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

===============================================================================
control-3.2.0      Release Date: 2019-04-01
===============================================================================

 ** new function sgrid

 ** New function: ss2ss
 
 ** Fixed many warnings

 ** added demos to rlocus, pzmap, bode, nichols, nyquist,
    impulse, lsim, ramp and step

 ** improved pzmap to plot with only zeros

===============================================================================
control-3.1.0      Release Date: 2018-03-11
===============================================================================
 
 ** New function: damp
 
 ** Changed nelem to numel and length to numel to remove the warnings
    during installation.

 ** The Impulse section has been rewritten, and a new file -- imp_invar.m -- 
    added. The old system tried to use the "zoh" to get an impulse response
    but that is impossible.

 ** The new function "imp_invar.m" converts a Laplace tranfer function
    to a discrete tranfer function.

 ** New functions: acker, dsort, esort.

 ** Function lsim now plots inputs as well.

 ** Class 'ss' should now work with complex inputs.

 ** Extend nyquist plots.

 ** Fixed warnings with newer Octave versions.

===============================================================================
control-3.0.0   Release Date: 2015-10-30   Release Manager: Lukas Reichlin
===============================================================================

 ** Substantial parts of the most vital internals have been rewritten
    and streamlined in order to improve compatibility and reliability.
    These changes to the codebase also result in better error handling
    in many situations and facilitate the integration of new features
    in upcoming releases.

 ** While many issues and pitfalls existing in previous releases of the
    control package have been fixed, the author isn't aware (at release date)
    of any regressions one might expect from a x.0.0 release.
    Note that the absence of regressions doesn't mean the absence of bugs.
    Especially it doesn't mean that *all* bugs from control-2.8.5 have been
    fixed.  Also note that to err is human, and software is written by humans.
    Therefore, any larger piece of software is likely to contain bugs.
    But the author is confident that he squashed considerably more bugs
    than he introduced in this release.  So if you waver between updating now
    and waiting for control-3.0.x, the author's suggestion is definitely 
    to update now.
    
 ** As always, your feedback is highly appreciated!  Even if you think that
    the control package is "fubar".  If you just follow the German saying
    "gesehen, gelacht, geloescht" - which roughly translates into
    "seen it, laughed at it, deleted it" - there's only little chance for
    improvement!  So if you think you've found a bug, please take the time
    to report your findings on Octave's bugtracker:
    
      <http://savannah.gnu.org/bugs/?group=octave>

    See the link below for more information about how to report problems:

      <http://octave.org/bugs.html>

 ** Your help is welcome!  There are many ways you can help the project
    fix the remaining problems, complete the documentation, and improve
    the overall user experience for both novices and experts alike.
    The author is aware of the fact that the documentation still leaves
    a lot to be desired.  As it is often the case with developers, writing
    documentation and tutorials is neither his key skill nor his favorite
    occupation.  After all, the author wrote the control package due to his
    personal interest in the field of control systems and not to make a
    living out of it.  The developer's profits are the insights and experience
    he obtained from his work.  Consequently, the control project doesn't ask
    you for money.  In case you like the control package and should feel like
    donating, then consider donating to GNU Octave or the Free Software
    Foundation according to the information on their websites.  Thank you!

 ** LTI models in general

    Improved handling of key-value pairs in all constructors for LTI models:
    
      tf (matrix, 'key1', val1, ...),  ss (a, b, 'key1', val1, ...)

    Support inheritance of LTI properties in many variations:

      tf (num, den, ltisys),  ss (a, b, ltisys, 'key1', val1, ...)
      sys.lti = ltisys,  zpk (z, p, k, Ts, 'var', 'q', 'lti', ltisys, ...)
    
    Accept [] as an alias for unspecified sampling time (Ts = -1):
    
      filt ('q^-1', []),  ss (a, b, c, d, []),  sys.Ts = []
    
    Automatic vector expansion for input and output names:
    
      sys.inname = 'str',  sys.outname = ''  (for MIMO systems)


 ** dss

    Fixed a long-standing but undiscovered bug where 'dss (sys)' overwrites
    sys.e with an identity matrix even if sys.e was not empty.  D'oh!

