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fonik
Joined: Jun 07, 2006
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 Posted: Tue Dec 11, 2007 4:21 pm Post subject:
Optocoupler 6N138 in MIDI circuits - what do they do? |
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i know nothing about that so it is even hard for me to frame a question.
i find this optocoupler right after the midi input of thomas henry's GMvoice or a midi2cv converter.
what does it actually do? the datasheet says "digital logic ground isolation". what does that mean? does it delink the ground of the incoming signal from the circuits ground reference? what for?
what is the midi signal? a pulse? why has it to go thru this optocoupler?
i hope these questions make any sense...
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Danno Gee Ray
Joined: Sep 25, 2005
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Location: Telford, PA USA
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| Posted: Tue Dec 11, 2007 4:33 pm Post subject:
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I'm not an engineer, but I'll give a go at my take on this.
The optoisolator serves to transfer ONLY the data of the midi signal intom the circuit. By transfering the data as an optical signal, none of the attendant characteristics of usual circuits come along with the signal. Impedence matches / mismatches, loading, capacitances, and all the other minutae of circuits getting lovey dovey with each other apply. Each circuit can do it's own thing withoput interference from the other. ONLY the data is shared. |
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para
Joined: Oct 16, 2006
Posts: 276
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| Posted: Tue Dec 11, 2007 5:29 pm Post subject:
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its just a switch that you can use to isolate the two half’s of the circuit.
steven |
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blue hell
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Joined: Apr 03, 2004
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Location: The Netherlands, Enschede
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| Posted: Tue Dec 11, 2007 5:48 pm Post subject:
Re: Optocoupler 6N138 in MIDI circuits - what do they do? |
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| fonik wrote: | | "digital logic ground isolation" |
That part is the key actually, isolating the ground circuits from sender and receiver helps to avoid ground loops. MIDI is a pulsed (digital) signal indeed and when any bit of the MIDI signal gets into an audio related ground loop you'll get some very nasty sound from it (due to the sharp signal edges).
Now if only the USB people would have been that smart 
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Jan
also .. could someone please turn down the thermostat a bit.
9 3 4 .. erm .. not 13 then? .. hmm, ah eight! .. yeah yeah as in 8647 .. 47 is an 88 .. pwew .. numbles! |
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fonik
Joined: Jun 07, 2006
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| Posted: Wed Dec 12, 2007 2:36 am Post subject:
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thanks for the replies, guys. appreciated. now i got the idea.
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matthias
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JovianPyx
Joined: Nov 20, 2007
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| Posted: Wed Dec 12, 2007 6:42 am Post subject:
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| A MIDI serial link is a current loop. Unlike RS232 which uses voltage to represent the bit states, a current loop uses current on and current off for bit states. The 6N138 input LED becomes part of the current loop. When current flows, the LED lights and illuminates the phototransistor. The circuitry on the phototransistor side then generates a TTL logic level that represents the bit states. This logic level can then be routed to a TTL UART or the UART input to a small MCU or in my case, it goes to an FPGA pin. From there, it's all magic. |
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fonik
Joined: Jun 07, 2006
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| Posted: Wed Dec 12, 2007 7:06 am Post subject:
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| ScottG wrote: | | From there, it's all magic. |
 
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EdisonRex
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| Posted: Wed Dec 12, 2007 9:58 am Post subject:
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| ScottG wrote: | | A MIDI serial link is a current loop. Unlike RS232 which uses voltage to represent the bit states, a current loop uses current on and current off for bit states. The 6N138 input LED becomes part of the current loop. When current flows, the LED lights and illuminates the phototransistor. The circuitry on the phototransistor side then generates a TTL logic level that represents the bit states. This logic level can then be routed to a TTL UART or the UART input to a small MCU or in my case, it goes to an FPGA pin. From there, it's all magic. |
This was one of the little brilliant things that Dave Smith and the rest of the MIDI group did, as it removes ground loops from the equation, as well as much RFI. Can you imagine how many problems we'd have with ground loops in a MIDI studio or live rig otherwise?
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urbanscallywag
Joined: Nov 30, 2007
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| Posted: Wed Dec 12, 2007 10:36 am Post subject:
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| EdisonRex wrote: | | This was one of the little brilliant things that Dave Smith and the rest of the MIDI group did, as it removes ground loops from the equation, as well as much RFI. Can you imagine how many problems we'd have with ground loops in a MIDI studio or live rig otherwise? |
I think I read somewhere it was Tom Oberheim who suggested an optocoupled signal. |
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EdisonRex
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| Posted: Wed Dec 12, 2007 11:53 am Post subject:
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| urbanscallywag wrote: | | EdisonRex wrote: | | This was one of the little brilliant things that Dave Smith and the rest of the MIDI group did, as it removes ground loops from the equation, as well as much RFI. Can you imagine how many problems we'd have with ground loops in a MIDI studio or live rig otherwise? |
I think I read somewhere it was Tom Oberheim who suggested an optocoupled signal. |
Phil Dodds (ex-ARP, then-CBS) has taken credit for the idea as well (as well as velocity in MIDI). You might have read one of the many articles in Mix, SOS, etc. Dave Smith, Tom Oberheim and Ikutaro Kakehashi were the original proponents, but they were not the only contributors. As it is, MIDI-DIN is still a quirky protocol, but it has outlived the 3.5" floppy disk, so I guess it's not all that bad.
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urbanscallywag
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| Posted: Wed Dec 12, 2007 11:55 am Post subject:
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| Interesting. I searched around and couldn't find where I read about Oberheim... |
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EdisonRex
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