rickman wrote:

Roger Hayter wrote on 11/19/2017 4:45 AM:
rickman wrote:

snip

I have seen no reasonable explanation of how a reed switch is any more
analog than a mechanical switch. They are both analog movements of a
mechanism. The only difference is one adjusts a magnetic field while the
other applies pressure to a spring which bends (again in an analog manner)
until it begins to make contact with another spring. Which will have more
jitter? Only a measurement will say and the reed switch has a much lower
time of bouncing, so will not possibly have a noticeable jitter in time of
actuation.

I agree that jitter is not a major issue, but I do think that with a
mechanical switch that human muscles and brain are directly part of the
analogue feedback loop controlling jitter in a way they may not be with
an intervening magnetic link. At least with a straight key, if springs
are part ot the system then your argument is more convincing. This is a
different issue from contact bounce, which occurs after the initial
contact closure.

Can you explain how the two differ? Just saying they are different isn't an
explanation.

A straight key is a simple proportional muscle movement with feedback
loop via the feel of the contact closure. A key that works via a spring
has a phase change to to the effective energy storage (inductance
analogue) of the spring. The brain is very good at correcting very
slight innacuracies if there is such simple motor feedback circuit,
musicians do it all the time. If there is a magnetic movement or an
optical circuit then a complex delay is included which is no longer a
very simple function of muscular position, Unless the magnetic reaction
of the reed switch is very reproducible or the optical link is very
fast. The human brain can probably have its otherwise very accurate
judgement of cadence harmed by a very few milliseconds of non-linearity.
I do agree it should be possible to arrange such circuits so they are
too fast to notice, but they would have to be very fast.





Of course any key has to have a spring for returning the key to home
position. Every key I've seen had at least two adjustments, home position
(and therefore the travel to contact closure) and spring tension.

The spring in a straight key varies the force but this is linear with
displacement and not out of phase as it is with a springy linkage.



Bounce time is still and issue because most debounce circuits use a simple
RC to resolve the bounce time and this adds delay to the contact closure
time as seen by the equipment.


Most debouncing circuits nowadays would be implemented by digital
device ignoring input for a certain time rather than a simple RC
circuit. Probably a microprocessor.


--

Roger Hayter