diff --git a/test/all.js b/test/all.js
index a21d2bd..c0a6053 100644
--- a/test/all.js
+++ b/test/all.js
@@ -4,7 +4,6 @@ require('../xm.js');
 require('../xmeffects.js');
 var XMPlayer = window.XMPlayer;
 
-tests = {};
 testdata = require('./testdata.js');
 
 // test TODO:
@@ -12,7 +11,7 @@ testdata = require('./testdata.js');
 //  - unit test for envelopes
 //
 // using assert passed to the test function that just logs failures 
-tests['test XM startup'] = function(assert) {
+exports['test XM startup'] = function(assert) {
   testdata.resetXMData();
   XMPlayer.nextTick();
   assert.equal(XMPlayer.cur_songpos, 0, 'advance to initial song position');
@@ -21,7 +20,7 @@ tests['test XM startup'] = function(assert) {
   assert.equal(XMPlayer.cur_row, 1, 'advance to row 1');
 };
 
-require('./instrument.js');
-require('./effects.js');
+exports['test instruments'] = require('./instrument.js');
+exports['test effects'] = require('./effects.js');
 
-if (module == require.main) require('test').run(tests);
+if (module == require.main) require('test').run(exports);
diff --git a/test/effects.js b/test/effects.js
index 2993de8..247fec6 100644
--- a/test/effects.js
+++ b/test/effects.js
@@ -7,7 +7,7 @@ var XMPlayer = window.XMPlayer;
 //  - porta+vol 5xy
 //  - etc etc
 
-tests['test 0xy arpeggio'] = function(assert) {
+exports['test 0xy arpeggio'] = function(assert) {
   var xm = testdata.resetXMData();
   // [pat][row][channel]
   xm.patterns[0][0][0] = [48, 1, -1, 0, 0x4f];  // C-4  1 -- 04f
@@ -26,7 +26,7 @@ tests['test 0xy arpeggio'] = function(assert) {
   assert.equal(ch.period, 1152 - 16*0, 'row 1 tick 0 period 0');
 };
 
-tests['test 1xx slide up'] = function(assert) {
+exports['test 1xx slide up'] = function(assert) {
   var xm = testdata.resetXMData();
   // [pat][row][channel]
   xm.patterns[0][0][0] = [48,  1, -1, 1, 0x01];  // C-4  1 -- 101
@@ -45,7 +45,7 @@ tests['test 1xx slide up'] = function(assert) {
   assert.equal(ch.period, 1152 - 3, 'row 1 tick 1 period -3');
 };
 
-tests['test 2xx slide down'] = function(assert) {
+exports['test 2xx slide down'] = function(assert) {
   var xm = testdata.resetXMData();
   // [pat][row][channel]
   xm.patterns[0][0][0] = [48,  1, -1, 2, 0x01];  // C-4  1 -- 201
@@ -55,16 +55,16 @@ tests['test 2xx slide down'] = function(assert) {
   var ch = xm.channelinfo[0];
   assert.equal(ch.period, 1152, 'row 0 tick 0 period 0');
   XMPlayer.nextTick();
-  assert.equal(ch.period, 1152 + 1, 'row 0 tick 1 period -1');
+  assert.equal(ch.period, 1152 + 1, 'row 0 tick 1 period +1');
   XMPlayer.nextTick();
-  assert.equal(ch.period, 1152 + 2, 'row 0 tick 2 period -2');
+  assert.equal(ch.period, 1152 + 2, 'row 0 tick 2 period +2');
   XMPlayer.nextTick();
-  assert.equal(ch.period, 1152 + 2, 'row 1 tick 0 period -2');
+  assert.equal(ch.period, 1152 + 2, 'row 1 tick 0 period +2');
   XMPlayer.nextTick();
-  assert.equal(ch.period, 1152 + 3, 'row 1 tick 1 period -3');
+  assert.equal(ch.period, 1152 + 3, 'row 1 tick 1 period +3');
 };
 
-tests['test 3xx portamento'] = function(assert) {
+exports['test 3xx portamento'] = function(assert) {
   var xm = testdata.resetXMData();
   // [pat][row][channel]
   xm.patterns[0][0][0] = [48,  1, -1, 0, 0x00];  // C-4  1 -- 000
@@ -83,3 +83,72 @@ tests['test 3xx portamento'] = function(assert) {
   assert.equal(ch.period, 1152 - 16, 'row 1 tick 2 period -16');
 };
 
