diff --git a/mutaext.py b/mutaext.py
new file mode 100644
index 0000000..799580e
--- /dev/null
+++ b/mutaext.py
@@ -0,0 +1,84 @@
+import mutagen
+import mutagen.id3
+import mutagen.easyid3
+
+# TODO: custom interface to tracknumber/tracktotal/disc...
+
+def popms(id3):
+    for k, v in id3.iteritems():
+        if k.startswith('POPM'):
+            yield k, v
+
+def byte2rating(b):
+    if b >= 224: return 5
+    if b >= 160: return 4
+    if b >= 96: return 3
+    if b >= 32: return 2
+    if b >= 1: return 1
+    return 0
+
+def rating2byte(r):
+    if r == 5: return 256
+    if r == 4: return 192
+    if r == 3: return 128
+    if r == 2: return 64
+    if r == 1: return 1
+    return 0
+
+def rating_get(id3, key):
+    try:
+        rating = id3['TXXX:RATING']
+    except KeyError:
+        try:
+            _, popm = popms(id3).next()
+        except StopIteration:
+            return []
+        else:
+            return [byte2rating(popm.rating)]
+    else:
+        return list(rating.text)
+
+def _canconv(r):
+    try:
+        if int(r) != str(int(r)):
+            return False
+        return int(r) >= 1 and int(r) <= 5
+    except ValueError, TypeError:
+        return False
+
+def rating_set(id3, key, val):
+    rating_delete(id3, key)
+    if _canconv(val):
+        popm = mutagen.id3.POPM()
+        popm.email = "Windows Media Player 9 Series"
+        popm.count = 0
+        popm.rating = rating2byte(int(val))
+        id3.add(popm)
+    else:
+        if 'TXXX:RATING' in id3:
+            del(id3['TXXX:RATING'])
+        id3.add(mutagen.id3.TXXX(encoding=3, desc='RATING', text=unicode(val)))
+
+def rating_delete(id3, key):
+    for k, v in popms(id3):
+        del(id3[k])
+    if 'TXXX:RATING' in id3:
+        del(id3['TXXX:RATING'])
+
+def tracknumber_get(id3, key):
+    # TODO: use for both track and disc
+    pass
+
+def tracknumber_set(id3, key, val):
+    pass
+
+def tracknumber_delete(id3, key):
+    pass
+
+replaygain_tags = ('replaygain_album_gain', 'replaygain_album_peak', \
+  'replaygain_track_gain', 'replaygain_track_peak')
+
+for tag in replaygain_tags:
+    mutagen.easyid3.EasyID3.RegisterTXXXKey(tag, tag)
+mutagen.easyid3.EasyID3.RegisterTXXXKey('sync', 'SYNC')
+mutagen.easyid3.EasyID3.RegisterKey('rating', rating_get, rating_set, rating_delete)
diff --git a/unsync.py b/unsync.py
index 72e986b..2cc1ffe 100755
--- a/unsync.py
+++ b/unsync.py
@@ -7,45 +7,7 @@ import os
 import os.path
 import sys
 import mutagen
-import mutagen.id3
-import mutagen.easyid3
-
-def popm_any(id3):
-    for k, v in id3.iteritems():
-        if k.startswith('POPM'):
-            return v
-
-def byte2rating(b):
-    if b >= 224: return 5
-    if b >= 160: return 4
-    if b >= 96: return 3
-    if b >= 32: return 2
-    if b >= 1: return 1
-    return 0
-
-def rating_get(id3, key):
-    try:
-        return list(id3['TXXX:RATING'])
-    except KeyError:
-        try:
-            return list(unicode(byte2rating(popm_any(id3).rating)))
-        except AttributeError:
-            return
-def rating_set(id3, key, val):
-    # TODO: test
-    try:
-        frame = id3["TXXX:RATING"]
-    except KeyError:
-        id3.add(mutagen.id3.TXXX(encoding=3, desc=u'RATING', text=val))
-    else:
-        frame.encoding = 3
-        frame.text = val
-def rating_delete(id3, key):
-    # TODO: delete POPM too?
-    del(id3["TXXX:RATING"])
-
-mutagen.easyid3.EasyID3.RegisterTXXXKey('sync', 'SYNC')
-mutagen.easyid3.EasyID3.RegisterKey('rating', rating_get, rating_set, rating_delete)
+import mutaext
 
 def G(d, k):
     try:
@@ -67,11 +29,8 @@ def filterext(paths, exts):
 def shouldsync(md):
     rating = G(md, 'rating')
     sync = G(md, 'sync')
-    if rating != None:
-        try:
-            rating = int(rating[0])
-        except ValueError:
-            rating = None
+    if rating:
+        rating = rating[0]
     if sync:
         sync = sync[0].lower()
     else:
@@ -93,7 +52,7 @@ def fixmetadata(md):
     if 'title' not in md:
         fn = os.path.basename(md.path)
         fn = os.path.splitext(fn)[0]
-        # TODO: attempt to infer trackNum/diskNum from fn
+        # TODO: attempt to infer trackNum/discNum from fn
         md['title'] = fn
 
 def findmatching(haystack, needle):
@@ -116,6 +75,28 @@ def findmatching(haystack, needle):
                 break
     return match
 
+def updatemetadata(mdold, mdnew):
+    modified = False
+    updatabletags = ('rating', 'sync', 'comment')
+    # TODO:
+    # composer      : probably safe
+    # genre         : might need deep equal?
+    # trackNum      : TRCK with optional /[total]
+    # discNum       : TPOS "    "        "
+    # total tracks  : ?
+    # total discs   : ?
+    # [replaygain]  : TXXX:* in id3
+    # albumArtist   : TPE2 in id3
+    for tag in updatabletags:
+        if tag in mdnew:
+            if not tag in mdold or mdnew[tag] != mdold[tag]:
+                mdold[tag] = mdnew[tag]
+                modified = True
+        elif tag in mdold:
+            del mdold[tag]
+            modified = True
+    return modified
+
 def run(args):
     if not len(args) in (2, 3):
         print("I need a path or two!", file=sys.stderr)
@@ -151,8 +132,9 @@ def run(args):
         md = mutagen.File(p, easy=True)
         match = findmatching(tosync, md)
         if match:
-            print("UPD", p)
-            # update tags here if updatemetadata returns true
+            if updatemetadata(md, match):
+                print("UPD", p)
+                # save here
         else:
             print("DEL", p)
             # delete files here