ZID OPERROR AND ACR PLTDEV CAUSED BY PERCEIVED CLRNC DSCNT AND MISSED READBACK AS A CARJ TAKES A DSCNT CLRNC OF A B190.

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I WAS WORKING 2 SECTORS COMBINED (BOTH FL230 AND BELOW) WITH 2 SEPARATE FREQS. ACFT Y WAS ON FREQ OVERFLT AT FL210. ACFT X CHKED ON AT FL240 AND WAS DSNDED TO FL210 INBOUND TO CVG (TFC WAS ACFT Y). ACFT Z CHKED ON (DIFFERENT FREQ THAN ACFT X) AT FL240 AND WAS DSNDED TO FL190. 2 OTHER ACFT WERE SHIPPED TO DIFFERENT FACILITIES AND ANOTHER ACFT WAS DSNDED. TFC WAS ISSUED TO ACFT Y (ACFT X) AND THEY REPLIED. TFC WAS THEN ISSUED TO ACFT X (ACFT Y) AND THERE WAS NO REPLY. SEVERAL SECONDS LATER THEY CALLED SAYING THAT I HAD CALLED THEM ACFT XX AND THAT THEY WERE ACFT X AND WAS THE 'NINETEEN CLRNC' FOR THEM (I DID NOT HEAR THE ALT QUESTION PART OF THEIR XMISSION). I XMITTED; 'SORRY; ACFT X; TFC 1 O'CLOCK EBOUND; ONE THOUSAND BELOW.' THEY REPLIED; 'ACFT X ROGER; LOOKING FOR TFC; DOWN TO ONE NINE OH.' I DID NOT HEAR THEM SAY DOWN TO FL190. I USUALLY DON'T LISTEN FOR A CORRECT ALT READBACK WHEN ISSUING TFC AND NO ALT WAS STATED. I TOOK MY EYES OFF OF THE SCOPE TO CHK THE CCLD SCREEN FOR TFC AND TO LOOK FOR AN ACFT THAT WAS INBOUND TO CVG WITH A MEDICAL SIT. THIS INFO HAD BEEN PHYSICALLY COORDINATED TO ME RIGHT AFTER I HAD ISSUED TFC TO ACFT X. ACFT X DSNDED TO FL190 WITHOUT A CLRNC; LOSING SEPARATION WITH ACFT Y. WHEN I LOOKED BACK AT THE SCOPE; ACFT X WAS AT FL199. I ASKED THEM TO VERIFY ASSIGNED ALT AND THEY STATED 'FL190.' DURING THE 5 MIN PERIOD AROUND THIS PLTDEV; I HAD CAUGHT 2 ALT READBACK ERRORS. AGAIN; I USUALLY DON'T LISTEN FOR CORRECT ALT READBACKS WHEN ISSUING TFC WHEN NO ALT WAS STATED. SUPPLEMENTAL INFO FROM ACN 537481: I HAVE NO RECOLLECTION OF ANY ATC CLRNC DEVS OR INQUIRIES; OR OF ANY TCASII TA'S COMMANDING AN ALTDEV DURING THIS SPECIFIC FLT.