 ** ss

    Support for the following special cases has been added:
    
      ss (sys, 'explicit')   % convert descriptor system to regular one
      ss (sys, 'minimal')    % equivalent to  minreal (ss (sys))

 ** tf, zpk, filt

    Restrict transfer function variable to the candidates 's', 'p' for
    continuous-time and 'z', 'q', 'z^-1', 'q^-1' for discrete-time models.
       
      s = tf ('s'),  z = tf ('z^-1', Ts),  tf (num, den, Ts, 'var', 'q')
       
    Previous control versions supported any string, which could be quite
    misleading, see bug #45371.  <http://savannah.gnu.org/bugs/?43586>

 ** bode, bodemag, nichols, nyquist, sigma

    Fixed bug #42495.  <http://savannah.gnu.org/bugs/?42495>
    It is now possible to specify a frequency vector of length 1, e.g.
    
      [mag, phase] = bode (sys, 1)

    Furthermore, the functions raise an error or warning if invalid arguments
    are passed to them.  Previously, invalid arguments were simply ignored,
    which could be confusing.

 ** step, impulse, initial, ramp
 
    The time response functions now raise an error or warning if invalid
    arguments are passed to them.  As it was the case for frequency response
    functions, they used to simply ignore invalid arguments.

 ** lsim

    Fixed bug #46247.  <http://savannah.gnu.org/bugs/?46247>
    (Reported by Lutz Mager and Piet Wertelaers)

    Support logical arrays as input signal.  Before, logical arrays were
    simply ignored without notice, which could lead to unexpected results.
    Additionally, lsim received the same improvements as the other time and
    frequency response functions regarding invalid arguments.

 ** pzmap

    Improved argument checks.

 ** augstate

    New function for state-space models.  Adds the state vector to the output
    vector.

 ** connect

    The function no longer raises an error for name-based interconnections
    if some systems have common input names and one of these inputs are to
    be kept.

 ** estim

    Add input/output/state names to the returned system.  Support name-based
    selection of inputs (arguments 'sensors' and 'known').
 
 ** sumblk

    Add signal names as input/output groups if argument  n > 1.
 
 ** minreal

    If state-space models already have a minimal realization, then no state
    transformations are applied to the returned model.  In other words, if the
    number of states cannot be reduced (with the chosen tolerance), the model
    is returned unchanged.  Note that if the user sets the tolerance to the
    maximum value of 1, every state-space model will be reduced to zero states.
    There's nothing wrong with that, it's just how numerical computation works.


===============================================================================
control-2.8.5   Release Date: 2015-10-01   Release Manager: Lukas Reichlin
===============================================================================

** nyquist, nichols
   Specifying a frequency range no longer errors out,
   i.e. nyquist (sys, {wmin, wmax})

** pid, pidstd
   New functions for improved Matlab compatibility.

** ss
   Support  ss (a, b, c, 0)  for Matlab compatibility.
   (Thanks to Thomas Vasileiou)

** tf
   Fixed a problem which caused the expression below to fail.

     tf (Boeing707) \ tf (Boeing707)


===============================================================================
control-2.8.4   Release Date: 2015-09-02   Release Manager: Lukas Reichlin
===============================================================================

** tf
   Fixed a bug which affected interconnections of MIMO transfer functions.
   (Reported by Piet Wertelaers)

** VLFamp
   Included new demo created by Thomas D. Dean.


===============================================================================
control-2.8.3   Release Date: 2015-07-01   Release Manager: Lukas Reichlin
===============================================================================

** LTI models & iddata datasets
   Support partial matching of property names.

** mpower
   Fixed bug #45336.  <http://savannah.gnu.org/bugs/?45336>
   Improved reliability for the power computation of MIMO transfer functions.
   (Reported by Nick Jankowski)

** set
   Fixed bug #45371.  <http://savannah.gnu.org/bugs/?45371>
   The iddata properties 'name', 'notes' and 'userdata' are now stored as
   expected.  (Thanks to Petr Ledvina)


===============================================================================
control-2.8.2   Release Date: 2015-06-15   Release Manager: Lukas Reichlin
===============================================================================

** tf
   Fixed a problem when converting certain MIMO descriptor state-space systems
   to transfer function models.

** minreal
   Fixed bug #43263.  <http://savannah.gnu.org/bugs/?43263>
   Affected were transfer function models.  (Thanks to Endre Kozma)

** n4sid, moesp, moen4
   Fixed bug #41716.  <http://savannah.gnu.org/bugs/?41716>
   Display a meaningful error message when the iddata dataset contains not
   enough samples per experiment.  (Reported by John W. Eaton)

** rlocus
   Fixed bug #44949.  <http://savannah.gnu.org/bugs/?44949>
   Fixed a problem with discontinuous plots for systems like the one below.
   (Thanks to Doug Stewart)

     sys = tf ([1 0 0 0 0 0 1], [1 0 0 0 1 0 0 0 0 1])
     rlocus (sys)


===============================================================================
control-2.8.1   Release Date: 2015-05-03   Release Manager: Lukas Reichlin
===============================================================================

** tf
   -- Revamped the modifications of TB01ZD from control version 2.8.0.
      The function now uses LAPACK routines to find the maximum norm element.
      (Thanks to Thomas Vasileiou)
   -- Improved error handling when converting state-space models.


