The GGLABS A520HD is the HDTV equivalent of the classic Commodore A520 TV modulator. Instead of encoding the Amiga video signal to a low quality composite output it converts the RGB output to a high quality YPbPr signal compatible with HDTVs with a component input. The A520HD connects to the Amiga 23-pin video port and is powered directly from the system.
The design is heavily based on our GS2HD released a few months ago.
The circuit is pretty straightforward. 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
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 operational amplifier. However to achieve good video quality a high speed opamp optimized for video applications is required. Suitable devices are Analog Devices AD8056ANZ and Texas Instruments LM6172IN.