diff --git a/index.html b/index.html
index 7dca2ce..ad8ed09 100644
--- a/index.html
+++ b/index.html
@@ -74,7 +74,7 @@ todo:
  - missing XM effects:
    - E3x, E4x, E5x, E6x, E7x, E9x, EDx, EEx
    - 7xy - tremolo
-   - Hxy, Kxx, Lxx, Pxy, Txy
+   - Kxx, Lxx, Pxy, Txy
  - render pattern with the wider fonts for fewer channels
  - fix occasional rendering/audio hiccups when switching songs
 </pre>
diff --git a/test/effects.js b/test/effects.js
index ed147ac..40ed393 100644
--- a/test/effects.js
+++ b/test/effects.js
@@ -224,6 +224,55 @@ exports['test Gxx global volume'] = function(assert) {
   assert.equal(XMPlayer.xm.global_volume, 0x40*2, 'global volume set to 0x40');
 };
 
+exports['test Hxy global volume slide'] = function(assert) {
+  var xm = testdata.resetXMData();
+  xm.tempo = 6;
+  xm.global_volume = 128;
+
+  xm.patterns[0] = [
+    [[48,  1, -1, 17, 0x0f]],  // C-4  1 -- H0f (slide down)
+    [[-1, -1, -1, 17, 0x90]],  // --- -- -- H90 (slide up)
+    [[-1, -1, -1, 17, 0x00]],  // --- -- -- H00 (continue same)
+    [[-1, -1, 0x30, 17, 0x11]],  // --- -- 30 H11 (invalid, do nothing)
+  ];
+  XMPlayer.nextTick();
+  assert.equal(xm.global_volume, 128, 'row 0 tick 0 vol 128');
+  XMPlayer.nextTick();
+  assert.equal(xm.global_volume, 98, 'row 0 tick 1 vol 98');
+  XMPlayer.nextTick();
+  assert.equal(xm.global_volume, 68, 'row 0 tick 2 vol 68');
+  XMPlayer.nextTick();
+  assert.equal(xm.global_volume, 38, 'row 0 tick 3 vol 38');
+  XMPlayer.nextTick();
+  assert.equal(xm.global_volume, 8, 'row 0 tick 4 vol 8');
+  XMPlayer.nextTick();
+  assert.equal(xm.global_volume, 0, 'row 0 tick 5 vol 0');
+  XMPlayer.nextTick();
+  assert.equal(xm.global_volume, 0, 'row 1 tick 0 vol 0');
+  XMPlayer.nextTick();
+  assert.equal(xm.global_volume, 18, 'row 1 tick 1 vol 18');
+  XMPlayer.nextTick();  // row 1 tick 2
+  XMPlayer.nextTick();  // tick 3
+  XMPlayer.nextTick();  // tick 4
+  XMPlayer.nextTick();  // tick 5
+  assert.equal(xm.global_volume, 90, 'row 1 tick 5 vol 90');
+  XMPlayer.nextTick();  // row 2 tick 0
+  assert.equal(xm.global_volume, 90, 'row 2 tick 0 vol 90');
+  XMPlayer.nextTick();  // row 2 tick 1
+  assert.equal(xm.global_volume, 108, 'row 2 tick 1 vol 108');
+  XMPlayer.nextTick();  // tick 2
+  XMPlayer.nextTick();  // tick 3
+  assert.equal(xm.global_volume, 128, 'row 2 tick 3 vol 128');
+  XMPlayer.nextTick();  // tick 4
+  XMPlayer.nextTick();  // tick 5
+  XMPlayer.nextTick();  // row 3 tick 0
+  assert.equal(xm.global_volume, 128, 'row 3 tick 0 vol 128');
+  XMPlayer.nextTick();  // tick 1
+  assert.equal(xm.global_volume, 128, 'row 3 tick 1 vol 128');
+  XMPlayer.nextTick();  // tick 2
+  assert.equal(xm.global_volume, 128, 'row 3 tick 2 vol 128');
+};
+
 exports['test E5x finetune override'] = function(assert) {
   var xm = testdata.resetXMData();
   // set an initial finetune so we know we're overriding it...
diff --git a/xmeffects.js b/xmeffects.js
index 0976b51..45b6319 100644
--- a/xmeffects.js
+++ b/xmeffects.js
@@ -191,6 +191,20 @@ function eff_t0_g(ch, data) {  // set global volume
   }
 }
 
+function eff_t0_h(ch, data) {  // global volume slide
+  if (data) {
+    // same as Axy but multiplied by 2
+    player.xm.global_volumeslide = (-(data & 0x0f) + (data >> 4)) * 2;
+  }
+}
+
+function eff_t1_h(ch) {  // global volume slide
+  if (player.xm.global_volumeslide !== undefined) {
+    player.xm.global_volume = Math.max(0, Math.min(player.max_global_volume,
+      player.xm.global_volume + player.xm.global_volumeslide));
+  }
+}
+
 function eff_t0_r(ch, data) {  // retrigger
   if (data & 0x0f) ch.retrig = (ch.retrig & 0xf0) + (data & 0x0f);
   if (data & 0xf0) ch.retrig = (ch.retrig & 0x0f) + (data & 0xf0);
@@ -244,7 +258,7 @@ player.effects_t0 = [  // effect functions on tick 0
   eff_t0_e,  // e
   eff_t0_f,  // f
   eff_t0_g,  // g
-  eff_unimplemented_t0,  // h
+  eff_t0_h,  // h
   eff_unimplemented_t0,  // i
   eff_unimplemented_t0,  // j
   eff_unimplemented_t0,  // k
@@ -283,7 +297,7 @@ player.effects_t1 = [  // effect functions on tick 1+
   eff_t1_e,  // e
   null,   // f
   null,  // g
-  eff_unimplemented,  // h
+  eff_t1_h,  // h
   eff_unimplemented,  // i
   eff_unimplemented,  // j
   eff_unimplemented,  // k