Philips 14AA3322/40B Guía de instalación descargar pdf (Pagina 14)

Generation of a transport stream for DVB and realisation of a HF-modulated DVB-signal

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9.1.2 Digital video:

Digital video is a type of digital recording system that works by using a digital rather than an

analog video signal. More specific, digital video is audio/visual in a binary format. Information is

represented as a sequence of digital data, rather than in a continuous signal as analog

information is. Digital video comprises a series of orthogonal bitmap digital images displayed in

rapid succession at a constant rate. In the context of video these images are called frames. We

measure the rate at which frames are displayed in frames per second. Since every frame is an

orthogonal bitmap digital image it comprises a raster of pixels. If it has a width of W pixels and a

height of H pixels we say that the frame size is WxH. Pixels have only one property, their color.

The color of a pixel is represented by a fixed number of bits. The more bits the more subtle

variations of colors can be reproduced. This is called the color depth of the video.

Because of the relatively high bit rate of uncompressed video, video compression is extensively

used. In the case of compressed video each frame requires a small percentage of the original

bits. Nowadays, there are a lot of different compression methods and algorithms. The most

common video compression formats are:

 .MP4

 .AVI

 .MKV

 .WMV

 .FLV

 .MPEG-2

There are two types of compressions. The constant and the variable. At the constant bitrate the

bits per second are kept in a constant rhythm. Each image in the video is compressed the same

and the details and quality of the image can drop in some cases. This type of compression is

usually used in video where we do not need a lot of details or we are not so interested in the

image. At the variable bitrate, the bits per second rhythm is changing. Each image in the video

is compressed differently. Images that have a lot of details and demand of a better quality have

smaller compression comparing to images with less details. This type of compression creates

bigger file sizes but better compressed image quality.

Also, there are two types of displaying the video. The interlaced and the progressive. At the

interlaced the image is projected in alternative sets of lines. The odd-numbered lines first and

then the even-numbered lines and again the odd-numbered lines and so on. While the

progressive display projects the image line by line.

During the digital era, starting in the early 1970s to early 1980s, there is a constant progress in

the quality of the digital videos. It started from simple digital videos of low quality that were

separated depending on the continent that you wanted to use it and nowadays there are videos

of extremely high quality playable everywhere in the world.

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Philips 14AA3322/40B Guía de instalación Pagina 14