On Sun, 2009-11-01 at 12:56 -0400, Raymond Martin wrote: My recollection is that when MIDI was originally standardised, they needed to specify a maximum length, and that 15M was chosen somewhat arbitrarily (No tests were done, no eye patterns examined). Now midi is actually at heart a 5mA current loop interface running at a fairly low baud rate (31.25 Kbaud IIRC), so should be good for far more then 15m without any problem, however nobody really knows and it will depend a bit on the environment and the receiver. Interference is unlikely to be an issue assuming good quality twisted pair cable (Cat 5 counts), but bulk capacitance just might ultimately limit the length. Regards, Dan.

On Mon, Nov 2, 2009 at 4:55 AM, Dan Mills <dmills(a)exponent.myzen.co.uk> wrote: My I offer some more information to clarify the notions of cable "quality" and signal "loss/degradation". Aside from the EMI issue, what limits performance here is not so much cable "quality" whatever that means but the unavoidable physical characteristics of the cable as a transmission line once its length causes a time delay that is significant relative to the rise/fall times of the signals it is carrying. The speed of electromagnetic propagation is fast, but it is not infinite. It takes time for a signal to travel from one end of the cable to the other. Plus, if the termination impedance is not matched to the cable impedance, then reflections will occur at either end of the cable causing unexpected signal variations. Good old physics screws up the imaginary fantasy of perfect 1's and 0's. A good reference on this is www.national.com/an/AN/AN-808.pdf particularly Figure 7 which is instructive even if you read none of the text but just look at the picture. The MIDI 1.0 spec keeps the cable length short to avoid addressing these issues. It makes no attempt to specify terminating impedances matched to cable impedance, or deal with any other long cable issues. Another thing about the that MIDI spec is where it says "optoisolators ... rise and fall times should be less than 2 microseconds" which is amusing because the total time of one midi bit is 3.2 microseconds. So don't imagine you have nice square bits driving the system even if your cable is zero length. So your optoisolators *might* be a limiting factor depending on their speed (ie age, cost). The transmitting equipment's ability to adequately drive current/voltage into the cable impedance *might* be another limiting factor. This is not intended to put you off. If you get yourself an oscilloscope and do some eye pattern tests (Fig 13 and Fig 15), you might discover your equipment and chosen cable works fine at the lengths you need. But this is the only way to know for sure. If you do decide to do this, you would need to keep in mind that your cable is terminated by a nonlinear impedance (the led in the receiver optocoupler) so that adds an extra complication because voltage is not proportional to current at that point in the system. You might need to scope the output (ie not the input) of the receiver optocoupler for errors. So that is why the MIDI spec says to keep the cables short. It avoids all these complicated issues, allows simple cheap circuitry and any cheap cable with unspecified characteristic impedance, and yet was fast enough for the audible performance latency requirement. By the way, the reason Cat5 cable gives better performance in *other* situations is because it has a defined characteristic impedance (100 ohms), and being known it can be matched in the design of the transmit/receive device impedances to minimise reflections. Cat5 cable does *not* provide that benefit when used to interconnect MIDI devices, because they are not designed for 100 ohm cable. Other relevant references are: http://en.wikipedia.org/wiki/Transmission_line http://en.wikipedia.org/wiki/Twisted_pair http://en.wikipedia.org/wiki/Category_5_cable And yes there is no need to run 5V down a MIDI cable. It is a current loop. David

On Mon, Nov 2, 2009 at 12:20 PM, Dan Mills <dmills(a)exponent.myzen.co.uk> wrote: Yes you are correct, I made a factor of 10 error. The copy of the MIDI 1.0 spec I looked at says "the interface operates at 31.25 kbaud" so I interpreted this as 31.25 10-bit symbols per second, which is one possible interpretation of baud, but this is not what they meant. The slippery old baud claims another victim. They meant "the interface operates at 31.25 kbit/s", so I made a mistake and thank you for clearing it up !! I broadly agree with your rule of thumb, and it is a useful addition to my attempt to give a better understanding of what issues are involved here, and it makes sense to add it. But nothing beats a test once you know what all the issues are, which was the point I was attempting to make. Also it would probably not hurt to improve the midi output's EMI rejection with some ferrite beads and bypass caps if there's going to be a longwire antenna hanging off it, even if it is made of twisted pair. Cheers David

Arnold Krille wrote: sure, i just numbered the wires, your count is based on rj45, but i meant the same. any midi has to carry 5v iirc. and midi is not balanced, so there is no signal+ and signal-. would be cool if there was - then 100m would be no problem :)

On Sun, Nov 01, 2009 at 07:23:27PM +0100, Jörn Nettingsmeier wrote: Midi does not carry a 5V supply. You need 5V to (or more) to create a midi output, but that voltage is required in the equipment providing the output only. A midi input is floating (the input of an optocoupler). Pin 4 is the '+' side, pin 5 is '-'. Usually one of the two has a series resistor (220R or so) to limit the current if accidentally connected to a low-impedance voltage source, and there may be a diode parallel to the optocoupler to protect against reversed voltage. For output, pin 4 is normally connected via a 220R series resistor to +5V, and pin 5 is the output of an open-collector logic gate able to sink at least 10 mA, again with 220R in series. That is the standard way to make a midi out. But since the input is floating you could reverse the active/passive parts, e.g. use an active high open collector output on pin 4 and connect pin 5 to ground (both again via 220R). You could even make it active in both states and balanced. But little is gained by doing that. Some midi receivers may rely on the 'standard' way to steal some power, but I don't think the standard provides for this. Ciao, -- FA Io lo dico sempre: l'Italia è troppo stretta e lunga.

On Sunday 01 November 2009, Arnold Krille wrote: Except that I have yet to see a piece of cat5 with shielding. -- Cheers, Gene "There are four boxes to be used in defense of liberty: soap, ballot, jury, and ammo. Please use in that order." -Ed Howdershelt (Author) The NRA is offering FREE Associate memberships to anyone who wants them. <https://www.nrahq.org/nrabonus/accept-membership.asp> "I will make no bargains with terrorist hardware." -- Peter da Silva

On Monday 02 November 2009, Arnold Krille wrote: FWIW, I have around 120 feet of unshielded cat5 stretched around my property, with 100mbit cards at all points, from a switch I can see here in my den, down into the basement and out through the same hole the telco cable comes in, under the back porch floor, and up the house wall to an anchor point on the porch roof, thence to an anchor point at the peak of a workshop shed I built 10 years ago, down to a 4 port switch in there, then from that another cable is currently running out the shop door and about 35 feet across the back yard to a window in the garage, and 20 some feet to a test machine. I can move something from here to that last machine at 10megabytes/second. Std, unshielded cat5. The piece from the house to the shed has been swinging in the wind for about 6 years now, a never ending source of amazement to me that it hasn't failed. My point is that the opto-isolation isn't required for ethernet since it is a transformer coupled differential circuit, therefore quite immune to longitudinal voltages caused by poor wiring practices, local high powered AM broadcasting signals etc. -- Cheers, Gene "There are four boxes to be used in defense of liberty: soap, ballot, jury, and ammo. Please use in that order." -Ed Howdershelt (Author) The NRA is offering FREE Associate memberships to anyone who wants them. <https://www.nrahq.org/nrabonus/accept-membership.asp> If this is timesharing, give me my share right now.