Michael J. Mahon wrote:
Prior to the //c, the toggle was AC-coupled to the Darlington driver
through an RC circuit with a time constant of about 5 milliseconds.
The effect is that no matter which way the toggle goes, after 5-10
milliseconds, it's back to its "rest" state: no current through speaker.
As long as the toggle is changing at least every 5-10 milliseconds
(100-200Hz), the AC-coupling makes little difference, and the speaker
sees each toggle as a full-scale current transition.
But if more than 5-10 ms. elapses between toggles, then what happens
on the *next* toggle depends on whether it is high-to-low or low-to-
high. If it is high-to-low, it will simply try to turn off the already
off Darlington driver, producing essentially no sound. If it is a
low-to-high transition, then it will produce a maximum "tick".
As a result, sounds with toggle intervals longer than 5-10 ms. or--
very importantly--short sounds with asymmetric pulses *separated* by
periods of silence *will* be affected, since the "attack" caused by
the first speaker pulse will either be heard or not, depending on the
initial state of the speaker toggle.
In the //c, the actual circuit of the audio op amp is not given, but
the diagram suggests that it is DC-coupled to the toggle, so it shows
no time-constant effects.
The above explanation, while excellent, is suffiently dense to those of
us without formal electronics training (I had to look up AC-coupled,
for instance) that I went back to Sather just to remind myself how
little he had written about the speaker. In his //e book the nearly
obsolete cassette port gets considerably more converage!
A version of the above explanation - expanded for the attentive layman
in the style of Sather - would form an invaluable supplement to his
works.
Still, I basically got that the AC-coupling acts as a high-pass filter,
and that its response time means that after 5-10ms the voltage on both
sides is equalized, resulting in the speaker "rest" state. I don't
pretend to understand the analog circuit, but the implications you
mention follow nicely.