diff --git a/CMakeLists.txt b/CMakeLists.txt
index d532da251..67d4ae966 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -711,6 +711,8 @@ endif (WIN32)
 
 # build a library of package functionality (without and optionally with OpenMP support)
 add_library (wsjt_cxx STATIC ${wsjt_CSRCS} ${wsjt_CXXSRCS})
+
+# build an OpenMP variant of the Fortran library routines
 add_library (wsjt_fort STATIC ${wsjt_FSRCS})
 if (${OPENMP_FOUND} OR APPLE)
   add_library (wsjt_fort_omp STATIC ${wsjt_FSRCS})
@@ -721,10 +723,12 @@ if (${OPENMP_FOUND} OR APPLE)
     )
   file (MAKE_DIRECTORY ${CMAKE_BINARY_DIR}/fortran_modules_omp)
   if (APPLE)
-    # nasty hack to use OpenMP only on Fortran files - assumes gfortran being used
+    # On  Mac  we don't  have  working  OpenMP  support in  the  C/C++
+    # compilers so  we have to manually  set the correct flags  to get
+    # OpenMP support in jt9.
     set_target_properties (wsjt_fort_omp
       PROPERTIES
-      COMPILE_FLAGS "-fopenmp"
+      COMPILE_FLAGS "-fopenmp"	# assumes GNU style Fortran compiler
       )
   endif (APPLE)
 endif (${OPENMP_FOUND} OR APPLE)
@@ -746,28 +750,29 @@ add_executable (jt9code lib/jt9code.f90 wsjtx.rc)
 target_link_libraries (jt9code wsjt_fort wsjt_cxx)
 
 add_executable (jt9 lib/jt9.f90 lib/jt9a.f90 lib/jt9b.f90 lib/jt9c.f90 ${jt9_CXXSRCS} wsjtx.rc)
-target_link_libraries (jt9 wsjt_fort wsjt_cxx ${FFTW3_LIBRARIES})
 qt5_use_modules (jt9 Core)
-
 if (${OPENMP_FOUND} OR APPLE)
-  add_executable (jt9_omp lib/jt9.f90 lib/jt9a.f90 lib/jt9b.f90 lib/jt9c.f90 ${jt9_CXXSRCS} wsjtx.rc)
   if (APPLE)
-    set_target_properties (jt9_omp
+    # On  Mac  we don't  have  working  OpenMP  support in  the  C/C++
+    # compilers so we  have to manually set the  correct linking flags
+    # and libraries to get OpenMP support in jt9.
+    set_target_properties (jt9
       PROPERTIES
-      COMPILE_FLAGS "-fopenmp"
+      COMPILE_FLAGS "-fopenmp"	# assumes GNU style Fortran compiler
       Fortran_MODULE_DIRECTORY ${CMAKE_BINARY_DIR}/fortran_modules_omp
-      LINK_LIBRARIES "gomp;gcc_s.1"
+      LINK_LIBRARIES "gomp;gcc_s.1" # assume GNU libgcc OpenMP
       )
   else (APPLE)
-    set_target_properties (jt9_omp
+    set_target_properties (jt9
       PROPERTIES
       COMPILE_FLAGS "${OpenMP_C_FLAGS}"
       LINK_FLAGS "${OpenMP_C_FLAGS}"
       Fortran_MODULE_DIRECTORY ${CMAKE_BINARY_DIR}/fortran_modules_omp
       )
   endif (APPLE)
-  target_link_libraries (jt9_omp wsjt_fort_omp wsjt_cxx ${FFTW3_LIBRARIES})
-  qt5_use_modules (jt9_omp Core)
+  target_link_libraries (jt9 wsjt_fort_omp wsjt_cxx ${FFTW3_LIBRARIES})
+else (${OPENMP_FOUND} OR APPLE)
+  target_link_libraries (jt9 wsjt_fort wsjt_cxx ${FFTW3_LIBRARIES})
 endif (${OPENMP_FOUND} OR APPLE)
 
 # build configuration dialog and transceiver interface test application
@@ -844,13 +849,6 @@ install (TARGETS jt9 jt65code jt9code
   BUNDLE DESTINATION ${WSJT_BIN_DESTINATION} COMPONENT runtime
   )
 
-if (${OPENMP_FOUND} OR APPLE)
-  install (TARGETS jt9_omp
-    RUNTIME DESTINATION ${WSJT_BIN_DESTINATION} COMPONENT runtime
-    BUNDLE DESTINATION ${WSJT_BIN_DESTINATION} COMPONENT runtime
-    )
-endif (${OPENMP_FOUND} OR APPLE)
-
 if (WSJT_INCLUDE_KVASD)
   install (PROGRAMS
     ${CMAKE_BINARY_DIR}/${KVASD_BINARY}
diff --git a/mainwindow.cpp b/mainwindow.cpp
index d2e38ec9f..32bece4ef 100644
--- a/mainwindow.cpp
+++ b/mainwindow.cpp
@@ -374,9 +374,19 @@ MainWindow::MainWindow(bool multiple, QSettings * settings, QSharedMemory *shdme
   QFile {m_config.temp_dir ().absoluteFilePath (".lock")}.open(QIODevice::ReadWrite);
 
   QStringList jt9_args {
-    "-s", QApplication::applicationName ()
-        , "-w", "1"                                     //FFTW patience
-        , "-m", "1"                                     //FFTW threads
+    "-s", QApplication::applicationName () // shared memory key,
+                                           // includes rig-name
+
+      , "-w", "2"               //FFTW patience
+
+      // The number  of threads for  FFTW specified here is  chosen as
+      // three because  that gives  the best  throughput of  the large
+      // FFTs used  in jt9.  The count  is the minimum of  (the number
+      // available CPU threads less one) and three.  This ensures that
+      // there is always at least one free CPU thread to run the other
+      // mode decoder in parallel.
+      , "-m", QString::number (qMin (qMax (QThread::idealThreadCount () - 1, 1), 3)) //FFTW threads
+
       , "-e", QDir::toNativeSeparators (m_appDir)
       , "-a", QDir::toNativeSeparators (m_dataDir.absolutePath ())
       , "-t", QDir::toNativeSeparators (m_config.temp_dir ().absolutePath ())