===============================================================================
control-2.8.0   Release Date: 2015-03-22   Release Manager: Lukas Reichlin
===============================================================================

** zero
   Compatibility fix in the C++ oct-file to support the upcoming Octave 4.0.
   (Thanks to Andreas Weber)

** isminimumphase
   Reverted changes from control-2.6.6.  According to the definition of
   Byrnes/Isidori, the function tests whether the system has asymptotically
   stable zero dynamics.  The poles are not tested.
   Note that the definition from Wikipedia is mixed up.
   For details, see the help text of the function (help @lti/isminimumphase)
   and the papers referenced therein.  (Thanks to Ulf Schaper)

** series
   Fixed a bug which gave unnecessary error messages when the function is
   called with 4 arguments.  (Thanks to Ulf Schaper)
   
** tf
   Resolved issues in MIMO state-space to transfer function conversion
   by implementing a more robust algorithm in SLICOT routine TB01ZD.
   The same algorithm is used in Octave's krylov function.
   (Thanks to Thomas Vasileiou)
   
** It is now possible to cross-compile the control package.
   (Thanks to John Donoghue)
   
** Support for 64bit indexing, i.e. Octave versions built with the --enable-64
   option.  (Thanks to Mike Miller)  

** The control package now depends on Octave version 3.8.0 or beyond.

** In order to comply with modified Octave Forge rules, the package no longer
   auto-loads by default.  See "help pkg" for details on auto-loading.


===============================================================================
control-2.6.6   Release Date: 2014-10-11   Release Manager: Lukas Reichlin
===============================================================================

** isminimumphase
   According to <http://en.wikipedia.org/wiki/Minimum_phase>, both the zeros
   and the poles of a minimum-phase system must be strictly inside the left
   complex half-plane (continuous-time case) or inside the unit circle
   (discrete-time case).  Previously, the poles were not tested.
   (Thanks to Endre Kozma)


===============================================================================
control-2.6.5   Release Date: 2014-06-16   Release Manager: Lukas Reichlin
===============================================================================

** LTI models
   -- The operator "times" (.*) no longer errors out when it is called with one
      SISO and one MIMO LTI system.  The operator behaves now the same way as
      Octave's built-in "times" for one scalar and one matrix.
   -- The function "repmat" is now overloaded for LTI systems.
   -- Added function "repsys" as an alias for "repmat" for the sake of Matlab
      compatibility.

** zero
   Fix crash in case of empty state-space models, e.g.

     zero (ss)


===============================================================================
control-2.6.4   Release Date: 2014-05-05   Release Manager: Lukas Reichlin
===============================================================================

** LTI models
   -- Fixed bug #42082.  <https://savannah.gnu.org/bugs/?42082>
      The operators mtimes (*), mldivide (\) and mrdivide (/) no longer error
      out when they are called with one SISO and one MIMO LTI system.  They now
      behave the same way as they do with one scalar and one matrix.
   -- Refuse struct arrays as input and output groups.
   -- Display non-empty properties of type 'name'.

** TF models
   -- Added TF-specific horzcat ([tf1, tf2]) and vertcat ([tf1; tf2]) operators
      which override the general LTI operators.  Their introduction avoids the
      conversion to state-space and back when MIMO transfer functions are
      concatenated horizontally and/or vertically.
   -- Added operator times (.*) to compute Hadamard/Schur product of two
      transfer function matrices.  This operator is useful to compute the
      Relative-Gain Array (RGA), e.g.
      
        G = tf (Boeing707)
        RGA = G .* inv (G).'
        RGA(0)


===============================================================================
control-2.6.3   Release Date: 2014-04-22   Release Manager: Lukas Reichlin
===============================================================================

** SS models
   For interconnections, support special case where the internal matrix
   (I - D*M) is singular, e.g.

     feedback (ss (-1, 1, 4, 1), '+')

** FRD models
   Fixed bug which let frequency response plots fail, e.g.
     
     bode (frd (tf (1, [1, 1])))

** tf
   Refuse denominators which are zero.

** c2d
   For zero-order hold approximation, use SLICOT MB05ND instead of Octave's
   "expm" to compute the matrix exponential.  Better accuracy is to be expected.

