just like analog TV signals do.

DTV has several advantages:

• The picture, even when displayed on a small TV, is better quality.
• A digital signal can support a higher resolution, so the picture will still look good when shown on a larger TV screen.
• The video can be progressive rather than interlaced — the screen shows the entire picture for every frame instead of every other line of pixels.
• TV stations can broadcast several signals using the same bandwidth. This is called multicasting.
• If broadcasters choose to, they can include interactive content or additional information with the DTV signal.
• It can support high-definition (HDTV) broadcasts.

DTV also has one really big disadvantage: Analog TVs can’t decode and display digital signals. When analog broadcasting ends, you’ll only be able to watch TV on your trusty old set if you have cable or satellite service transmitting analog signals or if you have a set-top digital converter.

This brings us to the first big misconception about HDTV. Some people believe that the United States is switching to HDTV — that all they’ll need for HDTV is a new TV and that they’ll automatically have HDTV when analog service ends. Unfortunately, none of this is true.

HDTV is just one part of the DTV transition. We’ll look at HDTV in more detail, including what makes it different from DTV, in the next section.

Important DTV DatesAs of March 1, 2007, all new TV sets in the U.S. had to have DTV tuners or be DTV ready. But ­the transition to digital TV isn’t complete. Feb. 17, 2009 is the proposed shutoff date for over-the-air analog broadcasts.

When the first high-definition television (HDTV) sets hit the market in 1998, movie buffs, sports fans and tech aficionados got pretty excited, and for good reason. Ads for the sets hinted at a television paradise with superior resolution and digital surround sound. With HDTV, you could also play movies in their original widescreen format without the letterbox “black bars” that some people find annoying.

HDTV Image Gallery

Photo courtesy Consumer Guide Products An 84-inch HDTV-ready plasma TV. See more HDTV pictures.

But for a lot of people, HDTV hasn’t delivered a ready-made source for transcendent experiences in front of the tube. Instead, people have gone shopping for a TV and found themselves surrounded by confusing abbreviations and too many choices. Some have even hooked up their new HDTV sets only to discover that the picture doesn’t look good.

Fortunately, a few basic facts easily dispel all of this confusion. In this article, we’ll look at the differences between analog, digital and high-definition, explain the acronyms and resolution levels and give you the facts on the United States transition to all-digital television. We’ll also tell you exactly what you need to know if you’re thinking about upgrading to HDTV.

Monitors vs. TVs

Your computer probably has a “VGA monitor” that looks a lot like a TV but is smaller, has a lot more pixels and has a much crisper display. The CRT and electronics in a monitor are much more precise than is required in a TV; a computer monitor needs higher resolutions. In addition, the plug on a VGA monitor is not accepting a composite signal — a VGA plug separates out all of the signals so they can be interpreted by the monitor more precisely. Here’s a typical VGA pinout:

• pin 1 - Red video
• pin 2 - Green video
• pin 3 - Blue video
• pin 4 - Ground
• pin 5 - Self test
• pin 6 - Red ground
• pin 7 - Green ground
• pin 8 - Blue ground
• pin 9 - No pin
• pin 10 - Digital ground
• pin 11 - Reserved
• pin 12 - Reserved
• pin 13 - Horizontal sync
• pin 14 - Vertical sync
• pin 15 - Reserved

This table makes the point that the signals for the three beams as well as both horizontal and vertical sync signals are all transmitted separately. See How Computer Monitors Work for details.

Digital TV

The latest buzz is digital TV, also known as DTV or HDTV (high-definition TV). DTV uses MPEG-2 encoding just like the satellite systems do, but digital TV allows a variety of new, larger screen formats.

Photo courtesy Sony Electronics Sony Wega 42″ XBR Plasma TV with built-in HDTV tuner

The formats include:

• 480p - 640×480 pixels progressive
• 720p - 1280×720 pixels progressive
• 1080i - 1920×1080 pixels interlaced
• 1080p - 1920×1080 pixels progressive

A digital TV decodes the MPEG-2 signal and displays it just like a computer monitor does, giving it incredible resolution and stability. There is also a wide range of set-top boxes that can decode the digital signal and convert it to analog to display it on a normal TV. For more information, check out How Digital Television Works.

Satellite TV Signals

Photo courtesy DirecTV Small-dish satellite system

Large-dish satellite antennas pick off unencoded or encoded signals being beamed to Earth by satellites. First, you point the dish to a particular satellite, and then you select a particular channel it is transmitting. The set-top box receives the signal, decodes it if necessary and then sends it to channel 3 or 4.Small-dish satellite systems are digital. The TV programs are encoded in MPEG-2 format and transmitted to Earth. The set-top box does a lot of work to decode MPEG-2, then converts it to a standard analog TV signal and sends it to your TV on channel 3 or 4. See How Satellite TV Works to learn more.

VCR and Cable Signals

VCRs are essentially their own little TV stations. Almost all VCRs have a switch on the back that allows you to select channel 3 or 4. The video tape contains a composite video signal and a separate sound signal. The VCR has a circuit inside that takes the video and sound signals off the tape and turns them into a signal that, to the TV, looks just like the broadcast signal for channel 3 or 4.The cable in cable TV contains a large number of channels that are transmitted on the cable. Your cable provider could simply modulate the different cable-TV programs onto all of the normal frequencies and transmit that to your house via the cable; then, the tuner in your TV would accept the signal and you would not need a cable box. Unfortunately, that approach would make theft of cable services very easy, so the signals are encoded in funny ways. The set-top box is a decoder. You select the channel on it, it decodes the right signal and then does the same thing a VCR does to transmit the signal to the TV on channel 3 or 4.

TV Broadcasts

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, like this:

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.