A DC9 FLC EXPERIENCES A LOSS OF COM FREQ WITH DCA TWR CTLR. THE TKOF IS DELAYED THEREBY CREATING THE NEED FOR AN ARR ACFT TO GAR. THE ATIS HAD ANNOUNCED 118.1 FOR TWR; THE CREW HAD USED 119.1 INSTEAD. THE FO COMPLAINED OF THE VOICE QUALITY OF THE ATIS.

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AFTER BEING CLRED BY GND TO CROSS RWY 33 AND TAXI TO RWY 18; I SWITCHED TO THE PUBLISHED TWR FREQ AT 119.1. TWR CALLED US ON 119.1 AND CLRED US INTO POS AND HOLD. THEY ADVISED US THAT WE WOULD DEPART AS SOON AS TFC LNDG ON RWY 21 HAD CLRED OUR RWY; AND THAT TFC SEVERAL MI OUT WAS LNDG ON RWY 18; SO BE READY TO GO. WHEN NO TKOF CLRNC CAME; WE SUSPECTED OUR RADIO HAD FAILED. WHILE THE CAPT SWITCHED RADIOS; I LOOKED TOWARD THE TWR FOR LIGHT GUN SIGNALS. THE CAPT WAS SUCCESSFUL CONTACTING THE TWR ON THE OTHER RADIO. THE ARRIVING ACFT WAS FORCED TO GAR. AFTER BEING CLRED TO GO; TWR SWITCHED US TO DEP. DEP ASKED US WHAT FREQ WE WERE USING TO COMMUNICATE WITH TWR. WE TOLD HIM AND HE COMMENTED; OK; THAT ONE'S BROKE. I BECAME SUSPICIOUS AND WHEN TIME PERMITTED; I LISTENED TO THE DCA ATIS AGAIN; AND AFTER SEVERAL ATTEMPTS HEARD THAT TWR WAS USING 118.1 AS THEIR FREQ. APPARENTLY TWR HAD BEEN EXPERIENCING PROBS WITH 119.1 AND WAS USING 118.1. ON THE GND I MISSED THIS ON THE ATIS; PROBABLY BECAUSE OF THE POOR QUALITY OF THE COMPUTER VOICE GENERATED BY THE DIGITAL ATIS. ALSO THE SIMILAR SOUND OF 118.1 AND 119.1. I ALSO QUESTION WHY THE TWR CALLED US AND CLRED US INTO POS ON 119.1 WHEN THEY KNEW IT WAS BAD. HAD THEY NOT CALLED OR RESPONDED ON THIS FREQ; WE WOULD HAVE INVESTIGATED AND DISCOVERED THE PROB PRIOR TO BEING CLRED INTO POS ON RWY 18. NOTAMS ON OUR WX PACKAGE DID NOT SHOW 119.1 AS BEING OTS. SUGGESTIONS: DIGITAL ATIS SHOULD BE REVIEWED FOR THE POOR QUALITY OF THE COMPUTERIZED VOICE. A FREQ SHOULD NOT BE USED IF KNOWN TO BE BAD. PLTS INSTINCTIVELY SWITCH BACK TO THEIR LAST ASSIGNED FREQ WHEN COM CAN'T BE ESTABLISHED. CALL GREATER ATTN TO A CHANGE ON ATIS.