1. Field of the Invention
The present invention relates to a video signal synthesizing system, and more particularly to a video signal synthesizing system for synthesizing the system's own video signal such as an analog video signal, digital image data, encoded, compressed image data and so on and external signals such as character signals from other image processors.
2. Description of the Related Art
As inexpensive high-speed circuits such as ISDNs are popularized and as semiconductor technology and image compression technology improve, multimedia transmission and display devices that can transmit images of picture level, characters, and graphics are realized. In these multimedia transmission and display devices, synthesizing images from different image devices such as character devices and graphic devices can present various kinds of screen services to greatly expand the functions of image processors.
The system for synthesizing the system's inner video signal or signals and video signals from external video signal generating devices usually comprises an analog-to-digital (A/D) converter and a memory for an inner video signal, an A/D converter and a memory for an external video signal, a memory for synthesizing the inner and external video signals, a digital-to-analog (D/A) converter, a display, and so on. In this system, an analog video signal generated from a system's own apparatus (the inner video signal) is once converted into a digital signal by the A/D converter and stored in the memory, and an analog video signal from an external apparatus (the external video signal) is also converted into a digital signal by the A/D converter and stored in the memory respectively.
The digital video signals stored in the memories are synthesized by the synthesizing memory and converted again into an analog signal by the D/A converter. The synthesized video signal is displayed on the display.
In this way, the external video signal from the external video devices and at least one internal video signal are converted into digital signals and stored in the two video memories respectively, and then synthesized in a digital area. Accordingly, this system requires memories for each signal and a synthesizing memory, thereby increasing the cost.
In addition, it is obvious that digitizing the analog video signals and then converting the digital signal into an analog signal again may cause quantization distortion and thereby reduce accuracy.