Overview of solve
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The only polarity distinguisher is the arrow at the bottom of the rightmost post.
That does permit you to decide the polarity of its German Whisper, but it doesn't spread.
The other two posts can be solved in A-I until polarity is fixed.
They must have at least one of the 4 or the 6, D or F, on them - ascertain which ends.
The middle post is busier, start on that one first: the monogomous letter can't go at the bottom, as teh lockout lines can't be filled.
So plonk 'D' at the top and pencil everything on the signs and in col 5.
No look at the leftmost post, and how column 1 in boxes 1 and 4 are restricted to [CDEFG] unless D's also at the top, so plonk that in.
Because [1289] is [ABHI], there's an [ABHI] quad in row 5, so r5c5 is G, and r7c5 is H above C.
This knocks the B off the box 5 lockout line, and forces A onto r3c2, which forces the B and the C too along the GW, and H and G get bounced up it.
Back on the middle post, the [AI] pair on the lockout line mean that r4c5 is B not A.
A+G can only be paired with C+E on the double arrow in the box.
A+H can only be paired with D+E on the other double arrow.
The lockout line at the top is now fully resolved DABCBI, which resolves teh box 1 one too.
Columns 2 and 5 can be completed
Box 2's H is in r1c6, andits C is in r1c4.
Row 7's F and row 8's E cannot be in the lockout lines, so are in column 9.
Moving attention to the rightmost post:
r7c8 cannot be a 3, as that puts too much pressure on teh lockou lines.
If it's 1, then F is 6, and if it's 2, then F is 6. So F is 6 and A is 1 etc.

And that's huge.

Box 6 has a tight dependency loop that forces r5c8 to be 3, so 8's above and 1/2 are on the lockout line.
Box 9 can be completed, which is worth a singsong, as can column 8, and by that point you've started your speedrun to the end.


Proof of uniqueness:
https://sigh.github.io/Interactive-Sudoku-Solver/?q=.Whisper%7E5%7ER2C5%7ER3C5%7ER4C5%7ER5C5%7ER6C5%7ER7C5%7ER8C5.Whisper%7E5%7ER7C2%7ER6C2%7ER5C2%7ER4C2%7ER3C2%7ER2C2%7ER1C2.Whisper%7E5%7ER4C8%7ER5C8%7ER6C8%7ER7C8%7ER8C8%7ER9C8.Lockout%7E4%7ER2C5%7ER2C6%7ER2C7%7ER3C7%7ER3C6%7ER3C5.Lockout%7E4%7ER4C5%7ER4C4%7ER4C3%7ER5C3%7ER5C4%7ER5C5.Lockout%7E4%7ER1C2%7ER1C3%7ER2C3%7ER2C2.Lockout%7E4%7ER5C8%7ER5C9%7ER6C9%7ER6C8.Lockout%7E4%7ER7C8%7ER7C7%7ER8C7%7ER8C8.DoubleArrow%7ER4C8%7ER4C7%7ER5C7%7ER5C8.DoubleArrow%7ER5C5%7ER5C6%7ER6C6%7ER6C5.DoubleArrow%7ER6C5%7ER6C4%7ER7C4%7ER7C5.DoubleArrow%7ER3C2%7ER3C1%7ER4C1%7ER4C2.Arrow%7ER9C7%7ER9C8%7ER9C9