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Digital TV / Digital Vs. Analog / Bandwidth Squeeze

Unlike many other countries, the United States is converting to both digital signals and high-definition pictures at the same time. Some countries are already broadcasting high-definition pictures, but they're using an analog signal. To send more picture detail, they just expanded the amount of frequency bandwidth for each. Broadcasters in the United States won't have the option to expand the size of their signal. They'll have to squeeze more picture detail into the same bandwidth they were using for analog television.

Bandwidth Squeeze

An advantage digital has over analog is that analog signals can't be compressed as well as a digital signal can. To transmit an image on analog television, every pixel is included in the signal. A standard NTSC screen includes 525 lines of 720 pixels, for a total of 378,000 pixels per frame. That's a lot, but it fits into the 6MHz bandwidth of a television channel. Japanese HDTV takes 20 MHz of bandwidth to send pictures with over 675,000 pixels. That's over two times as much signal to send a high definition picture and higher quality sound, but no other data.

In the United States, a standard ATSC (Advanced Television Systems Committee) screen can have up to 1080 lines of 1920 pixels each, or 2,073,600 pixels per frame. Somehow, more than five times as much information will need to squeeze into the same bandwidth of 6Mhz. That doesn't include the compressed audio or data. So how are they going to do that? The same way the compression software on your computer squeezes your files. Well, almost the same way.



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