MIDI Cable Length Q: I know the
MIDI specification says there is a maximum cable length of 50 feet (15m) but how far will it really go?
A: The only perfectly correct answer to this is to try it with the specific equipment and cable you want to use dressed where you want it to be and see if it works.
Here are the facts:
- Most MIDI equipment manufacturers have copied the examples (which are pretty good designs) from the original MIDI specification provided by the MIDI Manufacturers Association (MMA) for their MIDI IN and OUT circuits. There is no guarantee they have designed these circuits as well as they can, but most designs are good if not better than the MMA example circuits.
- The MIDI specification (especially if implemented using the example circuits and even more so if designed as well as possible) is inherently well conceived for transmission over long distances using typical cabling. The reasons for this are:
- The receiver is differential and optically isolated. It also has a back-EMF diode (if the equipment truly obeys the specification)
- The transmitter is fully balanced as long as it has been correctly designed. Early circuits used TTL logic with an open collector driver which does not work as well as current glue logic technology but that technology has essentially been obsolete for 15 years.
- Low voltages (5V maximum) and reasonably slow bit rates (31.25kHz) are used
- Digital data is transmitted via a current loop transfer function (rather than voltage level detection) and this is an inherently more robust protocol since noise sources are extremely unlikely to induce enough current (especially considering the very low impedance nature of the receiver circuit) to create errors
The process of
MIDI transmission is very similar (despite significant differences) to the transmission process that modems employ - a process which normally works quite well over many kilometres of small
gauge unshielded telephone
line pairs.
A major difference between the two lies in the fact that modems use very powerful error detection and correction algorithms which normally prevent incorrect data from being received.
MIDI does not have any such error detection capabilities.
Some
MIDI standards (such as Two-Phase Commit Show Control commands and File Transfer Protocols) have fail-safe error detection/correction built in but this is not generally true.
If any device could become dangerous if incorrect data were received, then a robust
MIDI link must be used.
The most robust
MIDI link we know of is our NetMIDI E-Show device which carries
MIDI over a
network.
Most other devices we have investigated which purport to be a 'long distance
MIDI transmission link' actually use a variation of the
EIA RS-422 standard which has a very long but definitely finite range.
We are also not aware of any such device which includes error detection/correction capabilities.
Because of this and since RS-422 uses
voltage level detection and it is technically more susceptible to induced noise errors than the
current loop design of
MIDI itself, we do not recommend such units.
In fact, we know of nothing which is in fact more robust than simply using well designed
MIDI equipment and interconnect cabling which follows standard procedures for data transmission integrity.
For those more familiar with audio than data, follow the procedures you would use with audio lines.
Specifically:
- Keep total loop impedances to a minimum within practical cost limits. The inherent impedance of a MIDI receiver is minimum 200 ohms so the total loop impedance should not exceed 20% of this value or 40 ohms
- Keep cables away from power lines or other wiring carrying high voltages, currents or especially noisy transmission cables
- Use twisted pair instead of untwisted pair and shielded rather than unshielded cable for maximum noise rejection - although these are definitely not essential and many installations use basic telephone wiring quite successfully over long distances
- Use good quality locking connectors in non-corrosive and non- condensing environments
Adhering to the above guidelines will result in the most reliable connection and the longer the run, the more important they become.
We have seen many installations in which some or all of the above have not been followed and
MIDI was flawlessly transmitted, so designing a successful installation involves many intangibles which may or may not affect the ultimate result.
Which is how we get back to the original and only genuine answer:
Try it with the specific equipment and cable you want to use dressed where you want it to be and see if it works.
One final caveat: if you want to use a '
MIDI powered' device such as the type made by
MIDI Solutions, you will have to use a third
conductor to carry
ground/earth from pin 2 of the
MIDI OUT or
THRU jack to the device even though
ground/earth is not required according to the official
MIDI specification. If you only utilize a signal pair to carry
MIDI in your installation, then you may have to add a
MIDI Solutions Power Adapter at the receiving end of your connection to provide local
power for '
MIDI powered' devices, but this is a very good solution to this problem.
The
MIDI Solutions
Power Adapter will probably extend the distance
MIDI can be carried over standard cables. Connect one to the
MIDI OUT of the sending
unit and another one to the
MIDI IN of the receiving
unit. If this does not extend the distance far enough, connect more at evenly spaced intervals through the length of the run.