Yamaha DX7 Technical Analysis

Re:Yamaha DX7 Technical Analysis by QuadrupleA on Thursday October 21, 2021 @11:02AM (#28945461)

I've been restoring a closely related DX11 recently - one interesting tidbit of over-engineering, the keyboard matrix for the tact switches on the front panel (not just the musical keys) has a diode for every switch, to apparently allow "chords" of 3 or more programming buttons to be held and scanned at once. Can't see why it's needed during programming of the patches, but I might be missing something. Maybe someone had a kickback deal with the diode vendor to ship an extra 40 of them per unit :).

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Re:Yamaha DX7 Technical Analysis by jacquesm on Thursday October 21, 2021 @12:03PM (#28946500)

I think without that diode there would be ambiguity about which switch is pressed, it keeps the rows and columns in the matrix separate so no current can flow back to other rows/colums when one key is pressed.

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Re:Yamaha DX7 Technical Analysis by QuadrupleA on Thursday October 21, 2021 @12:34PM (#28947009)

You can definitely do 2 simultaneous keys unambiguously without diodes; 3 keys is where you sometimes get "phantom keys" by bridging other rows / columns to each other. Depends on the matrix layout though, if you have say an 8 x 8 matrix for 64 keys and each pressed key is on the same column or row, you can read 8 simultaneously without any problems.

Most PC keyboards leverage this so they can skip diodes, and just route the matrix so that common simultaneous combinations (modifier keys, WASD for games etc.) don't interfere with each other.

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Re:Yamaha DX7 Technical Analysis by jacquesm on Thursday October 21, 2021 @12:45PM (#28947217)

Hm. I don't know about that, I thought that the problem already existed for 2 diagonally opposing sets of buttons: you can't tell the difference between the two.

so:

       | | | | | | 
    ---+-+-+-+-+-+-----
       | | | | | | 
    ---+-+-*-+-+-+-----        
       | | | | | | 
    ---+-+-+-*-+-+-----
       | | | | | |

and

       | | | | | | 
    ---+-+-+-+-+-+-----
       | | | | | | 
    ---+-+-+-*-+-+-----        
       | | | | | | 
    ---+-+-*-+-+-+-----
       | | | | | |

Would look identical to the CPU without diodes.

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Re:Yamaha DX7 Technical Analysis by TheOtherHobbes on Thursday October 21, 2021 @01:18PM (#28947750)

https://patents.google.com/patent/US5430443

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Re:Yamaha DX7 Technical Analysis by jacquesm on Thursday October 21, 2021 @01:30PM (#28947940)

Ah I see, clever!

So in essence because the diode solution requires rows times columns diodes and this one only has a couple of parts per row and per column it scales better and for larger keyboards will end up with a substantially reduced parts count and a simpler circuit.

That patent is quite a bit younger than the synth though, so it's kind of logical that that technique would not be used, and my hunch on why they used those diodes back then is likely correct, even if we have a smarter way of doing this now.

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Re:Yamaha DX7 Technical Analysis by QuadrupleA on Thursday October 21, 2021 @03:18PM (#28949591)

That should work fine - the CPU would scan / energize each row independently and then check which columns are connected.

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Re:Yamaha DX7 Technical Analysis by QuestingElf on Thursday October 21, 2021 @03:42PM (#28949882)

On a related note (pun intended) I once had a problem with one of these synths and its rubber contact strip. I would hit certain keys then hear a neighbor note or the same note an octave away. I thought there was a short in the scanning circuitry (I had resoldered some joints that appeared weak earlier by a ribbon connector for the keybed.)

Turns out the rubber contact strip was installed inverted. I think this one didn't have notches to make sure it was oriented correctly. After flipping it, notes sounded as expected.

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