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 Forum index » DIY Hardware and Software
DIY Atmel AVR based MIDI to CV converter
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mi_dach



Joined: Dec 17, 2005
Posts: 133
Location: Sweden

PostPosted: Wed Aug 02, 2006 11:47 am    Post subject: Reply with quote  Mark this post and the followings unread

Blue Hell wrote:
mi_dach wrote:
Or do I have it backwards, and the Thru is a copy of the out port?..


A copy of the input I think, directly in hardware and not routed through the processor. The MIDI standard used to have schematic hints for this.


My bad for not studying the specifications too closely Smile I was perhaps thinking more like a MIDI merge then, two inputs being joined into one output. I don't have enough gear at the moment to require the use of a Thru port, but it definitely seems useful for later, thanks for setting me straight.
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Blue Hell
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PostPosted: Wed Aug 02, 2006 11:57 am    Post subject: Reply with quote  Mark this post and the followings unread

Found a schematic example at : http://www.borg.com/~jglatt/tech/midispec/hardware.htm

This seems quite like the "original" to me, even the remarks about the opto isolator seem familliar, but probably its not as bad with modern devices as it was 20 years ago.

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jksuperstar



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PostPosted: Wed Aug 02, 2006 12:28 pm    Post subject: Reply with quote  Mark this post and the followings unread

Despite the fact that it's not "MIDI Spec Compliant", I like the soft-thru that's offered in most devices today. Since I have everything going in & out of the PC anyway, this function would've had to be done eventually anyway, and it allows me to daisy chain my devices without a need for a bunch of merging at the end. I highly recommend this method on top of the basic thru.
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State Machine
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Joined: Apr 17, 2006
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PostPosted: Thu Aug 03, 2006 7:19 am    Post subject: Reply with quote  Mark this post and the followings unread

Quote:
Despite the fact that it's not "MIDI Spec Compliant", I like the soft-thru that's offered in most devices today. Since I have everything going in & out of the PC anyway, this function would've had to be done eventually anyway, and it allows me to daisy chain my devices without a need for a bunch of merging at the end. I highly recommend this method on top of the basic thru.


The "soft" thru also adds a capability to filter or strip off unwanted MIDI data before it's passed onto the next device. Something the hardware thru is not able to do. You can do data "thinning" also so as to not tax the MIDI devices downstream. Using an AVR, the MIDI data latency is so low when using interrupt driven I/O software approach that it would behave very similar to the pure HW solution in terms of delay. Best case delay performance though is (320 uS + Interrupt Service Overhead). The 320 uS is equal to 10 bit times and is rather neglagable. The ISR overhead is the critical specification here and should be written "real time enough".
Very Happy

Bill
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State Machine
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PostPosted: Thu Aug 03, 2006 7:27 am    Post subject: Reply with quote  Mark this post and the followings unread

Quote:
This seems quite like the "original" to me, even the remarks about the opto isolator seem familliar, but probably its not as bad with modern devices as it was 20 years ago.


Yes, Jan. I usually use the 6N139 or 6N138 optocouplers. The inverters I use a usually 74HC04 type. To speed the circuit up a bit and lower latency, you can lower the optocoupler collector resistor to a range of 150-180 ohms. To high a value tends to "slew" the voltage too much.

Bill
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