r value decomposition algorithm 
<li><code>'ss'</code>: State space and singular value decomposition method (default)
<li><code>'tls'</code>: Hankel total least squares method
</ul>
<p>
The fourth input is an initial estimate for the number of damped sinusoids present in the <code>spectrum</code>. If an integer is provided that will be used otherwise the order can be estimated via:
</p>
<ul>
<li><code>'mdl'</code>: minimum description length (default) 
<li><code>'aic'</code>: Akaike information protocol
</u