Computable Minds -

How works the DTT?

Posted on: Oct, 19th 2010
DTT logo
Never I watch television, but all that is related to the digital technologies I find it interesting, so, today, I'm going to talk about the Digital Terrestrial Television (DTT).
Here you will discover all the advantadges and disadvantages that have the DTT.

In the DTT, the information comes encoded in a video format made of zeros and ones (for this reason is digital), in the same way that occurs in a computer. This kind of encoding allows compress the signal to its transmition, to improve better the use of the bandwidth.

In this way, an UHF channel where wad had only one channel of analogic television, now we can find a high definition channel (HD) or several channels of television multiplexed in standard quality (this is called multiplex) that, depending of the compression applied, will vary the quality.

A decoder is needed in order to read the format of the received data and watch the image in a television.

The great disadvantage of the digitial encoding compared to the analogic, is the way in that the noise affects to it. Although in the TDT the signal it's sended together to a codes of error detection and correction, that are applied in the decoder, these aren't infallible. When the number of erroneous bits is above of the quantity of errors that the algorithm can correct, the image can't be watched or is showed in the screen with squares decompressed bad, while in the analogical television, instead of don't watch nothing, we would get the image with snow.

The signal is compressed using the standards H.264 (for HD) y MPEG-2 (standard definition), from here comes the squares. These compression formats chop every image in squares to compress it by separate. The image only is stored totally in the key frames. In the rest, to compress further, store only the differences respect to the prior frame. That is to say, for every frame of a key frame it's calculated the difference that there are between this and the square that correspond in the following frame (differential compression) and saves a lot of space because usually in a video the images are very similar between them. This technique is combined with other compression technique called movement compression, that consist in do the correspondence between the actual square and the following, without assigning the same position, but calculating where its going to move. For this reason we usually see the square with errors, displacing it on the image during several frames until that arrives the following key frame.

Conclusion: The best is switch off the television and switch on the computer and read Computable Minds.

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