diff --git a/src/engine/platform/genesis.cpp b/src/engine/platform/genesis.cpp
index 90c221fc1..714447f39 100644
--- a/src/engine/platform/genesis.cpp
+++ b/src/engine/platform/genesis.cpp
@@ -74,33 +74,9 @@ void DivPlatformGenesis::tick() {
     if (i==2 && extMode) continue;
     if (chan[i].freqChanged) {
       chan[i].freq=(chan[i].baseFreq*(ONE_SEMITONE+chan[i].pitch))/ONE_SEMITONE;
-      if (chan[i].freq>=82432) {
-        chan[i].freqH=((chan[i].freq>>15)&7)|0x38;
-        chan[i].freqL=(chan[i].freq>>7)&0xff;
-      } else if (chan[i].freq>=41216) {
-        chan[i].freqH=((chan[i].freq>>14)&7)|0x30;
-        chan[i].freqL=(chan[i].freq>>6)&0xff;
-      } else if (chan[i].freq>=20608) {
-        chan[i].freqH=((chan[i].freq>>13)&7)|0x28;
-        chan[i].freqL=(chan[i].freq>>5)&0xff;
-      } else if (chan[i].freq>=10304) {
-        chan[i].freqH=((chan[i].freq>>12)&7)|0x20;
-        chan[i].freqL=(chan[i].freq>>4)&0xff;
-      } else if (chan[i].freq>=5152) {
-        chan[i].freqH=((chan[i].freq>>11)&7)|0x18;
-        chan[i].freqL=(chan[i].freq>>3)&0xff;
-      } else if (chan[i].freq>=2576) {
-        chan[i].freqH=((chan[i].freq>>10)&7)|0x10;
-        chan[i].freqL=(chan[i].freq>>2)&0xff;
-      } else if (chan[i].freq>=1288) {
-        chan[i].freqH=((chan[i].freq>>9)&7)|0x08;
-        chan[i].freqL=(chan[i].freq>>1)&0xff;
-      } else {
-        chan[i].freqH=(chan[i].freq>>8)&7;
-        chan[i].freqL=chan[i].freq&0xff;
-      }
-      writes.emplace(chanOffs[i]+0xa4,chan[i].freqH);
-      writes.emplace(chanOffs[i]+0xa0,chan[i].freqL);
+      int freqt=toFreq(chan[i].freq);
+      writes.emplace(chanOffs[i]+0xa4,freqt>>8);
+      writes.emplace(chanOffs[i]+0xa0,freqt&0xff);
     }
     if (chan[i].keyOn) {
       writes.emplace(0x28,0xf0|konOffs[i]);
@@ -132,6 +108,26 @@ int DivPlatformGenesis::octave(int freq) {
   return 1;
 }
 
+int DivPlatformGenesis::toFreq(int freq) {
+  if (freq>=82432) {
+    return 0x3800|((freq>>7)&0x7ff);
+  } else if (freq>=41216) {
+    return 0x3000|((freq>>6)&0x7ff);
+  } else if (freq>=20608) {
+    return 0x2800|((freq>>5)&0x7ff);
+  } else if (freq>=10304) {
+    return 0x2000|((freq>>4)&0x7ff);
+  } else if (freq>=5152) {
+    return 0x1800|((freq>>3)&0x7ff);
+  } else if (freq>=2576) {
+    return 0x1000|((freq>>2)&0x7ff);
+  } else if (freq>=1288) {
+    return 0x800|((freq>>1)&0x7ff);
+  } else {
+    return freq&0x7ff;
+  }
+}
+
 int DivPlatformGenesis::dispatch(DivCommand c) {
   if (c.chan>5) {
     c.chan-=6;
@@ -237,20 +233,29 @@ int DivPlatformGenesis::dispatch(DivCommand c) {
     }
     case DIV_CMD_NOTE_PORTA: {
       int destFreq=644.0f*pow(2.0f,((float)c.value2/12.0f));
+      int newFreq;
       bool return2=false;
       if (destFreq>chan[c.chan].baseFreq) {
-        chan[c.chan].baseFreq=chan[c.chan].baseFreq+c.value*octave(chan[c.chan].baseFreq);
-        if (chan[c.chan].baseFreq>=destFreq) {
-          chan[c.chan].baseFreq=destFreq;
+        newFreq=chan[c.chan].baseFreq+c.value*octave(chan[c.chan].baseFreq);
+        if (newFreq>=destFreq) {
+          newFreq=destFreq;
           return2=true;
         }
       } else {
-        chan[c.chan].baseFreq=chan[c.chan].baseFreq-c.value*octave(chan[c.chan].baseFreq);
-        if (chan[c.chan].baseFreq<=destFreq) {
-          chan[c.chan].baseFreq=destFreq;
+        newFreq=chan[c.chan].baseFreq-c.value*octave(chan[c.chan].baseFreq);
+        if (newFreq<=destFreq) {
+          newFreq=destFreq;
           return2=true;
         }
       }
+      if (!chan[c.chan].portaPause) {
+        if (octave(chan[c.chan].baseFreq)!=octave(newFreq)) {
+          chan[c.chan].portaPause=true;
+          break;
+        }
+      }
+      chan[c.chan].baseFreq=newFreq;
+      chan[c.chan].portaPause=false;
       chan[c.chan].freqChanged=true;
       if (return2) return 2;
       break;
diff --git a/src/engine/platform/genesis.h b/src/engine/platform/genesis.h
index e486c38f8..62dfcc58f 100644
--- a/src/engine/platform/genesis.h
+++ b/src/engine/platform/genesis.h
@@ -13,10 +13,10 @@ class DivPlatformGenesis: public DivDispatch {
       int freq, baseFreq, pitch;
       unsigned char ins;
       signed char konCycles;
-      bool active, insChanged, freqChanged, keyOn, keyOff;
+      bool active, insChanged, freqChanged, keyOn, keyOff, portaPause;
       int vol;
       unsigned char pan;
-      Channel(): freqH(0), freqL(0), freq(0), baseFreq(0), pitch(0), ins(-1), active(false), insChanged(true), freqChanged(false), keyOn(false), keyOff(false), vol(0), pan(3) {}
+      Channel(): freqH(0), freqL(0), freq(0), baseFreq(0), pitch(0), ins(-1), active(false), insChanged(true), freqChanged(false), keyOn(false), keyOff(false), portaPause(false), vol(0), pan(3) {}
     };
     Channel chan[10];
     struct QueuedWrite {
@@ -45,6 +45,7 @@ class DivPlatformGenesis: public DivDispatch {
     short pendingWrites[512];
 
