What Is the Difference Between Digital TV and HDTV?

FCC Official SDTV, EDTV, and HDTV Logos
FCC Official SDTV, EDTV, and HDTV Logos. Image provided by the FCC

The implementation of DTV and HDTV broadcasting via the DTV Transition that officially occurred on June 12, 2009, was a major historical event, as it changed the way TV content was broadcast and accessed by consumers in the U.S. However, there is some confusion as to what the terms DTV and HDTV actually refer to.

All HDTV is digital, but not all Digital TV is HDTV. In other words, the same bandwidth allocated for digital TV broadcasting can either used to supply a video signal (or several) and other services, or can be used to transmit a single HDTV signal.

Although there are technically 18 different resolution formats available for digital TV broadcasting, approved by the Advanced Standards Television Committee (ATSC), and all Digital TV tuners are required to decode all 18 formats, the practical application of DTV broadcasting has come down to 3 resolution formats: 480p, 720p, and 1080i.

480p

If you have a progressive scan DVD player and TV, you are familiar with 480p (480 lines of resolution, scanned progressively). 480p is similar to the same resolution of analog broadcast TV, but is transmitted digitally (DTV). It is referred to as SDTV (Standard Definition Television), but the image is scanned progressively, rather than in alternate fields as in analog TV transmission.

480p does provide an excellent picture (especially on smaller 19-29" screens). It is much more film-like than standard cable or even standard DVD output, but it only provides half the potential video quality of an HDTV picture, therefore its effectiveness is lost on larger screen sets (for example, 32 inches and up).

Although 480p is part of the approved DTV broadcasting scheme, it is not HDTV. This standard was included as one of the DTV broadcasting standards to provide broadcasters the option of providing multiple channels of programming in the same bandwidth as a single HDTV signal. In other words, 480p is just more of what you would see in an analog TV signal, with a slight increase in image quality.

720p

720p (720 lines of resolution scanned progressively) is also a digital TV format, but it is also considered as one of the HDTV broadcast formats.

As such, ABC and FOX use 720p as their HDTV broadcasting standard. Not only does 720p provide a very smooth, film-like image due to its progressive scan formula, but image detail is at least 30% sharper than 480p. As a result, 720p provides an acceptable image upgrade that is visible on both medium (32"- 39") size screens as well as larger screen sets. Also, even though 720p is considered high-definition, it takes up less bandwidth than 1080i, which is covered next.

1080i

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

However, the drawbacks of 1080i are:

  • 1080i takes up the most bandwidth of all the DTV broadcast formats.
  • 1080i is an interlaced signal, which means that the displayed image is made up of lines or pixel rows that are scanned/displayed alternately instead of progressively as in 480p and 720p.
  • 1080i cannot be displayed in its native form on an LCD, OLED, Plasma, or DLP TV. In order to display images transmitted in 1080i, those types of sets need to convert the 1080i signal to either 720p or 1080p in order to display the image on the TV screen.

In other words if you have a 1080p LCD or OLED TV, (or still have a Plasma or DLP TV) it will deinterlace the 1080i signal and display it as a 1080p image.

This process, if done well, removes any visible scan lines present in 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.

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 for this is that when Digital TV broadcast standards were approved, 1080p was not part of the equation. As a result TV broadcasters do not transmit over-the-air TV signals in 1080p.

More To Come - 4K and 8K

Now after sorting out the differences between all the standards involved in DTV broadcasting, don't relax just yet, as the next round of standards is expected to include 4K resolution, and, further down the road, 8K.

Initially it was thought broadcasting 4K and 8K resolution over-the-air would not be possible due the huge bandwidth requirements. However, there is ongoing testing that has resulted in the ability to fit all the increased information within the current physical broadcast infrastructure using newly refined video compression technologies that retain the quality result needed on the TV display end.

For a full rundown on the impending availability of 4K Ultra HD TV broadcasting, read our companion article ATSC 3.0 - The Next Step In TV Broadcasting

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 - 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.

However, when 8K TV broadcasts will available on a wide basis is anybody's guess, as in 2020, 4K TV broadcasting will still not be fully implemented - so making another jump to 8K will probably be another decade away, especially when considering that TV makers haven't made 8K TVs or content available to consumers yet - and even by 2020, such TVs will be small in number, if available at all. Of course, there would definitely be an shortage of 8K content to watch - not to mention another major investment required for TV broadcasters to purchase and install new equipment.