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MIDI Electrical Specification Diagram-CA33

Abstract:

This document updates the MIDI 1.0 Electrical Specification to include 3.3-volt signaling and optional RF grounding. This document replaces the Hardware section of the MIDI 1.0 Detailed Specification (Document Version 4.2), which is included in the MMA publication "The Complete MIDI 1.0 Detailed Specification". 


Background:

When the MIDI 1.0 Specification was written, 5-volt signaling was the industry standard for electronic circuits. The standard MIDI transmitter circuit in the Specification requires the use of 5 volts. However, today's industry standard is 3.3 volts, so there is a significant advantage in cost and complexity of newly-developed

devices to adapt the MIDI transmitter circuit for 3.3-volt signaling. In addition, the Specification requires that pin 2 of the MIDI In jack not be connected to ground to avoid ground loops. The MIDI cable shield thus depends on the ground from Pin 2 of the MIDI Out or MIDI Thru jack. This works well for audio frequency signals. However, it is less effective for high frequency (e.g. RF) interference. An optional solution to improve RF performance is to connect pin 2 of the MIDI In jack to the

local device ground through a small-valued capacitor. This maintains the ground-loop isolation at audio frequencies while providing a low-impedance path to ground at radio frequencies. Further, compliance with Electro-Magnetic Interference / Electro-Magnetic Compatibility (EMI/EMC) regulations may require filtering of RF interference on signal pins of the MIDI jacks as well. This update to the Specification adds optional ferrite bead RF filters to the signal pins. The update also adds optional

grounding provisions for the grounding shield connectors on the MIDI jacks.

Please refer to The Complete MIDI 1.0 Detailed Specification for more information on the subject matter.

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