     int octave(int freq);
+    int toFreq(int freq);
   
   public:
     void acquire(int& l, int& r);
diff --git a/src/engine/platform/genesisext.cpp b/src/engine/platform/genesisext.cpp
index f33572d73..5232ef942 100644
--- a/src/engine/platform/genesisext.cpp
+++ b/src/engine/platform/genesisext.cpp
@@ -99,20 +99,29 @@ int DivPlatformGenesisExt::dispatch(DivCommand c) {
     }
     case DIV_CMD_NOTE_PORTA: {
       int destFreq=644.0f*pow(2.0f,((float)c.value2/12.0f));
+      int newFreq;
       bool return2=false;
       if (destFreq>opChan[ch].baseFreq) {
-        opChan[ch].baseFreq=opChan[ch].baseFreq+c.value*octave(opChan[ch].baseFreq);
-        if (opChan[ch].baseFreq>=destFreq) {
-          opChan[ch].baseFreq=destFreq;
+        newFreq=opChan[ch].baseFreq+c.value*octave(opChan[ch].baseFreq);
+        if (newFreq>=destFreq) {
+          newFreq=destFreq;
           return2=true;
         }
       } else {
-        opChan[ch].baseFreq=opChan[ch].baseFreq-c.value*octave(opChan[ch].baseFreq);
-        if (opChan[ch].baseFreq<=destFreq) {
-          opChan[ch].baseFreq=destFreq;
+        newFreq=opChan[ch].baseFreq-c.value*octave(opChan[ch].baseFreq);
+        if (newFreq<=destFreq) {
+          newFreq=destFreq;
           return2=true;
         }
       }
+      if (!opChan[ch].portaPause) {
+        if (octave(opChan[ch].baseFreq)!=octave(newFreq)) {
+          opChan[ch].portaPause=true;
+          break;
+        }
+      }
+      opChan[ch].baseFreq=newFreq;
+      opChan[ch].portaPause=false;
       opChan[ch].freqChanged=true;
       if (return2) return 2;
       break;
diff --git a/src/engine/platform/genesisext.h b/src/engine/platform/genesisext.h
index a83995a14..56c2741de 100644
--- a/src/engine/platform/genesisext.h
+++ b/src/engine/platform/genesisext.h
@@ -8,10 +8,10 @@ class DivPlatformGenesisExt: public DivPlatformGenesis {
     int freq, baseFreq, pitch;
     unsigned char ins;
     signed char konCycles;
-    bool active, insChanged, freqChanged, keyOn, keyOff;
+    bool active, insChanged, freqChanged, keyOn, keyOff, portaPause;
     int vol;
     unsigned char pan;
-    OpChannel(): freqH(0), freqL(0), freq(0), baseFreq(0), pitch(0), ins(-1), active(false), insChanged(true), freqChanged(false), keyOn(false), keyOff(false), vol(0), pan(3) {}
+    OpChannel(): freqH(0), freqL(0), freq(0), baseFreq(0), pitch(0), ins(-1), active(false), insChanged(true), freqChanged(false), keyOn(false), keyOff(false), portaPause(false), vol(0), pan(3) {}
   };
   OpChannel opChan[4];
   public: