A circuit for displaying
graphics in a
digital television receiver includes an MPEG decoder which has as input a video
data stream in digital format, a central memory for storing decoded video data and
graphics data, a
video output unit and a
graphics output unit to which video data and graphics data, respectively, are fed from the central memory, a mixer to which an output
signal from the
video output unit is fed, and a switch through which the output
signal of the graphics output unit is selectively fed to the mixer.
Horizontal and vertical synchronization signals from the MPEG decoder may be fed to the
video output unit, while
horizontal and vertical synchronization signals from various sources (including those from the MPEG decoder) may be selectively fed through a
multiplexer to the graphics output unit. One buffer memory each may be placed before the video output unit and the graphics output unit, respectively, each memory unit buffering the corresponding data retrieved from the central memory. A
signal generator may be provided which generates
horizontal and vertical synchronization signals and provides these to the
multiplexer. Additional
horizontal and vertical synchronization signals may be fed to the
multiplexer from an external connector. The video output unit may be operated at a first
clock frequency, while the graphics output unit may be operated at a second
clock frequency. This helps to achieve essentially independent
processing of the graphics data and the video data. When the switch is closed, the first and second
clock frequencies may be the same, and the same horizontal and vertical synchronization signals may then be sent to both the video output unit and the graphics output unit.