my $startsmp = $mw->Button(-text => "Sample/Change Rate",
                                -command => sub{  
                                            if ( not defined $smp ){ $smp = $mw->repeat($srate,  \&samp)}
                                            else { 
                                                $smp->cancel;
                                                $smp = $mw->repeat($srate,  \&samp)
                                            }
                                }
                                );