A MIDI message which provides a crude method of monitoring the integrity of a MIDI connection.The transmitter sends the active sensing message approximately every 200 milliseconds when the line would otherwise be quiet (no data waiting to be sent). If the receiver does not receive either real data or an active sensing message for about 0.5 seconds, it assumes that some break in the connection has occurred. When this happens, the receiving synth acts as if an all notes off message had been sent; the purpose of this is to eliminate any possibility of stuck notes due to the connection break.In reality, this doesn’t work very well, and MIDI interfaces usually provide a capability to filter the active sensing messages out.

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