Summary of important user-visible changes for releases of the control package

===============================================================================
control-2.3.50   Release Date: 2012-xx-yy   Release Manager: Lukas Reichlin
===============================================================================

** Added new functions for frequency-weighted model and controller order
   reduction:
   
     bstmodred      btaconred
     btamodred      cfconred
     hnamodred      fwcfconred
     spamodred      spaconred

** Require Octave version 3.6.0 or better.  (The frequency response plotting
   commands have been simplified.  They now use the fixed "axis tight" command.
   This is a first step towards multiple systems in one plot,
   e.g. bode (sys1, sys2, sys3).  Furthermore, the code takes advantage of the
   new "arrayfun" function which became a faster oct-file instead of an m-file)

** get
   -- The get command doesn't need a return argument anymore.
      set (sys, "key", value)  is now equivalent to  sys = set (sys, "key", value).

** Revised package installation.  The new solution compiles the SLICOT library
   in a less barbaric way and brings along faster compile times.


===============================================================================
control-2.2.5   Release Date: 2012-02-09   Release Manager: Lukas Reichlin
===============================================================================

** Improved Matlab compatibility for frequency response commands.  It is now
   possible to specify a frequency range.  Example: bode (sys, {wmin, wmax})
   where wmin and wmax denote frequencies in rad/s.

** margin
   -- Fixed a variable name such that discrete-time models are plotted
      without an error. (Thanks to Renato Caldas)


===============================================================================
control-2.2.4   Release Date: 2012-01-07   Release Manager: Lukas Reichlin
===============================================================================

** Compatibility with Octave 3.6.0.  (The makefile must specify the libraries
   that mkoctfile needs to link.  Thanks to Marco Atzeri and Carlo De Falco)

** ctrbf, obsvf
   -- Added new functions to compute controllable and observable block
      Hessenberg realizations based on SLICOT TB01UD.
      (Thanks to Benjamin Fernandez and Alexandre Felipe)


===============================================================================
control-2.2.3   Release Date: 2011-12-07   Release Manager: Lukas Reichlin
===============================================================================

** Improved performance when computing the frequency response of transfer
   function models by orders of magnitude.  (I realized that "polyval" can
   evaluate a polynomial at several values at once in a vectorized manner.)

** bode, bodemag, nichols, sigma
   -- Fixed a hang when plotting pairs of purely imaginary poles or zeros.
      The hang was caused by nonsensical (Inf, NaN) axis values.
      (Reported by Laurent Tissier)

** Use single instead of double quotes when displaying names of TF and FRD
   models.


===============================================================================
control-2.2.2   Release Date: 2011-12-04   Release Manager: Lukas Reichlin
===============================================================================

** Improved error messages for place, ARE solvers and robust control commands.

** minreal, ss
   -- Fixed a crash for descriptor models.  minreal for dss models as well as
      conversion from non-proper transfer functions to descriptor state-space
      models should work now as expected.

** ss
   -- Revised default tolerance for transfer function to state-space conversion
      by SLICOT TD04AD.

** Better performance when computing the frequency response of SISO transfer
   function models.

** Reorganized tests.  Most tests have been moved from "ltimodels" to the files
   of the LTI methods being tested.  All available tests can be executed by
   "test_control".

** The NEWS file is handled correctly when using Octave version 3.6.
   Type "news("control")" or "news control" to display the news (3.6 only).


===============================================================================
control-2.2.1   Release Date: 2011-10-24   Release Manager: Lukas Reichlin
===============================================================================

** tf
   -- Fixed a nasty bug that prevented shortening of numerator and denominator
      polynomials consisting solely of zeros.
   -- MIMO support for descriptor state-space to transfer function conversion.
      Usage: tf_sys = tf (dss_sys)
   -- MIMO support for interconnections of non-proper transfer functions via
      internal conversion to state-space.

** ss
   -- Support conversion from non-proper transfer function to descriptor state-
      space.  Usage: dss_sys = ss (tf_sys) 

** c2d, d2c
   -- Support bilinear transformation of descriptor state-space models.

** inv
   -- Support the inversion of MIMO transfer functions.  Inverses of 2x2 TFs are
      computed directly, larger models are computed internally in state-space.

** place
   -- Return the number of fixed, assigned and uncontrollable poles in a single
      "info" struct instead of three individual output arguments. 

** rlocus
   -- Clarify usage statement in help string.
   -- Check whether system is SISO.

** MDSSystem
   -- Display bode plots of controllers instead of singular value plots of the
      closed loops.

** hsvd
   -- Added option "alpha" to specify the alpha-stability boundary for the
      eigenvalues of the state dynamics matrix A.

