Alterning colors on SECAM TV

[Windless]

Hi,

 

  I try to understand video signals. SECAM apparently takes signal colors from two succesive lines, which are send with a rotated phase, and average them. Hence the color resolution is halved, but any phase shifting that would occure during transmission is canceled.

 

  So I wanted to try generating pairs of different line to see what the resulting color would be, and I not completely understand what I see :

 - color are not completely mixed, odd and even lines have slightly different (hard to tell on the picture, but easily noticed IRL. On the picture you see that the diagonal -which has same color on each line- is more uniform than the other squares

- alterning colors A and B do not give the same result as alterning B and A (e.g. column 4 line 6 is orange, when column 6 line 4 is green)

- and most abviously : the first third of the screen is purple (even on the area which just "altern" color zero and color zero...)

 

What can explain these results ?

Is the purple thing some artefact of an auto adjustement of the phase / amplitude ? Is my TV broken ? (non alterning pictures look very normal !)

For some historical copy-pastin reasons, my code uses color $4E to $40 for the columns and $0E to $00 for the lines. The SECAM 2600 only uses the luminosity and should completely ignore the four high bits, but could the activation of the COL delay circuit induce some small dephasing / amplitude variation on the LUM pins that, once converted to SECAM, give these colors ? (these 2600 has an NTSC TIA, I've not tried it on my PAL-TIA SECAM 2600 yet)

 

For reference,I attach how STELLA shows the same code runing in SECAM mode : really different from what I get.

testcolors4k.4k testcolors.asm

Edit : do not believe any comment in the code, I just randomly copy pasted and edited pieces of other programs.

Edited May 6, 2023 by Windless