C182 PLT ATTEMPTS TO CLARIFY DEP INSTRUCTIONS AFTER TKOF CLRNC; USES INCORRECT RWY; AND IS CORRECTED BY BFI TWR.

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PROB WITH PARALLEL RWYS THAT USE DISCRETE FREQS BUT SOMETIMES USE 1 CTLR BROADCASTING ON BOTH FREQS. GND CLRED ME TO TAXI TO A9 FOR A VASHON DEP ON RWY 31L. ONCE READY FOR TKOF; I CONTACTED TWR ON 120.6 STATING 'SKYLANE #XYZ HOLDING SHORT A9 READY FOR VASHON DEP.' TWR CAME BACK WITH A TKOF CLRNC WITH MY COMPLETE CALL SIGN; '#XYZ CLRED TKOF RWY 31L; N DEP.' I STARTED ROLLING ONTO RWY 31L WHILE MAKING MY CLRNC CALL TO CORRECT TWR ABOUT MY VASHON DEP. I APPARENTLY STATED 'XYZ CLRED TKOF RWY 31R VASHON DEP.' TWR CORRECTED MY RWY CLRNC; I APOLOGIZED AND RESTATED HIS CLRNC; AND HE SAID NO PROB. DEPARTED NORMALLY USING VASHON DEP (FIRST TIME USING THIS PROC; THOUGH LAND AT BFI REGULARLY). THERE WAS NO OTHER EXCHANGE WITH THE TWR DURING THE 10 MINS I REMAINED ON BFI FREQ 120.6. EVEN THOUGH I BELIEVE THE MINOR MISCOMS WERE PROPERLY CORRECTED AND TKOF WAS AS APPROVED; IT FELT ODD. I WONDERED IF THE TWR CTLR CONFUSED MY CESSNA WITH ANOTHER CESSNA ON RWY 31R. HE INTENDED TO CLR HIM FOR TKOF ON RWY 31R BUT USED MY CALL SIGN. THIS FLT WAS MIDDAY WHEN BFI TENDS TO HAVE DEDICATED CTLRS FOR EACH RWY. HOWEVER; IF ONE CTLR CTLS BOTH RWYS; THEN HIS TKOF CLRNC IS BROADCAST ON BOTH 120.6 AND 118.3. THEREFORE; I COULD HAVE HEARD A CLRNC WITH MY CALL SIGN INTENDED FOR A DIFFERENT CESSNA USING RWY 31R. THIS SCENARIO SEEMS UNLIKELY AS TXWY A9 IS VERY CLOSE TO THE TWR AND MY LARGE CALL SIGN NUMBERS WOULD HAVE BEEN EASY TO REACH FROM THE TWR. HOWEVER; IT STRIKES ME THAT THE ADVANTAGE OF USING 1 CTLR TO WORK PARALLEL RWYS AND BROADCAST ON 2 FREQS SIMULTANEOUSLY COMES WITH A RISK. IF THE CTLR CONFUSES WHICH PLANE IS ON WHICH RWY; THE TKOF CLRNC WITH YOUR CALL SIGN IS STILL HEARD REGARDLESS OF WHICH RWY YOU ARE USING AND WHICH FREQ YOU HAVE TUNED. IF THE CTLR CONFUSED THE PLANE BUT BROADCAST THE CLRNC ONLY FOR THE CORRECT RWY ON THAT RWY'S DISCRETE FREQ; THEN THE PLANE ON THE OTHER PARALLEL USING A DIFFERENT FREQ WOULD NOT HEAR THE BOGUS CLRNC AT ALL. SEPARATE FREQS FOR PARALLEL RWYS SEEM GOOD. HAVING THE TWR BROADCAST SIMULTANEOUSLY ON BOTH FREQS SEEMS BAD. DISCRETE FREQS IMPROVE SAFETY.