plot frequency responses better

1 files changed, 59 insertions, 50 deletions

diff --git a/volume_pot/volume.pl b/volume_pot/volume.pl
index 07d7d49..3dc0cd3 100755
--- a/volume_pot/volume.pl
+++ b/volume_pot/volume.pl
@@ -35,88 +35,97 @@ sub par($$) {
 @fns  = ();
 @frqs = ();
 
-$rac = 470000.;
+# calc gain for taps at te,td frequeny f and knob position k (0-10)
 
-$te = 0.20;    # terminal e
-$td = 0.66;    # terminal d
+sub calc_gain($$) {
+    my ( $f, $k ) = @_;
+
+    my $te = 0.4;    # terminal e
+    my $td = 0.6;    # terminal d
+
+    my $rac = 470000.;
 
-for ( $f = 20. ; $f < 20010. ; $f = $f * sqrt(10.) ) {
-    #
     #terminal c is input
     #terminal b is output
     #terminal a is ground
 
-    $cnf = -i * ( 1. / ( 0.000000001 * 2 * pi * $f ) );
+    my $cnf = -i * ( 1. / ( 0.000000001 * 2 * pi * $f ) );
 
-    $ld = 18000. + ( ( 1. / 47. ) * $cnf );     #18k+47nF
-    $le = 4700. +  ( ( 1. / 250. ) * $cnf );    #4.7k+250nF
+    # loads on terminals d and e
+    my $ld = 18000. + ( ( 1. / 47. ) * $cnf );     #18k+47nF
+    my $le = 4700. +  ( ( 1. / 250. ) * $cnf );    #4.7k+250nF
 
     #compute resistances of track
+    my $rae = $rac * logpot($te);
+    my $rad = $rac * logpot($td);
 
-    $rae = $rac * logpot($te);
-    $rad = $rac * logpot($td);
-
-    $red = $rad - $rae;
+    my $red = $rad - $rae;
 
-    $rdc = $rac - $rad;
+    my $rdc = $rac - $rad;
 
-    $tot = $rae + $red + $rdc;
+    my $tot = $rae + $red + $rdc;
 
     print "Track resistances: $rae + $red + $rdc  = $tot\n";
 
-    # next compute the efective resistances of each leg at HF
+    # next compute the efective impedances of each leg
 
-    $erae = par( $rae,         $le );
-    $erad = par( $erae + $red, $ld );
-    $ered = $erad - $erae;
-    $erac = $erad + $rdc;
-    $erdc = $erac - $erad;
+    my $erae = par( $rae,         $le );
+    my $erad = par( $erae + $red, $ld );
+    my $ered = $erad - $erae;
+    my $erac = $erad + $rdc;
+    my $erdc = $erac - $erad;
 
     print "Effective total HF resistance: $erae + $ered + $erdc = $erac\n";
 
-    $pe  = $erae / $erac;
-    $pd  = $erad / $erac;
-    $ped = $pd - $pe;
-    $pda = 1 - $pd;
+    # compute the potentials for a 1V input
 
-    $fn = sprintf( "volume-%d.dat", int( $f + .1 ) );
-    push @fns,  $fn;
-    push @frqs, int( $f + .1 ) . " Hz";
-
-    open F, ">", $fn;
+    my $pe  = $erae / $erac;
+    my $pd  = $erad / $erac;
+    my $ped = $pd - $pe;
+    my $pda = 1 - $pd;
 
-    for ( my $v = 0.01 ; $v < 1 ; $v += 0.01 ) {
+    my $v = $k / 10.;
+    my $hf;
 
-        $lf = logpot($v);
-
-        if ( $v < $te ) {
-            $hf = $pe * logprop( $v, 0, $te );
-        }
-        elsif ( $v < $td ) {
-            $hf = $pe + ( $ped * logprop( $v, $te, $td ) );
-        }
-        else {
-            $hf = $pd + ( $pda * logprop( $v, $td, 1. ) );
-        }
-
-        print F 10. * $v, " ", abs($hf), " ", $lf, "\n";
+    if ( $v < $te ) {
+        $hf = $pe * logprop( $v, 0, $te );
+    }
+    elsif ( $v < $td ) {
+        $hf = $pe + ( $ped * logprop( $v, $te, $td ) );
+    }
+    else {
+        $hf = $pd + ( $pda * logprop( $v, $td, 1. ) );
     }
 
-    close F;
+    return abs($hf);
 }
 
 open P, "|gnuplot";
 print P "set term png size 1280,1024\n";
 print P "set output 'volume.png'\n";
+print P "set logscale x\n";
 print P "set logscale y\n";
-print P "set xlabel 'knob position'\n";
+print P "set yrange [:1.1]\n";
+print P "set xlabel 'frequency/Hz'\n";
 print P "set ylabel 'gain'\n";
 print P "plot ";
-for $i ( 0 .. $#fns ) {
-    print P " '"
-      . $fns[$i]
-      . "' using 1:2 with lines title '"
-      . $frqs[$i] . "',";
+
+for ( my $k = 1 ; $k < 10.5 ; ++$k ) {
+    my $gmax = 0;
+    for ( my $f = 20 ; $f < 20000.1 ; $f *= 1.1 ) {
+        my $g = calc_gain( $f, $k );
+        $gmax = $g if $g > $gmax;
+    }
+    my $fn = "volume-" . $k . ".dat";
+    open F, ">", $fn;
+    for ( my $f = 20 ; $f < 20000.1 ; $f *= 1.1 ) {
+        my $g = calc_gain( $f, $k );
+        print F $f, " ", $g, " ", $g / $gmax, "\n";
+    }
+    close F;
+
+    print P " '" . $fn . "' using 1:2 with lines title '" . $k . "',";
+
 }
 print "\n";
 close P;