EXHIBIT 12.1

 
 
                                         COMPUTATION OF RATIO OF EARNINGS TO FIXED CHARGES

                                              ----------------------------------------------------------------------------------
                                                   SIX MONTHS ENDED                             YEAR ENDED
                                                       JUNE 30,                                 DECEMBER 31,
                                              ---------------------------   ----------------------------------------------------
                                                    1997           1998         1993      1994      1995      1996      1997      
                                                -----------    ------------   --------  --------  --------  --------  --------  
                                                                                   (In thousands)
                                                                                                        
Ratio of earnings to fixed charges:

  Earnings (deficiency):                          

    Income (loss) from continuing
      operations before taxes                     $38,702        $(25,206)    $18,812   $28,343   $38,081   $59,828   $60,278
                                                 ---------       ---------   --------- --------- --------- --------- ---------
  Fixed Charges:                           
    Interest charges                                  104             210         887     3,172     3,188     2,774       293
    Interest factor of operating rents              1,000           1,639         738       877     1,240     1,871     2,142  
                                                 ---------       ---------   --------- --------- --------- --------- ---------
  Total fixed charges                               1,104           1,849       1,625     4,049     4,428     4,645     2,435  
                                                 ---------       ---------   --------- --------- --------- --------- ---------  
  Earnings (deficiency), as adjusted              $39,806        $(23,357)    $20,437   $32,392   $42,509   $64,473   $62,713 
                                                 =========       =========   ========= ========= ========= ========= =========
Ratio of earnings to fixed charges                  36.1x             N/A (1)   12.6x      8.0x      9.6x     13.9x     25.8x 
                                                 =========       =========   ========= ========= ========= ========= =========
 

(1) For the six months ended June 30, 1998, there was a deficiency of earnings
    available to cover the fixed charges amounting to $25.2 million. Excluding
    the charge of $68.2 million for purchased in-process technology associated
    with the acquisition of Voicetek Corporation, the ratio of earnings to fixed
    charges for the six months ended June 30, 1998 would have been 24.2x.