In 1953, the National Television Systems Committee (NTSC) devised an American standard for the video signal that’s still used today. Understanding this standard offers insight as to how your camcorder records and delivers a video signal. The NTSC standard has served us rather well during a century filled with the most rapid technological changes in human history, but with the arrival of HDTV, is it in its last days?
Look at that Screen
If you look closely at your television picture, you’ll see that it’s not a continuous image but rather a series of horizontal lines. Phosphors–compounds that emit light when they are hit with an electron beam–coat the face of your picture tube. An electron beam traces lines across the picture tube (525 of them every 30th of a second), lighting up the phosphors and generating a picture. The moving pictures that you record with your camcorder are actually a series of still images. When these stills run at a fast enough speed, your eye perceives them as one continuous moving image. But even 30 frames per second is not fast enough. The resulting picture still appears to have a little flicker.
Engineers solved this problem by dividing each frame into two fields–one containing the odd number of lines and the other containing the even number of lines. The electron beam traces out the odd numbered lines, then goes back to the top and puts the even lines in between. This process is called interlaced scanning. Now the system displays 60 fields per second, and the viewer can no longer see the flicker.
A Closer Look
The black and white part of a frame of video, called the luminance information, is represented in the NTSC signal as a continuously varying voltage. When a camcorder records a video image, the luminance information gets recorded one horizontal scan line at a time. Light-colored areas of the picture correspond to high voltages, while darker areas correspond to lower voltages.
There are 525 horizontal lines in a single frame of video but you cannot see all of them. The first 40 lines, 20 from each field, are reserved for non-image information. The frame around the picture tube cuts off a few more, reducing the number to 480. This is a serious limitation; the result is that the smallest vertical detail you can resolve is the distance between these horizontal lines.
As the beam reaches the right side of the picture, it shuts off before jumping back to the left side for another line. Consequently, an extraneous trace will not be visible. The horizontal blanking interval shuts off the beam. When the beam reaches the bottom of the screen after it displays every other horizontal line, it shuts off before it goes back to the top of the screen to begin another field. This is caused by the vertical blanking interval.
Figure 1 shows one line of video picture information as seen on a waveform monitor. The square trough is the sync pulse. Your video monitor or TV set uses this to locate scan lines correctly, so that the video plays on the screen with correct timing. Some of the other features of the signal picked up colorful labels from the 1950s. The area before the sync pulse is the front porch and the area after it is the back porch. When NTSC engineers added the color burst reference to the back porch, it left an area which they called the breezeway, between the sync pulse and the color burst. The actual luminance information follows the back porch.
The color burst is a reference signal consisting of eight cycles of a sine wave. It recovers the color, or chrominance, information encoded in another sine wave, which itself is combined with the NTSC signal by means of a process called amplitude modulation. This modulated color signal.
A disadvantage of NTSC technology is that when all of these signals are carried on the same wire, they tend to interfere with each other. Y/C cables allow the S-VHS and Hi8 formats to achieve an increase in picture quality by simply separating the luminance and chrominance signals, but the NTSC monitor’s limited resolution (480 lines) is another disadvantage. Finally, NTSC video relies on an analog signal that degrades with each copy made. All too often, the result is a frustrating lack of detail (see Generation Loss sidebar).
HDTV–the end of NTSC?
The new HDTV specification offers high resolution imaging like never before. Instead of a mere 480 scan lines, some HDTV sets have up to 1080 lines of vertical resolution. Like your computer’s monitor, some forms of HDTV use progressive scanning (see Progressive Scan sidebar). Moreover, HDTV is digital, which means it is much more impervious to noise. So should you throw away your collection of NTSC gear? Not yet.
The digital world may exist some day but you will roll a lot of analog tape before the NTSC signal disappears.
Some have compared the introduction of HDTV to the arrival of color TV in the early 50s. Broadcasting television in black and white began in 1941. Twelve years later, color television was introduced. Because stations had been broadcasting in black and white (and stores marketing black and white television sets), there was concern over obsolescence. Rendering all black and white television sets obsolete would have made the industry unpopular, so there was a need to allow both black and white and color sets to use the same signal.
Black and white television sets were certainly not made obsolete, many are still in use today, nearly 50 years later. But the greater satisfaction of watching TV in full color has led millions of consumers away from their "old fashioned" black and white sets. In the same way, that new NTSC TV you just bought, or the old one you’ve had around, won’t be obsolete for a long, long time. When and if you purchase a HDTV, an inexpensive set-top box will convert the HDTV signal to NTSC for viewing on your current TV.