What's the Difference Between Digital TV and HDTV?

Sorting out the state of digital TV broadcasting

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The implementation of DTV (digital television) and HDTV (high-definition television) broadcasting in 2009 via the DTV transition changed how television content is broadcast to and accessed by consumers in the U.S. It's also given rise to an alphabet soup of terms — among them, DTV and HDTV.

All HDTV Broadcasting Is DTV, but Not All DTV Broadcasting Is HDTV

The same bandwidth allocated for DTV broadcasting can be used to supply multiple standard resolution digital channels (SDTV) and other services, or to transmit one or two full HDTV signals.

The Advanced Standards Television Committee (ATSC) made 18 different resolution formats available for digital TV broadcasting, and all built-in and external digital TV tuners are required to decode all 18 formats. The practical application of DTV broadcasting, however, has come down to just three resolutions: 480p, 720p, and 1080i.

480p (SDTV)

The 480p resolution of SDTV (standard definition television) is similar to that of analog broadcast TV but is transmitted digitally (DTV). The image is made up of 480 lines or pixel rows of resolution scanned progressively, rather than in alternate fields as in analog TV transmission.

This provides a good picture, especially on smaller 19- to 29-inch screens. It's much more film-like than standard cable or even standard DVD output, but it provides only half the potential video quality of an HDTV picture. This means that its effectiveness is lessened on large screen sets (TVs with screen sizes 32 inches and up).

Although 480p is part of the approved DTV broadcasting standards, it is not HDTV. It was included to give broadcasters the option of providing multiple channels of programming and services within the same channel bandwidth allocation as a single HDTV signal. It's similar to what you would see in an analog TV signal, with a slight increase in image quality.

720p

Another DTV format, 720p (720 lines of resolution scanned progressively) is also considered HDTV.

ABC and FOX use 720p as their HDTV broadcasting standard. This resolution provides a smooth, film-like image due to its progressive scan implementation, and image detail is at least 30 percent sharper than 480p. It provides an acceptable image upgrade for medium (32- to 39-inch) and larger screens. Also, even though 720p is considered high-definition, it takes up less bandwidth than 1080i, which is covered next.

1080i

The most commonly used HDTV format for over-the-air TV broadcasting is1080i (1,080 lines of resolution scanned in alternate fields of 540 lines each). PBS, NBC, CBS, and CW (as well as satellite programmers TNT, Showtime, HBO, and other pay services) use it as their HDTV broadcast standard. Although there is still debate as to whether 1080i is that much better than 720p in actual viewer perception, 1080i technically provides the most detailed image of all the 18 approved DTV broadcast standards. The visual impact of 1080i, however, is lost on screen sets smaller than 32 inches.

Here some additional 1080i facts:

  • 1080i takes up the most bandwidth of all the DTV broadcast formats.
  • 1080i is an interlaced signal. The image signal is made up of alternating lines or pixel rows instead of progressive lines or rows as in 480p and 720p.
  • 1080i cannot be displayed in its native form on an LCD, OLED, Plasma, or DLP TV. To display 1080i signals, those types of sets have to convert the 1080i signal to 720p or 1080p.

If you have a 1080p LCD, OLED, plasma, or DLP TV, it will deinterlace the 1080i signal and display it as a 1080p image. This process, if done well, removes all visible scan lines from the interlaced 1080i image, resulting in very smooth edges. By the same token, if you have a 720p HDTV, your TV will deinterlace and downscale the 1080i image to 720p for screen display.

Here's what all those acronyms mean:

  • LCD: liquid crystal display
  • OLED: organic light-emitting diode
  • DLP: digital light processing
  • HDTV: high-definition television
  • DTV: digital television
  • SDTV: standard-definition television

What About 1080p?

Although 1080p is used for Blu-ray, cable, and Internet streaming, it is not used in over-the-air TV broadcasting. The reason is that, when DTV broadcast standards were approved, 1080p was not initially included.

More to Come: 4K and 8K

Although DTV broadcasting is the current standard, the next round of standards includes 4K resolution; farther down the road, we'll see 8K.

Initially, the industry's position was that broadcasting 4K and 8K resolution over the air would not be possible because of the huge bandwidth requirements. Ongoing testing, however, has resulted in refined video compression and other technologies that work with a minimal increase in bandwidth. The new standards that will include 4K are referred to as ATSC 3.0 or NextGen TV broadcasting.

As TV stations make the necessary equipment and transmission upgrades, and TV makers start incorporating new tuners into TVs and plug-in set-top boxes, consumers will be able to access 4K TV transmissions. However, unlike the hard date that was required to transition from analog to digital/HDTV broadcasting, the transition to 4K will be slow and is voluntary as of right now.

The implementation of 4K TV broadcasting is definitely lagging behind other methods of accessing 4K content, such as through internet streaming services, including Netflix and Vudu, as well as via the physical Ultra HD Blu-ray Disc format. DirecTV also offers limited 4K satellite feeds.

Meanwhile, although the major effort is to bring 4K to TV broadcasting, Japan is also pushing ahead with its 8K Super Hi-Vision TV Broadcasting format, which also includes up to 22.2-channel audio. Super Hi-Vision has been in testing for well over a decade and is expected to be fully ready for wide use by 2020, pending final standards approval.

Just when 8K TV broadcasts will available on a wide basis, however, is anybody's guess. 4K TV broadcasting will still not be fully implemented in 2020, so making another jump to 8K is probably another decade away, The first 8K TVs are just beginning to be made available. Of course, there needs to 8K content to watch, and TV broadcasters would have to make major equipment investments.