Could you tell me the diffreence between 0x56[3:0] and 0x56[5:4]?
0x56[5:4]:picture aspect ratio.
0x56[3:0]:active format aspect ratio.
How should I do when I want to set a 4:3 or 16:9 output?
Both picture aspect ratio and active format aspect ratio are defined in CEA-861-E, Appendix H. Basically picture aspect ratio is as you would expect. Active format aspect ratio just conveys more information about letter box bar sizes and locations.
You only need to set format aspect ratio if the source knows it else leave it at the default of '0000'.
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