Running Status

Since MIDI data is transmitted serially, it is possible that musical events which originally occurred at the same time and must be sent one at a time in the MIDI data stream may not actually be played at exactly the same time. With a data transmission rate of 31.25 Kbit/s and 10 bits transmitted per byte of MIDI data, a 3-byte Note On or Note Off message takes 960µs, about 1 ms to be sent, which is generally short enough that the events are perceived as having occurred simultaneously. In fact, for a person playing a MIDI instrument keyboard, the time skew between playback of notes when 10 keys are pressed simultaneously should not exceed 10 ms, and this would not be perceptible.

However, MIDI data being sent from a sequencer can include a number of different parts. On a given beat, there may be a large number of musical events which should occur simultaneously, and the delays introduced by serialization of this information might be noticeable. To help reduce the amount of data transmitted in the MIDI data stream, a technique called "running status" may be employed.

Running status considers the fact that it is very common for a string of consecutive messages to be of the same message type. For instance, when a chord is played on a keyboard, 10 successive Note On messages may be generated, followed by 10 Note Off messages. When running status is used, a status byte is sent for a message only when the message is not of the same type as the last message sent on the same Channel. The status byte for subsequent messages of the same type may be omitted (only the data bytes are sent for these subsequent messages).

The effectiveness of running status can be enhanced by sending Note On messages with a velocity of zero in place of Note Off messages. In this case, long strings of Note On messages will often occur. Changes in some of the MIDI controllers or movement of the pitch bend wheel on a musical instrument can produce a staggering number of MIDI Channel voice messages, and running status can also help a great deal in these instances.