Thursday, 02 May 2019
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.
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.
The length of the cable is critical as well. IMA specifications suggest an absolute maximum cable length of 50 feet on account of of the method of data transmission through the cable. The entire length of a MIDI chain (discussed below) is unlimited, however, provided that none of the links are longer than 50 feet. The optimal maximum length for cable is about 20 feet, and most commercially manufactured cable comes in five to ten foot lengths.
A MIDI chain describes a series of one-way connections in a MIDI setup. The elemental chain is a single-link chain. The MIDI Outside port of one device is connected to the MIDI IN port of a second. In this configuration, a key pressed on the first unit will cause both units to sound. Pressing a key on the second unit, however, only causes the second unit to sound. Many instruments may be chained together using a series of single links to connect the units. In this condition, the Gone of the first unit is connected to the second, the THRU of the second is connected to the IN of a third, and so on. If all the units are locate to receive on the same channel, pressing a key on the first one will cause all the units to sound. Pressing a key on any of the other units will only activate the sound of that unit.
A MIDI loop is a special configuration of a MIDI chain. The single element loop is made of two interconnecting links. This was the configuration used in the debut of the MIDI system. The Outside port of the first unit is connected to the IN port of the second, and the Outside port of the second is connected to the IN port of the first. In this condition, as described earlier, a key pressed on either unit causes both units to sound, provided they are on the same channel. A MIDI feedback loop does NOT exist here, as the data going into the second unit from the first is not duplicated in the Gone port of the second going back into the first. Here, we have two one-way links connected, not a multi-link chain.