[sdiy] Info request (longshot) - Technics keyboard mech/encoder

[Michael Bacich]

On Jul 31, 2006, at 3:35 AM, Steve Lenham wrote:

> 2. The key contacts themselves are not the usual spring-wire-plus- 
> two-busbars that I was expecting to see; instead, there is a row of  
> individual rubber buttons sitting on the PCB. Normally I would  
> associate that sort of thing with cheapo keyboard mechanisms, but  
> this is from a multi-thousand pound/dollar/euro instrument which I  
> know for a fact plays very well and has first-class velocity  
> sensitivity. It's not obvious how that sensing is done - anybody  
> know? (Obviously I could learn more by taking it to pieces, and  
> probably will in the end, but perhaps someone knows already).

There are most likely a set of two rubber contact switches per key.   
They are made in such a way that one of the two contacts always makes  
contact before the other one does.  The CPU rapidly scans the  
contacts and when it notices that one of them has made contact, it  
starts a timer that times how long it takes for the second contact on  
that key to make contact.  The harder you play, the shorter that time  
will be.  That's because with the Newtonian physics to which these  
keyboards slavishly adhere, harder equals faster (indeed, that's why  
they call it "velocity").  Therefore, short times between the two  
contacts equal high velocity, and long times equal low velocity.   
Almost all modern keyboards use this two-rubber-contacts system --  
even very expensive professional instruments.

Some keyboards use a slight variation on this system -- if the  
keyboard has metal spring contacts, it's possible to design it so  
that when the key is at rest, the spring is in contact with one bus  
bar, and when you play the key it loses contact with that bus, then  
makes contact with a second bus.  This is sometime referred to as a  
"break-make" contact system, whereas the rubber contacts are always a  
"make-make" system.  The CPU scans the "break" bus and when it  
notices a contact has been broken with that first bus, it times how  
long it takes for that key to make contact with the second "make"  
bus.  As with the previous example, it translates "break-make" short  
times into high velocities and long times into low velocities.  These  
metal key contact systems were pretty much superior in every way to  
the rubber contact systems, but they are considerably more expensive  
to build, so they have fallen by the wayside.  The Yamaha DX7 series  
had an excellent spring-metal-contact velocity sensing keyboard.  Now  
all of Yamaha's keyboards have rubber key contacts.