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Digital Video Fundimentals

Understanding what digital video is first requires an understanding of its ancestor – broadcast television or analogue video. The invention of radio demonstrated that sound waves can be converted into electromagnetic waves and transmitted over great distances to radio receivers. Likewise, a television camera converts the colour and brightness information of individual optical images into electrical signals to be transmitted through the air or recorded onto video tape. Similar to a movie, television signals are converted into frames of information and projected at a rate fast enough to fool the human eye into perceiving continuous motion. When viewed by an oscilloscope, the unprojected analogue signal looks like a brain wave scan – a continuous landscape of jagged hills and valleys, analogous to the ever-changing brightness and colour information.

There are three forms of TV signal encoding:

  • most of Europe uses the PAL system
  • France, Russia and some Eastern European countries use SECAM, which only differs from the PAL system only in detail, although sufficient to make it incompatible
  • the USA and Japan use a system called NTSC.

With PAL (Phase-Alternation-Line) each complete frame is drawn line-by-line, from top to bottom. Europe uses an AC electric current that alternates 50 times per second (50Hz), and the PAL system ties in with this to perform 50 passes (fields) each second. It takes two passes to draw a complete frame, so the picture rate is 25 fps. The odd lines are drawn on the first pass, the even lines on the second. This is known as interlaced, as opposed to an image on a computer monitor which is drawn in one pass, known as non-interlaced. Interlaced signals, particularly at 50Hz, are prone to unsteadiness and flicker, and are not good for displaying text or thin horizontal lines.

PCs, by contrast, deal with information in digits – ones and zeros, to be precise. To store visual information digitally, the hills and valleys of the analogue video signal have to be translated into the digital equivalent – ones and zeros – by a sophisticated computer-on-a-chip, called an analogue-to-digital converter (ADC). The conversion process is known as sampling, or video capture. Since computers have the capability to deal with digital graphics information, no other special processing of this data is needed to display digital video on a computer monitor. However, to view digital video on a traditional television set, the process has to be reversed. A digital-to-analogue converter (DAC) is required to decode the binary information back into the analogue signal.

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