diff --git a/core/vb.c b/core/vb.c
index d0dfe96..f5dc369 100644
--- a/core/vb.c
+++ b/core/vb.c
@@ -360,10 +360,11 @@ struct VB {
             uint8_t shift;    /* Sweep shift amount */
 
             /* Other state */
-            uint32_t clocks;  /* Clocks until modification */
-            uint8_t  modmask; /* Modifications masked */
-            uint16_t next;    /* Next frequency value */
-            int      sample;  /* Current sample index */
+            uint32_t clocks;   /* Clocks until modification */
+            uint8_t  modmask;  /* Modifications masked */
+            uint8_t  freqmask; /* Frequency mask */
+            uint16_t next;     /* Next frequency value */
+            int      sample;   /* Current sample index */
         } freqmod;
 
         /* Channel 6 noise generator */
diff --git a/core/vsu.c b/core/vsu.c
index 971e507..3f52da9 100644
--- a/core/vsu.c
+++ b/core/vsu.c
@@ -83,11 +83,11 @@ static void vsuNextFreqMod(VB *sim, Channel *chan) {
     /* Modulation */
     else {
         next = (int32_t) chan->freq.written +
-            sim->vsu.modulation[sim->vsu.freqmod.sample];
-        if (next < 0)
-            next = 0;
-        if (next > 2047)
-            next = 2047;
+            sim->vsu.modulation[sim->vsu.freqmod.sample] & 0x7FF;
+        if (sim->vsu.freqmod.freqmask == 1)
+            next = (next & 0x700) | (chan->freq.written & 0x0FF);
+        else if (sim->vsu.freqmod.freqmask == 2) {
+            next = (next & 0x0FF) | (chan->freq.written & 0x700);
     }
 
     /* Configure state */
@@ -257,8 +257,10 @@ static void vsuWriteFQH(VB *sim, int index, uint16_t value) {
     value = value << 8 & 0x0700;
     chan->freq.current = (chan->freq.current & 0x00FF) | value;
     chan->freq.written = (chan->freq.written & 0x00FF) | value;
-    if (index == 4)
-        vsuNextFreqMod(sim, chan);
+    if (index != 4)
+        return;
+    sim->vsu.freqmod.freqmask = 2;
+    vsuNextFreqMod(sim, chan);
 }
 
 /* Write a value to S*FQL */
@@ -266,8 +268,10 @@ static void vsuWriteFQL(VB *sim, int index, uint8_t value) {
     Channel *chan = &sim->vsu.channels[index];
     chan->freq.current = (chan->freq.current & 0x0700) | value;
     chan->freq.written = (chan->freq.written & 0x0700) | value;
-    if (index == 4)
-        vsuNextFreqMod(sim, chan);
+    if (index != 4)
+        return;
+    sim->vsu.freqmod.freqmask = 1;
+    vsuNextFreqMod(sim, chan);
 }
 
 /* Write a value to S*INT */
@@ -510,6 +514,7 @@ static void vsuReset(VB *sim) {
     sim->vsu.freqmod.func     = 0;
     sim->vsu.freqmod.interval = 0;
     sim->vsu.freqmod.modmask  = 0;
+    sim->vsu.freqmod.freqmask = 0;
     sim->vsu.freqmod.next     = 0;
     sim->vsu.freqmod.rep      = 0;
     sim->vsu.freqmod.sample   = 0;
@@ -536,11 +541,14 @@ static void vsuWrite(VB*sim,uint32_t address,int type,int32_t value,int debug){
     }
 
     /* Unmapped */
-    if (address & 3)
+    if (address & 1)
         return;
 
     /* Working variables */
-    address &= 0x7FF;
+    address &= 0x7FC;
+
+    /* Clear frequency mask */
+    sim->vsu.freqmod.freqmask = 0;
 
     /* Wave memory */
     if (address < 0x280) {