+exports['test 4xy vibrato'] = function(assert) {
+  var xm = testdata.resetXMData();
+  // vibrato 4xy: speed x, depth y
+  // full cycle is 64/speed
+  // [pat][row][channel]
+  xm.patterns[0] = [
+    [[48,  1, -1, 4, 0x81]],  // C-4  1 -- 481
+    [[-1, -1, -1, 4, 0x02]],  // --- -- -- 402
+    [[-1, -1, -1, 4, 0x10]],  // --- -- -- 410
+    [[-1, -1, -1, 4, 0x00]],  // --- -- -- 400
+    [[-1, -1, -1, 0, 0x00]],  // --- -- -- 000 - no vibrato
+    [[-1, -1, -1, 4, 0x00]],  // --- -- -- 400 - resume vibrato @ pos 0
+  ];
+  // I should really be testing ch.doff directly
+  XMPlayer.xm.tempo = 3;
+  var ch = xm.channelinfo[0];
+  XMPlayer.nextTick();  // row 0 tick 0
+  var p0 = ch.doff;
+  assert.equal(ch.periodoffset, 0, 'row 0 tick 0 periodoffset=0');
+  XMPlayer.nextTick();  // row 0 tick 1
+  // compute logical period p from actual play frequency
+  var p = 16*12 * Math.log(p0 / ch.doff) / Math.log(2);
+  assert.equal(p.toFixed(3), "0.707", 'row 0 tick 1 period +0.707');
+  XMPlayer.nextTick();  // row 0 tick 2
+  p = -16*12 * Math.log(ch.doff / p0) / Math.log(2);
+  assert.equal(p.toFixed(3), "1.000", 'row 0 tick 2 period +1.000');
+  XMPlayer.nextTick();  // row 1 tick 0
+  p = -16*12 * Math.log(ch.doff / p0) / Math.log(2);
+  assert.equal(p.toFixed(3), "1.414", 'row 1 tick 0 period +1.414');
+  XMPlayer.nextTick();  // row 1 tick 1
+  p = -16*12 * Math.log(ch.doff / p0) / Math.log(2);
+  assert.equal(p.toFixed(3), "0.000", 'row 1 tick 1 period +0');
+  XMPlayer.nextTick();  // row 1 tick 2
+  p = -16*12 * Math.log(ch.doff / p0) / Math.log(2);
+  assert.equal(p.toFixed(3), "-1.414", 'row 1 tick 2 period -1.414');
+  XMPlayer.nextTick();  // row 2 tick 0
+  p = -16*12 * Math.log(ch.doff / p0) / Math.log(2);
+  assert.equal(p.toFixed(3), "-2.000", 'row 2 tick 0 period -2.000');
+  XMPlayer.nextTick();  // row 2 tick 1
+  p = -16*12 * Math.log(ch.doff / p0) / Math.log(2);
+  assert.equal(p.toFixed(3), "-1.990", 'row 2 tick 1 period -1.990');
+  XMPlayer.nextTick();  // row 2 tick 2
+  p = -16*12 * Math.log(ch.doff / p0) / Math.log(2);
+  assert.equal(p.toFixed(3), "-1.962", 'row 2 tick 2 period -1.962');
+  XMPlayer.nextTick();  // row 3 tick 0
+  p = -16*12 * Math.log(ch.doff / p0) / Math.log(2);
+  assert.equal(p.toFixed(3), "-1.914", 'row 3 tick 0 period -1.914');
+  XMPlayer.nextTick();  // row 3 tick 1
+  p = -16*12 * Math.log(ch.doff / p0) / Math.log(2);
+  assert.equal(p.toFixed(3), "-1.848", 'row 3 tick 1 period -1.848');
+  XMPlayer.nextTick();  // row 3 tick 2
+  p = -16*12 * Math.log(ch.doff / p0) / Math.log(2);
+  assert.equal(p.toFixed(3), "-1.764", 'row 3 tick 2 period -1.764');
+  XMPlayer.nextTick();  // row 4 tick 0
+  p = -16*12 * Math.log(ch.doff / p0) / Math.log(2);
+  assert.equal(p.toFixed(3), "0.000", 'row 4 tick 0 period 0 - no vibrato');
+  XMPlayer.nextTick();  // row 4 tick 1
+  XMPlayer.nextTick();  // row 4 tick 2
+  // I actually don't know whether vibrato is supposed to reset when the effect
+  // goes away or whether it should resume. Resuming is simpler to implement so
+  // that's what I'm assuming here...
+  XMPlayer.nextTick();  // row 5 tick 0
+  p = -16*12 * Math.log(ch.doff / p0) / Math.log(2);
+  assert.equal(p.toFixed(3), "-1.663", 'row 5 tick 0 period -1.663 - vibrato resume');
+  XMPlayer.nextTick();  // row 5 tick 1
+  p = -16*12 * Math.log(ch.doff / p0) / Math.log(2);
+  assert.equal(p.toFixed(3), "-1.546", 'row 5 tick 1 period -1.546');
+};
+
diff --git a/test/instrument.js b/test/instrument.js
index 929ee7e..a7d399b 100644
--- a/test/instrument.js
+++ b/test/instrument.js
@@ -4,7 +4,7 @@ var XMPlayer = window.XMPlayer;
 //  - test volume envelope (sustain, release)
 //  - test panning envelope
 
