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Solutions for a very high number of inputs

Former Member

I have a rather ambitious project to get started learning about microcontrollers and embedded systems. I have cheaply acquired an old organ console that is currently analog that i want to convert to digital/MIDI. The MIDI out part seems straightforward enough from a bit of Google searching, but I have no idea how to deal with the seemingly massive number of inputs I need. The current console is wired with a common rail that spans each keyboard, voice bank, etc., then each key is individually wired. The straightforward approach to this would seem to indicate that I need 300+ input connections. I understand the concept of shift registers for pulling in all of those lines, but I'm worried about being able to read them fast enough to get me the nearly real-time response I need to make it a usable musical instrument. Thoughts?

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  • 0

    It's not so hard,

     

    I'll assume that 1mS latency is good  enough, so you need  a 300kHz clock on your shift register, and thats assuming that you use just one serial stream for all your inputs.

     

    A bigger problem will be wiring it all up, if I were doing this I would design  a little board with about 32 inputs (using  a 64 pin micro) and put these where physically convenient to reduce the amount of wire. Then link the little boards to a cenrtal controller using CAN, RS485, or whatever you prefer.

     

    An STM32F100R4T6B is £1.98 from farnell, has 64 pins so will give you 32 inputs and a serial link with no need for shift registers. You'll need to design a board but its not too hard.

     

    Let me know if you fancy going this way and I might be able to help a bit more.

     

    MK

  • 0 in reply to michaelkellett

    I have never faced a problem like this but I would like to propose a different approach :

    Multiplexing.

    It's how the PC handles its keyboard.

    Each organ normal keyboard has 88 keys that could be handled by a 11 bits of input and 8 bits outputs (19 lines).

    The full organ could be handles by a 18 x 18 matrix (324 keys).

    One of the advantages of this approach is that the cabling would be done in the keyboard itself with fewer lines going to the processor(s).

  • 0 in reply to Aryldo

    Aryldo Russo wrote:

     

    I have never faced a problem like this but I would like to propose a different approach :

    Multiplexing.

    It's how the PC handles its keyboard.

    Each organ normal keyboard has 88 keys that could be handled by a 11 bits of input and 8 bits outputs (19 lines).

    The full organ could be handles by a 18 x 18 matrix (324 keys).

    One of the advantages of this approach is that the cabling would be done in the keyboard itself with fewer lines going to the processor(s).

    That's a good solution if you have isolated switches.  In my 1977 design the switches had a common ground, so multiplexing had to be done using ICs.

     

    For a polyphonic keyboard each switch needs a diode.  http://en.wikipedia.org/wiki/Keyboard_matrix_circuit

     

    BTW, organ manuals typically have 61 keys (5 octaves plus an extra C).  Modern piano keyboards have 88 keys.

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  • 0 in reply to Aryldo

    Aryldo Russo wrote:

     

    I have never faced a problem like this but I would like to propose a different approach :

    Multiplexing.

    It's how the PC handles its keyboard.

    Each organ normal keyboard has 88 keys that could be handled by a 11 bits of input and 8 bits outputs (19 lines).

    The full organ could be handles by a 18 x 18 matrix (324 keys).

    One of the advantages of this approach is that the cabling would be done in the keyboard itself with fewer lines going to the processor(s).

    That's a good solution if you have isolated switches.  In my 1977 design the switches had a common ground, so multiplexing had to be done using ICs.

     

    For a polyphonic keyboard each switch needs a diode.  http://en.wikipedia.org/wiki/Keyboard_matrix_circuit

     

    BTW, organ manuals typically have 61 keys (5 octaves plus an extra C).  Modern piano keyboards have 88 keys.

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