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Video Equalizing Pulses

| | Tuesday, July 28, 2009

Equalizing Pulses
During the vertical blanking interval, the sync generator puts out equalizing pulses. Equalizing pulses occur both before and after the vertical sync signal. The equalizing pulses that occur before the vertical sync are called pre-equalizing pulses. Those that occur after vertical sync are called post-equalizing pulses. Equalizing pulses in video assure continued synchronization during vertical retrace as well as proper interlace of the odd and even fields.
Lines 1 through 9 in each field actually consist of pre-equalizing pulses, vertical sync pulses, and post-equalizing pulses. The 6 preequalizing pulses break up the first 3 lines of a field into 6 half-lines.The next 3 lines consist of 6 vertical sync pulses. Lines 7, 8, and 9 are separated by post-equalizing pulses.

Depending on whether it is the odd or even field, there will be 6 postequalizing pulses, but either 5 or 6 half-lines. In the even field, there are only 5 half-lines. The first half-line of inactive video is called line 9. In the odd field, there are 6 post-equalizing pulses and 6 halflines, so that the first full line of inactive video is called line 10.

There are other ways of defining fields. Each field consists of 2621/2 lines. The odd field begins with a whole line of active video on line 21 and ends with a half-line of video. The even field is defined as starting active video with a half-line on line 20 and ending with a whole line of video. In either case, each field is handled individually, and line counting is done within each field.
It is the equalizing pulses that allow the system to distinguish the odd from the even fields and therefore interlace the two proper fields together to create one frame. If the fields were not properly interlaced, it would be possible to be off by one field in the interlace process.

Color Subcarrier
With the advent of color television, a new signal was introduced to carry the color information. This signal, known as the color subcarrier, became the most important signal of the sync generator. Most sync generators combine color subcarrier with horizontal sync, vertical sync, blanking, and a black video signal to produce a composite signal called black burst or color black. The color subcarrier signal, or any of the synchronizing or blanking pulses, can be taken as a separate output from a sync generator. However, the combination of sync pulses in a black burst signal is much more useful.

The frequency of the color subcarrier is 3,579,545 cycles per second. This frequency must be maintained within plus or minus 10 cycles per second. If this frequency changes, the rate of changecannot be greater than one cycle per second every second. The exactness of this specification has to do with the sensitivity of the human eye to changes in color. As this color subcarrier signal is the reference for color information, any change in the frequency would cause a shift in the color balance. The color subcarrier is also used as the main reference signal for the entire video signal. If the colorsubcarrier is incorrect, then all the signals in the television system will be affected.