Yamaha MEP4

MIDI event processor

Yamaha MEP4 midi patch rack
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1986 (Mar.)
Initially sold for ¥48,000

The Yamaha MEP4 is a powerful and highly flexible MIDI event processor. It can be programmed to modify MIDI data in many useful ways that include creating up to four-way keyboard splits, filtering out certain MIDI messages, changing one type of MIDI data into another, and even creating time delay effects. The MEP4 contains four internal processors, which can be routed in any combination to its four programmable outputs.

In 1986 the reviews named it the most flexible and powerful MIDI signal processor currently available. The MEP4 allows to modify any MIDI signal in virtually every way. Data can be separated on different MIDI channels and sent to independent MIDI OUT terminals, for example. A MIDI message can be changed to a different message: for example changing a pitch bend value into a modulation message. Notes can be transposed on a specified MIDI channel up or down by any amount. The MEP4 is even capable of delaying MIDI signals to create a variety of echo effects. Delay and transposition can be combined to create automatic arpeggios.

Anything of these commands can be done through a modern DAW, that's true, but having a box with a well defined set of modifications available still sounds like a very interesting option for creativity purposes.


  • Data Presetter: initialize and control the receiving MIDI keyboards, tone generators, etc. when you choose one of the MEP4's memories.
  • Channel Filter: reject or pass specified MIDI channels.
  • Message Filter: reject or pass specified MIDI messages.
  • Data Modifier: convert any input MIDI message into virtually any other MIDI message.
  • Delay Processor: delays the output from each processor up to 3 seconds to create unique echo or doubling effects.
  • Output Assigner: assigns output from each of the four MIDI processors to any of the four MIDI OUT terminals. Also allows modification of MIDI channel numbers.
Yamaha MEP4 brochure cover
Yamaha MEP4 brochure cover
Cover of After Touch June 1986
Cover of After Touch June 1986
MEP4 on Music Technology, Dec 1987
MEP4 on Music Technology, Dec 1987


Memory capacity

60 programs

Connection terminals and interfaces

Foot Switch

Dimensions and weight

480w x 45.2h x 282d (mm)
3.3Kg / 7lbs. 3oz.