** connect
   -- Improved argument checking for legacy connection matrix.
   -- Document usage of index-based interconnections.

** inv
   -- Handle i/o names and groups correctly.
   -- Conserve the meaning of states by using the Matlab-compatible formula
      instead of the SLICOT-compatible one.
   -- Always use conversion to state-space and back for the inversion of
      MIMO transfer functions, even for 2x2 systems.
   
** pole
   Compute poles of descriptor state-space models with SLICOT AG08BD instead
   of Octave's built-in "eig" function.
   
** zero
   The gain of descriptor state-space models is now computed correctly.

   (Special thanks to Thomas Vasileiou for all the fixes above)

** rlocus
   -- Function "rlocus" now supports the fltk graphics toolkit.
      (Thanks to Doug Stewart)
   -- Fixed bug #41820.  <https://savannah.gnu.org/bugs/?41820>
      Function "rlocus" no longer errors out for certain systems like

        rlocus (tf ([1, 4, 4], [1, 8, 1, 8, 0]))

** transpose, ctranspose
   Delete i/o groups of transposed (sys.') or conjugate transposed (sys')
   LTI models.

** IDDATA identification datasets
   Raise an error if non-real-valued input or output matrices are assigned to
   time-domain datasets via the "set" function or subscripted assignment.


===============================================================================
control-2.6.2   Release Date: 2014-02-02   Release Manager: Lukas Reichlin
===============================================================================

** hinfsyn, mixsyn
   By default, hinfsyn and mixsyn compute now an optimal instead of a
   suboptimal controller.  Furthermore, Matlab compatibility is improved if
   hinfsyn/mixsyn is called with more than three/four input arguments.
   (Thanks to Thomas Vasileiou for the gamma iteration)
   The old behavior (sub-optimal controller) can be restored by the expressions
   
     hinfsyn (P, nmeas, ncont, 'method', 'sub')
     mixsyn (G, W1, W2, W3, 'method', 'sub')

** h2syn, hinfsyn, mixsyn, ncfsyn
   For Matlab compatibility, the third return argument is now norm "gamma".
   The info struct is now returned as fourth argument.

     [K, N, gamma, info] = *syn (G, …)

** mktito
   New function to partition plant for robust control.  "mktito" is also used
   by function "augw".  If a plant is partitioned that way, one can omit the
   inputs "nmeas" and "ncon" when calling the functions "hinfsyn" and "h2syn".

** ss
   The conversion of realizable descriptor systems to regular state-space form
   is now possible, even when the descriptor matrix "E" is singular.
   (Thanks to Thomas Vasileiou and Bruce Minaker)


===============================================================================
control-2.6.1   Release Date: 2013-12-10   Release Manager: Lukas Reichlin
===============================================================================

** LTI models
   Raise an error if the user tries to create an ingroup or outgroup with
   indices larger than the number of inputs m or outputs p.

** thiran
   New function to approximate a continuous-time delay by a discrete-time
   allpass Thiran filter.  (Thanks to Thomas Vasileiou and Spencer Jackson)

** end
   "end" indexing for LTI and IDDATA objects works now as expected.


===============================================================================
control-2.6.0   Release Date: 2013-10-24   Release Manager: Lukas Reichlin
===============================================================================

** LTI models
   -- Support for name-based channel selection in various commands.
      Example:  sys = Boeing707, sys('pitch', {'rudder', 'thrust'})
      instead of  sys(2, [2, 1]).
   -- New properties "ingroup" and "outgroup" to pool several channels:
      sys.ingroup.controls = [3, 1], sys.outgroup.measurements = [1, 2]
      sys('measurements', 'controls')  instead of  sys([1, 2], [3, 1]).
   -- Support names instead of indices in commands like "feedback" and "xperm".

** connect
   Support name-based interconnections.  This means that the LTI models passed
   to "connect" will be interconnected automatically if they have their inputs
   and outputs named accordingly.

** sumblk
   New function to build summing junctions for name-based interconnections
   by "connect".  Example: Sum = sumblk ('e = r - y + n')

** tf
   State-space to transfer function conversions of static gains, 
   e.g. "tf (ss (5))", no longer error out.  (Thanks to Thomas Vasileiou)

** IDDATA identification datasets
   Support for name-based output, input and experiment selection similar to
   LTI models.


