vertical retrace การใช้

• Vertical retrace is comparatively slow, occurring over a span of time required for several tens of horizontal scans.
• In analog CRT TVs, setting brightness to maximum typically made the vertical retrace visible as zigzag lines on the picture.
• The tracks had to be cut during a vertical retrace, without disturbing the odd-field / even-field ordering.
• This is called " the vertical retrace interval "-and there is a signal called'VSYNC'that determines when that is happening.
• Traditionally, one of the principles of video / computer game programming is to avoid altering the computer's video buffer except during the vertical retrace.
• The sync pulses occupy the whole of line interval of a number of lines at the beginning and end of a scan; no picture information is transmitted during vertical retrace.
• Only during those periods when the video chip is displaying the screen border or putting out the horizontal and vertical retrace signals is the CPU able to run at full speed.
• Once the last scan line is written, the vertical component of the magnetic field continues to increase by the equivalent of a few percent of the total height before the vertical retrace takes place.
• The player would seek the short distance to the next segment of a storyline during the vertical retrace interval by adjusting the tracking mirror, allowing perfectly continuous video even as the player switched storylines under control of the game's computer.
• Video recorders ( as opposed to camcorders ) don't suffer from frequency drift with a ( compliant ) input signal, as they constantly resync using special information embedded in the PAL or NTSC signal ( the horizontal and vertical retrace signals ).
• The pulse sequence is designed to allow horizontal sync to continue during vertical retrace; it also indicates whether each field represents even or odd lines in interlaced systems ( depending on whether it begins at the start of a horizontal line, or midway through ).
• The physics of the CRT require that a finite time interval be allowed for the spot to move back to the start of the next line ( " horizontal retrace " ) or the start of the screen ( " vertical retrace " ).
• While the video signal can easily hide the head-switching point in the invisible vertical retrace section of the signal, so that the exact switching point is not very important, the same is obviously not possible with a continuous audio signal that has no inaudible sections.
• The horizontal retrace is accounted for in the time allotted to each scan line, but the vertical retrace is accounted for as " phantom lines " which are never displayed but which are included in the number of lines per frame defined for each video system.
• With a three-buffer swap chain in set "'5 "', drawing of frame B can start without having to wait for frame A to be copied to video memory, reducing the chance of a delayed frame missing its vertical retrace . ]]
• Most software DVD players do not assist with switching display modes, and even if it is switched manually, they hardly synchronize frame updating with the display's vertical retrace periods . ( There is only soft synchronization using hardware double buffering, which is not enough to match hardware players in the stability of playback .)
• For this reason, it is necessary to shut off the electron beam ( corresponding to a video signal of zero luminance ) during the time it takes to reorient the beam from the end of one line to the beginning of the next ( " horizontal retrace " ) and from the bottom of the screen to the top ( " vertical retrace " or " vertical blanking interval " ).