diff --git a/core/vb.c b/core/vb.c
index ef0790b..12b7d3e 100644
--- a/core/vb.c
+++ b/core/vb.c
@@ -509,7 +509,7 @@ VBAPI vbOnRead vbGetReadCallback(VB *sim) {
 
 /* Retrieve the audio samples buffer */
 VBAPI void* vbGetSamples(VB *sim, uint32_t *capacity, uint32_t *position) {
-    if (capacity == NULL) {
+    if (sim->vsu.out.samples == NULL) {
         if (capacity != NULL)
             *capacity = 0;
         if (position != NULL)
diff --git a/core/vsu.c b/core/vsu.c
index db5fc14..283efa4 100644
--- a/core/vsu.c
+++ b/core/vsu.c
@@ -75,12 +75,13 @@ static void vsuNextFreqMod(VB *sim, Channel *chan) {
 
 /* Process one channel */
 static void vsuEmulateChannel(VB *sim, int index, uint32_t clocks) {
-    uint32_t bit;    /* Pseudorandom bit */
-    Channel *chan;   /* Channel handle */
-    uint32_t level;  /* Stereo output level */
-    uint32_t sample; /* Input sample */
-    uint32_t until;  /* Clocks to process sub-channel components */
-    int      e;      /* Iterator */
+    uint32_t bit;     /* Pseudorandom bit */
+    Channel *chan;    /* Channel handle */
+    int      freqmod; /* Frequency modifications enabled */
+    uint32_t level;   /* Stereo output level */
+    uint32_t sample;  /* Input sample */
+    uint32_t until;   /* Clocks to process sub-channel components */
+    int      e;       /* Iterator */
 
     /* Select channel */
     chan = &sim->vsu.channels[index];
@@ -107,39 +108,42 @@ static void vsuEmulateChannel(VB *sim, int index, uint32_t clocks) {
     /* Process all clocks */
     do {
 
+        /* Frequency modifications are active */
+        freqmod = 
+            index == 4           &&        /* Channel 5 */
+            sim->vsu.freqmod.enb &&        /* Modifications enabled */
+            sim->vsu.freqmod.interval != 0 /* Modifications valid */
+        ;
+
         /* Clocks until next state change */
         until = clocks;
-        if (chan->clocks < until) /* Next sample */
+        if (chan->clocks < until)
             until = chan->clocks;
-        if (
-            chan->env.enb &&         /* Modifications enabled */
-            chan->env.clocks < until /* Next modification */
-        ) until = chan->env.clocks;
-        if (
-            chan->int_.auto_ &&       /* Shutoff enabled */
-            chan->int_.clocks < until /* Shutoff */
-        ) until = chan->int_.clocks;
-        if (
-            index == 4                     && /* Channel 5 */
-            sim->vsu.freqmod.enb           && /* Modifications enabled */
-            sim->vsu.freqmod.interval != 0 && /* Modifications valid */
-            sim->vsu.freqmod.clocks < until   /* Next modification */
-        ) until = sim->vsu.freqmod.clocks;
+        if (chan->env.enb && chan->env.clocks < until)
+            until = chan->env.clocks;
+        if (chan->int_.auto_ && chan->int_.clocks < until)
+            until = chan->int_.clocks;
+        if (freqmod && sim->vsu.freqmod.clocks < until)
+            until = sim->vsu.freqmod.clocks;
 
         /* Manage clocks */
-        clocks -= until;
+        clocks       -= until;
+        chan->clocks -= until;
+        if (chan->env.enb)
+            chan->env.clocks -= until;
+        if (chan->int_.auto_)
+            chan->int_.clocks -= until;
+        if (freqmod)
+            sim->vsu.freqmod.clocks -= until;
 
         /* Automatic shutoff */
-        if (
-            chan->int_.auto_ && /* Shutoff enabled */
-            chan->int_.clocks == until
-        ) {
+        if (chan->int_.auto_ && chan->int_.clocks == 0) {
             chan->int_.enb = 0;
             return;
         }
 
         /* Next sample */
-        if (chan->clocks == until) {
+        if (chan->clocks == 0) {
 
             /* Wave */
             if (index != 5) {
@@ -161,10 +165,7 @@ static void vsuEmulateChannel(VB *sim, int index, uint32_t clocks) {
         }
 
         /* Envelope modification */
-        if (
-            chan->env.enb && /* Modifications enabled */
-            chan->env.clocks == until
-        ) {
+        if (chan->env.enb && chan->env.clocks == 0) {
             if (chan->env.dir == 0 && chan->env.value != 0)
                 chan->env.value--;
             else if (chan->env.dir == 1 && chan->env.value != 15)
@@ -172,16 +173,11 @@ static void vsuEmulateChannel(VB *sim, int index, uint32_t clocks) {
             else if (chan->env.rep)
                 chan->env.value = chan->env.reload;
             else chan->env.enb = 0;
-            chan->env.clocks = (uint32_t) chan->env.interval * 307220;
+            chan->env.clocks = ((uint32_t) chan->env.interval + 1) * 307220;
         }
 
         /* Frequency modification */
-        if (
-            index == 4                     && /* Channel 5 */
-            sim->vsu.freqmod.enb           && /* Modifications enabled */
-            sim->vsu.freqmod.interval != 0 && /* Modifications valid */
-            sim->vsu.freqmod.clocks == until
-        ) {
+        if (freqmod && sim->vsu.freqmod.clocks == 0) {
             chan->freq.current = sim->vsu.freqmod.next;
             vsuNextFreqMod(sim, chan);
             if (!chan->int_.enb)
@@ -222,7 +218,7 @@ static void vsuWriteEV1(VB *sim, int index, uint8_t value) {
         case 4: /* Channel 5 */
             sim->vsu.freqmod.enb  = value >> 6 & 1;
             sim->vsu.freqmod.func = value >> 4 & 1;
-            sim->vsu.freqmod.rep  = value >> 4 & 1;
+            sim->vsu.freqmod.rep  = value >> 5 & 1;
             vsuNextFreqMod(sim, chan);
             break;