GS2HD - Apple IIgs RGB to HDTV Component Adapter

Submitted by GG on Thu, 01/13/2022 - 15:25
Project Status

GS2HD E01The apple IIgs, like many computers from the 80's and early 90's, has a analog RGB output at a 15KHz horizontal frequency (same as NTSC/PAL). The majority of modern monitors and TVs will not sync to anything below 31.5KHz on their VGA input. If you have a TV with a SCART connector it is possible to wire the RGB directly to the TV. Most TVs in USA, however, use the YPbPr component instead of SCART making a direct connection not possible. GS2HD converts the IIgs RGB output to a YPbPr signal compatible with HDTVs with a component input.

The schematics is identical to the GS2HD-D01. Layout changes were necessary to replace the DIP-8 operational amplifiers with a SMD package. U1A, U2A and U3A buffer and amplify the RGB signals (standard non inverting configuration with a gain of 2). U1B, U2B and U3B perform the matrix transformation from RGB to YPbPr according to the standard coefficients for SDTV.

Y = 0.299 * R + 0.587 * G + 0.114 * B
Pb = -0.169 * R - 0.331 * G + 0.500 * B
Pr = 0.500 * R - 0.419 * G - 0.081 * B

U5, L1, L2 and the associated capacitors provide a cleaned up power rails for the sensitive video circuits.

The basic circuit will work with pretty much any dual operational amplifier with standard foorptint. However to achieve good video quality a high speed opamp optimized for video applications is required. Suitable devices are ST Microelectronics TSH82 and Texas Instruments LM6172.

GS2HD fullscreenGS2HD ComparisonDetails

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