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VGA Front of Screen Flexible Dongle Solution

IP.com Disclosure Number: IPCOM000015115D
Original Publication Date: 2002-Jun-11
Included in the Prior Art Database: 2003-Jun-20

Publishing Venue

IBM

Abstract

VGA Front of Screen Flexible Dongle Solution. Monitors connected to Personal Computers require a vertical and horizontal sync signal to keep the screens stable. These signals are usually driven by circuits that use TTL voltage levels. TTL logic levels have a high voltage of 2.4 volts. Some new computer system use video logic that uses 3.3 volts. Sometimes these circuits do not drive the sync signals to 2.4 volts or do not have enough current drive for some monitors. To solve this problem without changing the video circuits or video card, a solution was developed that incorporates a buffer into a flexible dongle. This dongle improves the sync signals by buffering them to TTL voltage levels. Every monitor presents a unique load to its attached video card. Monitors often degrade signal quality on the HSYNC signal, introducing unwanted characteristics such as steps or slope reversals on HSYNC. When these signal degrading characteristics are introduced near a monitor's triggering/sampling threshold, unwanted Front of Screen characteristics such as jitter or screen tearing will occur. This is not a frequently seen problem with video cards that use 5V signaling because with the increased voltage amplitude their is a smaller chance that the unwanted characteristics will fall within a monitor's sampling point. Thus, incorporating 5V buffer circuitry to video cards that use 3.3V signaling eliminates the majority of Front of Screen problems. Schematics are attached in the vgadongle1.pdf file. vgadongle1.pdf