What is ATSC Digital?

John Lister

ATSC digital is the current set of standards for broadcasting television in the United States. It replaced the previous standards, NTSC, in June 2009. The most notable introductions are support for widescreen formats, high-definition pictures and full digital surround sound. Standards in this context refers to the technology used, not the editorial content or quality of programming.

TV stations in the US are required to transmit HD digital signals.
TV stations in the US are required to transmit HD digital signals.

The NTSC standards had been in place since the 1940s when they were developed to resolve differences in formats between rival broadcasters. Adopting a single set of standards increased the potential audience as people could be sure all sets could cope with all broadcasts. It also made it easier for programmers to resell programming for reruns on different networks.

A digital TV antenna can be hooked to a television and used to pick up over-the-air ATSC broadcasts.
A digital TV antenna can be hooked to a television and used to pick up over-the-air ATSC broadcasts.

NTSC standards covered the main broadcasting technology issues of the time. These included the number of lines on the screen, the rate at which these were refreshed, and the way in which material from film was transferred to TV broadcasts. The standards also covered the system used to encode the color information and the way audio and video information was arranged within the available broadcast frequency.

ATSC digital standards were developed in readiness for the replacement of analog television broadcasts with digital broadcasting. Most of the new standards do not deal inherently with the broadcasts being in digital format. Instead they address issues raised by the greater range of options that are open to broadcasters because digital broadcasting is more efficient at carrying information. These standards also cover high definition broadcasts, which are simply those which exceed a set resolution.

One key difference with ATSC digital standards are that there is support for a much wider variety of sizes and shapes of pictures. These include both widescreen and 4:3 ratio pictures. Within each these different resolutions there is usually support for both progressive and interlaced scanning and two or more different framerates. The standards deal with the resolution of a picture rather than the physical size of a TV screen. This is because two different sized screens may be showing the same number of pixels, with the increase in size being down to the pixels being larger rather than the picture being more detailed.

The ATSC digital standards also add Dolby Digital AC-3 as the main supported audio format. This format allows the broadcast of full surround sound known as 5.1 format. The NTSC standards had only covered stereo audio signals.

It is important to note that virtually all televisions, however old, should be able to display pictures broadcast using ATSC digital standards. When it comes to over-the-air broadcasts, there is no inherent technical reason why any aerial cannot pick up an ATSC signal. However, either the television itself or a set-top box will be needed to decode the information in an ATSC signal.

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