Memory efficient video graphics subsystem with vertical filtering and

Abstract

A graphics subsystem converts a first graphics data stream for display on a computer monitor having a first refresh rate into a second graphics data stream for a television monitor having a second, slower refresh rate. The graphics subsystem has a first memory for storing one horizontal scan line of pixel data and a second memory for storing one half of a horizontal scan line of pixel data. Multiplexers direct data to a first summing circuit from an input port and from the first memory itself, so that a first horizontal line of input pixel data is initially stored in the first memory and a second horizontal line of input pixel data is combined with the first horizontal line of data by the first summing circuit, and the resulting combined pixel data is stored in back into the first memory. A controller sends the combined pixel data from the first memory to a second summing circuit while a next horizontal line of input pixel data is received. The second summing circuit combines that next horizontal line of input pixel data with data from the first memory so as to generate vertically averaged pixel data that is then stored in the second memory. The vertically averaged pixel data in the second memory is sent to an output port at a rate of no less than one half the rate at which pixel data is being received at the input port.