Many color television receivers include on-screen display circuitry for displaying video characters on a television screen. For example, the channel number can be displayed so that a viewer can readily verify to which channel the receiver is tuned. Such displays are typically generated by replacing normal video information with appropriately synchronized character signals developed by an alpha-numeric character generator ROM (CG ROM) in the receiver, so that the character information is displayed on a given portion of the television (or kinescope screen). The information displayed can include time of day information as well as channel information, for example, by employing appropriate electronic control circuits in the receiver. Examples of on-screen character display systems are found in U.S. Pat. No. 3,984,828, inventor Beyers; U.S. Pat. No. 4,354,202, inventor Harlan; and U.S. Pat. No. 5,150,107, inventor Kurisu.
As discussed in Kurisu, a summary of the television transmission process will now be described. A linear scanning process is used to break down a television picture for transmission. The image information in the resulting video signal is used at the receiver to control an electron gun in a kinescope, where the gun sweeps across a screen along nearly horizontal parallel lines that together make up the television picture.
When the video signal is processed at the receiver, it requires a means of synchronizing with the televised scene exactly as scanned by the transmitter camera tube. The speed of the receiver scanning line must duplicate that of the transmitter scanning line so that the top of the scene appears at the top of the screen and not elsewhere. When the horizontal beam reaches the end of the bottom line of the televised scene, it must retrace back to the beginning of the top line without being seen, simultaneously at both transmitter and receiver. During the retrace, the electron beam at the receiver must be blanked off (turned off) by a high amplitude signal that turns off the electron gun while the scanning circuits retrace the beam. The electron gun is similarly turned off during the vertical retrace, although for a longer time interval.
The scanning process requires means of coordinating the transmitter and receiver. To accomplish this objective, the transmission system generates synchronized signals to be used by the receiver so that it stays in steps with the transmitter. Two sets of synchronization signals are transmitted--horizonal (HSYNC) pulses and vertical (VSYNC) pulses. During each horizonal retrace, a HSYNC pulse is transmitted which is not seen on the screen since the beam is turned off. Similarly, the VSYNC pulse transmitted during each vertical retrace is not visible.