Data Compression: The Complete Reference

6.1 Analog Video 639achieving an effective refresh rate of 48 fps. Modern movie projectors have very brightlamps and can even use a triple-blade shutter, for an effective refresh rate of 72 fps.The frequency of electric current in Europe is 50 Hz, so television standards usedthere, such as PAL and SECAM, employ a refresh rate of 25 fps. This is convenient fortransmitting a movie on television. The movie, which was filmed at 24 fps, is shown at25 fps, an undetectable difference.The frequency of electric current in the United States is 60 Hz, so when televisioncame, in the 1930s, it used a refresh rate of 30 fps. When color was added, in 1953, thatrate was decreased by 1%, to 29.97 fps, because of the need for precise separation ofthe video and audio signal carriers. Because of interlacing, a complete television pictureis made of two frames, so a refresh rate of 29.97 pictures per second requires a rate of59.94 frames per second.It turns out that the refresh rate for television should be higher than the rate formovies. A movie is normally watched in darkness, whereas television is watched in alighted room, and human vision is more sensitive to flicker under conditions of brightillumination. This is why 30 (or 29.97) fps is better than 25.The electron beam can be turned off and on very rapidly. It can also be deflectedhorizontally and vertically by two pairs (X and Y) of electrodes. Displaying a singlepoint on the screen is done by turning the beam off, moving it to the part of the screenwhere the point should appear, and turning it on. This is done by special hardware inresponse to the analog signal received by the television set.The signal instructs the hardware to turn the beam off, move it to the top-leftcorner of the screen, turn it on, and sweep a horizontal line on the screen. While thebeam is swept horizontally along the top scan line, the analog signal is used to adjustthe beam’s intensity according to the image parts being displayed. At the end of thefirst scan line, the signal instructs the television hardware to turn the beam off, move itback and slightly down, to the start of the third (not the second) scan line, turn it on,and sweep that line. Moving the beam to the start of the next scan line is known as aretrace. The time it takes to retrace is the horizontal blanking time.This way, one field of the picture is created on the screen line by line, using just theodd-numbered scan lines (Figure 6.1c). At the end of the last line, the signal containsinstructions for a frame retrace. This turns the beam off and moves it to the startof the next field (the second scan line) to scan the field of even-numbered scan lines(Figure 6.1d). The time it takes to do the vertical retrace is the vertical blanking time.The picture is therefore created in two fields that together make a frame. The pictureis said to be interlaced.This process is repeated several times each second, to refresh the picture. Thisorder of scanning (left to right, top to bottom, with or without interlacing) is calledraster scan. The word raster is derived from the Latin rastrum, meaning rake, since thisscan is done in a pattern similar to that left by a rake on a field.A consumer television set uses one of three international standards. The standardused in the United States is called NTSC (National Television Standards Committee),although the new digital standard (Section 6.3.1) is fast becoming popular. NTSCspecifies a television transmission of 525 lines (today, this would be 2 9 = 512 lines, butsince television was developed before the advent of computers with their preference forbinary numbers, the NTSC standard has nothing to do with powers of two). Because