I am investigating a similar project. The project is a MIDI knob box. I will
start with 7bit accuracy but would like the ability to scale up to 14bit
later. Currently I am considering an A\D converter (external) and an analog
MUX. However digital knobs would be much better as:
1) I don't have the conversion time overhead.
2) I don't have to worry about an analog section and isolating it etc.
3) No A\D accuracy problems (these would have been eased byt the 7\14 bit
accuracies meaning I can dump LSBs

However rotary encoders have the following problems:
1) Dealing with multiple active knobs
    While an analog pot holds a value indefinitely allowing round robin
sampling a digital knob must be constantly monitored. As I want up to 16 (if
not 32) knobs I need to MUX the knobs in some way. I could use a priority
decoder but then I risk missing a turn. What I really need is some hardware
handling the clicks and adding a knob address to a FIFO buffer for the PIC
to deal with. But how would you wire this up to add the address for a click
on the right line? Is there such a thing as an Encoder without priority.
i.e. if requests come in on 2 lines it lets you deal with both sequentially?

2) Accuracy
    In 7bit data mode you would ideally be able to configure it so 1 turn
(or thereabouts) gave the full 7bit range. This would require 128 positions
per rotation. Digikey have a device with such accuracy but it costs $51.37
which is a little out of my price range to say the least. Obviously I don't
need a full 128 positions as I can scale the value. But I don't want to
scale it too much or smoothing of the values will become neccesary. Anything
above 64CPR would be fine but a 64CPR knob is also $50. The only knob within
my price range is a 24CPR knob at $5ea and even this is pushing it.

Can anyone suggest a digital knob solution with a reasonable cost and a
method of dealing with multiple active knobs.

Tom.
-----Original Message-----
From: pic microcontroller discussion list
[mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Jeff Barlow
Sent: Saturday, September 11, 1999 4:07 PM
To: PICLIST@MITVMA.MIT.EDU
Subject: Re: A/D Challenge


Why not just replace the pots with rotary encoders and get rid of all the
analog stuff?