===============================================================================
control-2.4.5   Release Date: 2013-10-07   Release Manager: Lukas Reichlin
===============================================================================

** Compatibility to GNU Octave 3.8
   Replaced line continuation marker "\" by "..." to avoid deprecated syntax
   warning.  (Thanks to John W. Eaton)

** db2mag, mag2db
   New functions to convert Decibels to Magnitude and vice versa.

** d2d
   New function for resampling of discrete-time models.


===============================================================================
control-2.4.4   Release Date: 2013-08-11   Release Manager: Lukas Reichlin
===============================================================================

** zero
   -- Fix documentation.  Function "zero" computes the invariant zeros, not the
      transmission zeros when it is called with one input argument.
   -- Added new options to compute the system, transmission, input decoupling
      and output decoupling zeros besides the already existing invariant zeros.
      Examples: z = zero (sys, 'system'), z = zero (sys, 'transmission')
   -- Return an "info" struct as third output argument.  It contains additional
      rank information as well as Kronecker indices and the numbers of infinite
      elementary divisors.  Example: [z, k, info] = zero (sys)
   (Special thanks to Ferdinand Svaricek)

** resample
   The "resample" function for "iddata" identification datasets now handles
   unspecified sampling times (tsam = -1) correctly.

** Time response functions (impulse, initial, lsim, ramp, step)
   Fix a thinko introduced with control-2.4.3.  In some cases, several
   responses were mistakingly plotted on the same subplot.
   (Thanks to Thomas Vasileiou)

** Various documentation enhancements and fixes.


===============================================================================
control-2.4.3   Release Date: 2013-07-12   Release Manager: Lukas Reichlin
===============================================================================

** resample
   The resample function for iddata sets now adjusts the sampling time
   correctly after resampling.

** Time response functions (impulse, initial, lsim, ramp, step)
   Don't use subplot for SISO models.  This allows the user to plot different
   responses on a single figure by calling subplot.  (Thanks to Matthias Meier)

** h2syn, hinfsyn, mixsyn, ncfsyn
   Third return argument is now an info struct.
   [K, N, info] = *syn (G, …)

** optiPID
   Beef up documentation.


===============================================================================
control-2.4.2   Release Date: 2013-02-16   Release Manager: Lukas Reichlin
===============================================================================

** Transfer function to state-space conversion (Thanks to Thomas Vasileiou):
   -- Fix crash.
   -- Fix bug if denominators are zero.

** minreal 
   Fix crash in minreal for state-space systems.  (Thanks to Thomas Vasileiou)

** sigma
   The plot types 1, 2, 3 were no longer supported since the introduction of
   multi-system plots in control-2.3.54.  The help text is now updated
   accordingly and cruft is removed from the codebase.

** mixsyn, ncfsyn, Madievski
   Beef up help text.


===============================================================================
control-2.4.1   Release Date: 2012-11-01   Release Manager: Lukas Reichlin
===============================================================================

** impulse, initial, lsim, step
   These time response functions now error out correctly when the user tries to
   plot multiple systems in one window with mismatching numbers of inputs and
   outputs.  The problem was caused by Octave's built-in function "size_equal"
   which gave false positives for LTI objects.  The problem is solved by
   overloading the built-in function with a new LTI method "size_equal".

** ramp
   Added a new time domain analysis function "ramp" which plots/computes the
   system's response to a ramp signal.

** c2d, d2c
   Added matched pole/zero discretization method "matched".
   Usage:  Gd = c2d (Gc, Ts, 'matched'),  Gc = d2c (Gd, 'matched')

** doc/control.pdf
   Revised PDF manual.  There is now a function index at the end of the manual.
   Improved docstrings in several functions.


===============================================================================
control-2.4.0   Release Date: 2012-09-26   Release Manager: Lukas Reichlin
===============================================================================

** Multiplot feature for time-domain plotting:
   
     impulse    lsim
     initial    step
     
   Supported are now function calls like
   impulse (sys1, sys2, ...), step (sys1, 'b', sys2, 'r', ...)
   lsim (sys1, '-k', sys2, '--k', u), initial (sys, sys2, x0)
   The multiplot feature for frequency-domain plotting has already been
   introduced with control-2.3.54.