-tests['test note on'] = function(assert) {
+exports['test note on'] = function(assert) {
   var xm = testdata.resetXMData();
   // [pat][row][channel]
   xm.patterns[0][0][0] = [48, 1, 0x10 + 0x33, 0, 0];  // C-4  1 33 000
@@ -17,7 +17,7 @@ tests['test note on'] = function(assert) {
   assert.equal(ch.off, 0, 'trigger offset');
 };
 
-tests['test instrument trigger'] = function(assert) {
+exports['test instrument trigger'] = function(assert) {
   var xm = testdata.resetXMData();
   // [pat][row][channel]
   xm.patterns[0][0][0] = [48, 1, 0x10 + 0x33, 0, 0];  // C-4  1 33 000
@@ -37,7 +37,7 @@ tests['test instrument trigger'] = function(assert) {
   assert.equal(ch.off, 0, 'set offset=0');
 };
 
-tests['test note trigger'] = function(assert) {
+exports['test note trigger'] = function(assert) {
   var xm = testdata.resetXMData();
   // [pat][row][channel]
   xm.patterns[0][0][0] = [48, 1, 0x10 + 0x33, 0, 0];  // C-4  1 33 000
diff --git a/test/testdata.js b/test/testdata.js
index ed04dec..0ca0568 100644
--- a/test/testdata.js
+++ b/test/testdata.js
@@ -22,6 +22,7 @@ exports.resetXMData = function() {
     mute: 0,
     volE: 0, panE: 0,
     retrig: 0,
+    vibratopos: 0,
     vibratodepth: 1,
     vibratospeed: 1,
   });
diff --git a/xm.js b/xm.js
index 5b28d9f..ff31034 100644
--- a/xm.js
+++ b/xm.js
@@ -226,10 +226,9 @@ function nextRow() {
       if (ch.effect != 9) ch.off = 0;
       ch.release = 0;
       ch.envtick = 0;
-      ch.vibratopos = 0;
       ch.env_vol = new EnvelopeFollower(inst.env_vol);
       ch.env_pan = new EnvelopeFollower(inst.env_pan);
-      if (ch.note != undefined) {
+      if (ch.note) {
         ch.period = periodForNote(ch, ch.note);
       }
     }
@@ -287,25 +286,31 @@ EnvelopeFollower.prototype.Tick = function(release) {
 
 function nextTick() {
   player.cur_tick++;
+  var j, ch;
+  for (j = 0; j < player.xm.nchan; j++) {
+    ch = player.xm.channelinfo[j];
+    ch.periodoffset = 0;
+  }
   if (player.cur_tick >= player.xm.tempo) {
     player.cur_tick = 0;
     nextRow();
   }
-  for (var j = 0; j < player.xm.nchan; j++) {
-    var ch = player.xm.channelinfo[j];
+  for (j = 0; j < player.xm.nchan; j++) {
+    ch = player.xm.channelinfo[j];
     var inst = ch.inst;
-    ch.periodoffset = 0;
-    if (player.cur_tick != 0) {
+    if (player.cur_tick !== 0) {
       if(ch.voleffectfn) ch.voleffectfn(ch);
       if(ch.effectfn) ch.effectfn(ch);
     }
     if (isNaN(ch.period)) {
-      console.log(prettify_notedata(player.xm.patterns[player.cur_pat][player.cur_row-1][j]),
+      console.log(prettify_notedata(
+            player.xm.patterns[player.cur_pat][player.cur_row-1][j]),
           "set channel", j, "period to NaN");
     }
-    if (inst == undefined) continue;
-    if (ch.env_vol == undefined) {
-      console.log(prettify_notedata(player.xm.patterns[player.cur_pat][player.cur_row-1][j]),
+    if (inst === undefined) continue;
+    if (ch.env_vol === undefined) {
+      console.log(prettify_notedata(
+            player.xm.patterns[player.cur_pat][player.cur_row-1][j]),
           "set channel", j, "env_vol to undefined, but note is playing");
       continue;
     }
@@ -655,6 +660,7 @@ function load(arrayBuf) {
       mute: 0,
       volE: 0, panE: 0,
       retrig: 0,
+      vibratopos: 0,
       vibratodepth: 1,
       vibratospeed: 1,
     });
diff --git a/xmeffects.js b/xmeffects.js
index ef429fa..5832d3d 100644
--- a/xmeffects.js
+++ b/xmeffects.js
@@ -61,13 +61,14 @@ function eff_t0_4(ch, data) {  // vibrato
   if (data >> 4) {
     ch.vibratospeed = data >> 4;
   }
-  eff_t1_4(ch, data);
+  eff_t1_4(ch);
 }
 
 function eff_t1_4(ch) {  // vibrato
   ch.periodoffset = Math.sin(ch.vibratopos * Math.PI / 32) * ch.vibratodepth;
   if (isNaN(ch.periodoffset)) {
-    console.log("vibrato periodoffset NaN?", ch.vibratopos, ch.vibratodepth);
+    console.log("vibrato periodoffset NaN?",
+        ch.vibratopos, ch.vibratospeed, ch.vibratodepth);
     ch.periodoffset = 0;
   }
   ch.vibratopos += ch.vibratospeed;