<html>
<head>
<meta http-equiv="content-type" content="text/html; charset=utf-8">
</head>
<body bgcolor="#FFFFFF" text="#000000">
<p>Hi there</p>
<p>In Scilab I've used interp1 to calculate a cubic spline
interpolation, like this:<span style="color:rgb(0,0,0);"><br>
</span></p>
<p><span style="color:rgb(0,0,0);">a</span><span
style="color:rgb(74,85,219);">(</span><span
style="color:rgb(255,170,0);">:</span><span
style="color:rgb(0,0,0);">,</span><span
style="color:rgb(188,143,143);">1</span><span
style="color:rgb(74,85,219);">)</span> <span
style="color:rgb(92,92,92);">=</span> <span
style="color:rgb(174,92,176);text-decoration:underline;">interp1</span><span
style="color:rgb(74,85,219);">(</span><span
style="color:rgb(50,185,185);">log</span><span
style="color:rgb(74,85,219);">(</span><span
style="color:rgb(0,0,0);">f3</span><span
style="color:rgb(74,85,219);">)</span><span
style="color:rgb(0,0,0);">,</span><span
style="color:rgb(0,0,0);">a1</span><span
style="color:rgb(0,0,0);">,</span><span
style="color:rgb(50,185,185);">log</span><span
style="color:rgb(74,85,219);">(</span><span
style="color:rgb(0,0,0);">f</span><span
style="color:rgb(74,85,219);">)</span><span
style="color:rgb(0,0,0);">,</span><span
style="color:rgb(188,143,143);">'</span><span
style="color:rgb(188,143,143);">spline</span><span
style="color:rgb(188,143,143);">'</span><span
style="color:rgb(74,85,219);">)</span><span
style="color:rgb(0,0,0);">;</span></p>
<p>a = amplitude (magnitude). f3 is a frequency (27000 linear spaced
data), a1 is the original data, f is the resampled 1200
frequencies (log-spaced), where I need the spline to interpolate
some data for me.</p>
<p>Above should work OK, but it's not perfect, compared to a Fortran
script. The fortran script calculates with its own cubic spline
routine, utilizing LAPACK (DGTTRF and DGTTRS) to solve for
polynomial coefficients.</p>
<p><br>
</p>
<p>The question is - above code line with the interp1 spline, which
kind of spline is it?</p>
<p>Digging into interp, I see multiple options. Digging into splin,
I also see multiple options.</p>
<p>I looks like the interp1 is using "natural" spline - is this
correct?</p>
<p>It's strange because the splin help documentation doesn't
recommend this. It says: Don't use the natural type unless the
underlying function have zero second end points derivatives.<br>
</p>
<p>This might be my problem.</p>
<p>The Scilab help for interp1 doesn't give any examples, but does
mention I can add an "extrap" method. Could this be any of the
suggestions in the splin documentation. For example, could I
write:</p>
<p><span style="color:rgb(0,0,0);">a</span><span
style="color:rgb(74,85,219);">(</span><span
style="color:rgb(255,170,0);">:</span><span
style="color:rgb(0,0,0);">,</span><span
style="color:rgb(188,143,143);">1</span><span
style="color:rgb(74,85,219);">)</span> <span
style="color:rgb(92,92,92);">=</span> <span
style="color:rgb(174,92,176);text-decoration:underline;">interp1</span><span
style="color:rgb(74,85,219);">(</span><span
style="color:rgb(50,185,185);">log</span><span
style="color:rgb(74,85,219);">(</span><span
style="color:rgb(0,0,0);">f3</span><span
style="color:rgb(74,85,219);">)</span><span
style="color:rgb(0,0,0);">,</span><span
style="color:rgb(0,0,0);">a1</span><span
style="color:rgb(0,0,0);">,</span><span
style="color:rgb(50,185,185);">log</span><span
style="color:rgb(74,85,219);">(</span><span
style="color:rgb(0,0,0);">f</span><span
style="color:rgb(74,85,219);">)</span><span
style="color:rgb(0,0,0);">,</span><span
style="color:rgb(188,143,143);">'</span><span
style="color:rgb(188,143,143);">spline</span><span
style="color:rgb(188,143,143);">'</span><span
style="color:rgb(0,0,0);">,</span><span
style="color:rgb(188,143,143);">'</span><span
style="color:rgb(188,143,143);">not-a-knot</span><span
style="color:rgb(188,143,143);">'</span><span
style="color:rgb(74,85,219);">)</span><span
style="color:rgb(0,0,0);">;</span></p>
<p>?</p>
<p>P.S. Since not-a-knot is mentioned as the default for the splin
function, I think it should also be made the default for interp1
... just my two cents.<br>
</p>
<p>/Claus<br>
</p>
</body>
</html>