diff --git a/libs/backends/coreaudio/coreaudio_backend.cc b/libs/backends/coreaudio/coreaudio_backend.cc
index f074ad4c3c..85bd2355bc 100644
--- a/libs/backends/coreaudio/coreaudio_backend.cc
+++ b/libs/backends/coreaudio/coreaudio_backend.cc
@@ -17,6 +17,21 @@
  * Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
  */
 
+
+/* use an additional midi message parser
+ *
+ * coreaudio does packetize midi. every packet includes a timestamp.
+ * With any real midi-device with a phyical layer
+ * 1 packet = 1 event (no concurrent events are possible on a cable)
+ *
+ * Howver, some USB-midi keyboards manage to send concurrent events
+ * which end up in the same packet (eg. 6 byte message: 2 note-on).
+ *
+ * An additional parser is needed to separate them
+ */
+#define USE_MIDI_PARSER
+
+
 #include <regex.h>
 #include <sys/mman.h>
 #include <sys/time.h>
@@ -105,12 +120,11 @@ CoreAudioBackend::CoreAudioBackend (AudioEngine& e, AudioBackendInfo& info)
 	, _processed_samples (0)
 	, _port_change_flag (false)
 #ifdef USE_MIDI_PARSER
-	, _event(0)
-	, _first_time(true)
 	, _unbuffered_bytes(0)
 	, _total_bytes(0)
 	, _expected_bytes(0)
 	, _status_byte(0)
+	, _parser_bytes(0)
 #endif
 {
 	_instance_name = s_instance_name;
@@ -1719,16 +1733,23 @@ CoreAudioBackend::process_callback (const uint32_t n_samples, const uint64_t hos
 #ifndef USE_MIDI_PARSER
 			midi_event_put((void*)mbuf, time, data, size);
 #else
+			assert (size < 128);// coremidi limit per packet
+			bool first_time = true; // this would need to be rememberd per port.
 			for (size_t mb = 0; mb < size; ++mb) {
-				if (_first_time && !(data[mb] & 0x80)) {
-					// expect a status byte at the beginning or every Packet
-					assert (0);
+				if (first_time && !(data[mb] & 0x80)) {
+					/* expect a status byte at the beginning or every Packet.
+					 *
+					 * This parser drops messages spanning multiple packets
+					 * (sysex > 127 bytes).
+					 * see also libs/backends/alsa/alsa_rawmidi.cc
+					 * which implements a complete parser per port without this limit.
+					 */
 					continue;
 				}
-				_first_time = false;
+				first_time = false;
 
 				if (midi_process_byte (data[mb])) {
-					midi_event_put ((void*)mbuf, time, _parser_buffer, _event._size);
+					midi_event_put ((void*)mbuf, time, _parser_buffer, _parser_bytes);
 				}
 			}
 #endif
diff --git a/libs/backends/coreaudio/coreaudio_backend.h b/libs/backends/coreaudio/coreaudio_backend.h
index 4f4e844442..0d598f586a 100644
--- a/libs/backends/coreaudio/coreaudio_backend.h
+++ b/libs/backends/coreaudio/coreaudio_backend.h
@@ -445,8 +445,6 @@ class CoreAudioBackend : public AudioBackend {
 			return NULL;
 		}
 
-#define USE_MIDI_PARSER
-
 #ifdef USE_MIDI_PARSER
 
 		bool midi_process_byte (const uint8_t);
@@ -462,13 +460,13 @@ class CoreAudioBackend : public AudioBackend {
 
 		void midi_prepare_byte_event (const uint8_t byte) {
 			_parser_buffer[0] = byte;
-			_event.prepare(1);
+			_parser_bytes = 1;
 		}
 
 		bool midi_prepare_buffered_event () {
 			const bool result = _unbuffered_bytes == 0;
 			if (result) {
-				_event.prepare (_total_bytes);
+				_parser_bytes = _total_bytes;
 			}
 			_total_bytes = 0;
 			_unbuffered_bytes = 0;
@@ -479,25 +477,12 @@ class CoreAudioBackend : public AudioBackend {
 			return result;
 		}
 
-		struct ParserEvent {
-			size_t _size;
-			bool _pending;
-			ParserEvent (const size_t size)
-				: _size(size)
-				  , _pending(false) {}
-
-			void prepare (const size_t size) {
-				_size = size;
-				_pending = true;
-			}
-		} _event;
-
-		bool    _first_time;
 		size_t  _unbuffered_bytes;
 		size_t  _total_bytes;
 		size_t  _expected_bytes;
 		uint8_t _status_byte;
-		uint8_t _parser_buffer[1024];
+		uint8_t _parser_buffer[128];
+		uint8_t _parser_bytes;
 #endif
 
 }; // class CoreAudioBackend