Abstract: A synchronization system aligns video signals without the use of a phase locked loop. One embodiment includes a delay line and a selection circuit. A clock signal with a desired frequency for a pixel clock is applied to the delay line to generate a series of delayed signals at taps on the delay line. When a transition in a horizontal sync signal occurs, the selection circuit senses delayed signals and selects a delayed signal having a transition aligned relative to the transition in the horizontal sync signal. This delayed signal is a pixel clock signal which is not subject to frequency fluctuation of a phase locked loop. Selecting a new delayed signal at each horizontal blanking period keeps the pixel clock for each line of video aligned to the horizontal sync signal.

Abstract: In the preferred embodiment, the RGB analog signals generated by a VGA card in a personal computer are applied to a first input of an analog multiplexer. The digital video data stored in a video memory buffer on a video card is converted to RGB analog signals using a D/A converter, and these analog signals are applied to a second input of the analog multiplexer. An analog comparator compares a preselected color key to the analog output of the VGA card. When there is a match, the analog comparator controls the multiplexer to pass the analog video data to the monitor for display. When the color key is not detected, the data from the VGA card is transmitted to the monitor for display. Other features for improving the video overlay precision and the quality of the displayed image are also described.

Abstract: A digital to analog converter is described for a multimedia system in which the magnitude of the analog output is controlled by signals applied to level select inputs of the digital to analog converter. Digital signals to be converted to an analog output are connected to a different set of inputs. This converter is advantageous for use in a display system where the brightness of the display may be independently controlled by the signals applied to the level select inputs. In this way, the digital information signals to be converted to an analog signal need not be modified to increase or decrease the brightness of the display.

Abstract: This method is most advantageous when an analog multiplexer is controlled to pass either analog graphics signals or analog motion video signals to a multimedia display screen. In the preferred embodiment, this method includes the steps of outputting a digital color key from a graphics memory buffer at a designated time and for a designated period and then converting this color key to an analog signal. This analog color key signal is then detected during this designated period and stored to obtain a stored color key value. The graphics memory is then controlled to output the graphics signals to be displayed on a multimedia screen. These graphics signals include a digital color key for controlling a multiplexer to display a motion video window on the multimedia screen in place of the color key. The outputted graphics signals are converted into analog signals, and these analog signals are compared to the stored color key value for detecting whether a color key is being outputted in the graphics data stream.