** Anderson, Madievski, MDSSystem, optiPID
   Updated example scripts to use new multiplot feature.

** sensitivity
   Fixed a problem with plotting, introduced with control-2.3.54.

** doc/control.pdf
   Updated PDF manual.


===============================================================================
control-2.3.54   Release Date: 2012-09-15   Release Manager: Lukas Reichlin
===============================================================================

** Multiplot feature for frequency-domain plotting:
   
     bode       nichols     pzmap
     bodemag    nyquist     sigma

   The functions above support calls like 
   sigma (sys1, sys2, ...), nyquist (sys1, 'b', sys2, 'r', ...),
   bode (sys1, '-k', sys2, '-.k', sys3, ':k', {wmin, wmax}).
   Time domain functions are not yet multiplot ready, but they will follow in
   control-2.4.0.

** plot
   A selection of experiments from iddata identification datasets can be
   plotted by  plot (dat, exp)  instead of  plot (dat(:,:,:,exp)).

** sensitivity
   Fixed a problem where an error was raised about an undefined function
   "issiso".

** All SLICOT function names have now leading and trailing underscores to
   emphasize their private nature.


===============================================================================
control-2.3.53   Release Date: 2012-08-27   Release Manager: Lukas Reichlin
===============================================================================

** Added new functions for system identification, including:
   
     arx        iddata       ifft
     moen4      detrend      nkshift
     moesp      fft          plot
     n4sid      filter       resample

** sensitivity
   Added new function to compute and plot the sensitivity margin Ms.

** feedback
   Fixed an argument check which caused false positive error messages.
   It was a copy-paste mistake affecting non-square systems.
   (Thanks to Tony Olivo)


===============================================================================
control-2.3.52   Release Date: 2012-06-25   Release Manager: Lukas Reichlin
===============================================================================

** Fixed a silly mistake in MIMO transfer function to state-space conversion.
   The bug has been introduced with control-2.3.51.
   (Thanks to Jim Rawlings for providing the test case)

** dlqe, lqe
   Added new functions for linear quadratic estimators.
   (Thanks to Megan Zagrobelny)

** Upon request of the Debian maintainers, the tex-files  control.tex  and
   functions.texi  for generating  control.pdf  are included in the doc folder,
   next to  control.pdf.  Note that  functions.texi  is generated automatically
   by the scripts in the control/devel folder and the package generate_html.
   They are not included in the control package and can be found on the
   OctaveForge SVN server.

** Minor improvements in various help texts.


===============================================================================
control-2.3.51   Release Date: 2012-06-03   Release Manager: Lukas Reichlin
===============================================================================

** filt, filtdata, tf
   -- Added function "filt" to specify disrete-time transfer functions in DSP
      format, i.e. z^-1.
   -- Added function "filtdata" to return any type of discrete-time LTI model
      in DSP format.
   -- tf models have a new property "inv".  To display a discrete-time TF sys
      in z^-1, set  sys.inv=true.  In order to switch to z, set  sys.inv=false.
      "filt" sets property "inv" to true (z^-1) by default, while "tf" uses
      false (z) as default value.
      
** ctranspose
   Conjugate transpose or pertransposition of LTI objects.
   Used by Octave for "sys'".  For a transfer-function matrix G, G' denotes the
   conjugate of G given by G.'(-s) for a continuous-time system or G.'(1/z) for
   a discrete-time system.  The frequency response of the pertransposition of G
   is the Hermitian (conjugate) transpose of G(jw),
   i.e. freqresp (G', w) = freqresp (G, w)'.
   WARNING: Do NOT use this for dual problems, use the transpose "sys.'"
   (note the dot) instead.

** test_control
   Add a few remarks to the help text regarding the severity of failing tests.

** Makefile fixed to work with non-standard linker options e.g on
   Apple.

** The conversion to state-space of multi-input transfer functions with common
   row denominators is now handled more efficiently.


===============================================================================
control-2.3.50   Release Date: 2012-03-06   Release Manager: Lukas Reichlin
===============================================================================

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

** Anderson, Madievski
   -- Added two examples for controller reduction.  The m-files are named after
      the authors of the corresponding papers.

** fitfrd
   -- Added function to fit frequency response data with a state-space model.

** set
   -- The set command doesn't need a return argument anymore in order to save
      the modified values.  set (sys, "key", value)  is now equivalent to
      sys = set (sys, "key", value).

** 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)

** Revised package installation and cleanup efforts under the hood.
   The new solution compiles the SLICOT library in a less barbaric way and
   creates only a single oct-file containing all the SLICOT routines.
   This also brings along faster compile times.
   (Special thanks to Hans Buchmann, Carlo De Falco and Michael Goffioul for
   their advice)

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


===============================================================================
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.


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