** isctrb, isobsv
   -- Return number of controllable/observable states as a second output
      argument.

** doc/control.pdf
   -- Added preface to PDF manual.


===============================================================================
control-2.2.0   Release Date: 2011-09-26   Release Manager: Lukas Reichlin
===============================================================================

** ss
   -- Transfer function to state-space conversion uses now SLICOT TD04AD.
      Conversion of MIMO models is now supported.  Usage: ss_sys = ss (tf_sys)

** tf
   -- Support for interconnections of MIMO transfer functions.  This is done by
      an internal conversion to a minimal state-space representation.  With the
      current tf2ss and ss2tf conversions, only proper transfer function are
      supported.  This limitation does not exist for SISO transfer functions.
   -- Fixed a cellfun statement that caused problems on MinGW32 builds and
      possibly some others.  (Reported by Bernhard Weller)

** pole, zero
   -- Computation of poles and zeros of MIMO transfer functions is now possible
      via conversion to state-space.  Please note that the state-space
      realization of SLICOT TD04AD is a minimal one.  Therefore certain poles
      and zeros might be missing.

** zpk, zpkdata
   -- Included wrappers that create transfer function models from zero-pole-gain
      data (zpk) and zero-pole-gain data from lti models (zpkdata).  They are
      stop-gap measures for compatibility until ZPK models are implemented.

** tfdata
   -- "vector" option added.  For SISO models, it returns numerator and
      denominator directly as column vectors instead of cells containing a
      single column vector.

** doc/control.pdf
   -- Revised PDF manual.


===============================================================================
control-2.1.55   Release Date: 2011-09-07   Release Manager: Lukas Reichlin
===============================================================================

** c2d
   -- Support for "tustin" and "prewarp" method added.
   -- Transfer functions are now supported via the state-space methods.
   -- Improved Texinfo string.

** d2c
   -- Discrete to continuous-time conversion added.  However, support is
      limited to the zero-order hold, tustin and pre-warping methods.
   
** Conversion from descriptor to regular state-space is now performed by SLICOT
   routine SB10JD.  Better numerical results are to be expected over the
   previous naive inversion formula.  This conversion is used internally for
   ssdata and some other functions.


===============================================================================
control-2.1.54   Release Date: 2011-08-22   Release Manager: Lukas Reichlin
===============================================================================

** tf
   -- State-space to transfer function conversion uses now SLICOT TB04BD.
      Conversion of MIMO models is now supported.  Usage: tf_sys = tf (ss_sys)
   -- Display an empty line between title and numerator for better readability.
   -- Display whether model is static, continuous- or discrete-time.

** A PDF manual is included for the first time.  It is located inside the "doc"
   folder.  It has been generated automatically from the Texinfo help strings
   and is not yet completely sorted out.


===============================================================================
control-2.1.53   Release Date: 2011-08-08   Release Manager: Lukas Reichlin
===============================================================================

** ncfsyn
   -- Added support for McFarlane/Glover loop shaping design procedure.
      "ncfsyn" stands for Normalized Coprime Factor Synthesis.

** MDSSystem
   -- Added example script which demonstrates the usage of the robust control
      commands "mixsyn" and "ncfsyn".

** Texinfo help strings of several functions have been extended, although
   documentation still leaves a lot to be desired.


===============================================================================
control-2.1.52   Release Date: 2011-07-27   Release Manager: Lukas Reichlin
===============================================================================

** hsvd
   -- Use scaling unless state-space model property "scaled" is set to true.

** norm
   -- Use scaling for computation of L-infinity norm unless state-space model
      property "scaled" is set to true.
      
** minreal
   -- Use scaling for state-space and descriptor state-space models unless
      property "scaled" is set to true.
   -- More accurate results are to be expected for descriptor state-space
      models by performing only those reduction phases where effective order
      reduction occurs. This is achieved by saving the system matrices before
      each phase and restoring them if no order reduction took place.
  
** zero
   -- Use scaling for state-space and descriptor state-space models unless
      property "scaled" is set to true.

** frdata
   -- The frequency response is now returned correctly as an array and not as a
      vector, unless the "vector" option is set and the system is single-input
      single-output.
   -- Added help text.


===============================================================================
control-2.1.51   Release Date: 2011-07-21   Release Manager: Lukas Reichlin
===============================================================================

** frd
   -- Support for Frequency Response Data (frd) measurement "models".


===============================================================================
control-2.1.50   Release Date: 2011-07-06   Release Manager: Lukas Reichlin
===============================================================================

** ss
   -- Support for property "scaled".  By default, it is set to "false".

** prescale
   -- Scaling for state-space models (SLICOT TB01ID) and descriptor models
      (SLICOT TG01AD).
   
** freqresp
   -- Scale state-space models using @lti/prescale.m if property "scaled" is
      set to "false".  Frequency response commands now perform automatic
      scaling unless model property "scaled" is set to "true".


===============================================================================
control-2.0.2   Release Date: 2011-03-18   Release Manager: Lukas Reichlin
===============================================================================

** lsim
   -- Fixed a logical error that refused valid initial state vectors.  It was
      due to a thinko introduced with the changes in control-2.0.1.
      (Thanks to Rob Frohne)


===============================================================================
control-2.0.1   Release Date: 2011-03-06   Release Manager: Lukas Reichlin
===============================================================================

** lsim
   -- Support time vectors not starting at zero. (Thanks to Rob Frohne)
   -- Improved help text.

** zero
   -- The gain of descriptor state-space models is now computed correctly.
      (fingers crossed)


===============================================================================
control-2.0.0   Release Date: 2011-02-08   Release Manager: Lukas Reichlin
===============================================================================

** First official release.


===============================================================================