Handbook for Sound Engineers

926 Chapter 25

elements would today be wrapped up in programmable
CPLD and FPGA (field-programmable gate array)
packages for flexibility and to minimize part count and
board space. The discrete logic is left detailed here,
however, on the basis that it would be useful to have an
idea of what sort of logic would need to be programmed
into these parts!
A distinction is made in Figs. 25-96 and 25-97
between the analog signal switches and their digital
control electronics not purely because of the differing
disciplines but for clarity’s sake; that is, to avoid too
many lines running all over the place on the drawing.
Each top-panel switch is a momentary-action touch
switch with an associated LED indicator (with the
exception of the function mode switch). The toggle
push-on and push-off characteristic is provided by the
basic debouncer/flip-flop circuit, as shown in Fig.
25-99. This action is not only fun, play-worthy, and
therefore, fashionable, it also scores in a couple of other
important respects:

Cost. The combination of a small, mechanically simple,
nonlatching, push-to-make switch and a fairly small
number of silicon bits is much less expensive than
latching pushbutton switches.

Versatility. Using electronic latching rather than

mechanical catches makes remote/automatic function

presetting and triggering comparatively simple.

25.15.7 Switch Debouncing

Debouncing is removing the ragged edges from a

switching signal. Switch contacts do not simply make

contact when pressed and break contact on release. The

two bits of metal grind against each other or bounce a

few times while moving together or apart, resulting in a

series of ragged, spiky “almost contacts” rather than

simply touch or not touch.

Ordinarily, this doesn’t matter too much, but, if the

switch is feeding a bistable flip-flop (as here), the fun

begins. Flip-flops are usually edge triggers; on a posi-

tive-going transition, another pulse flops it back and so

on. A string of rapid, unpredictable pulses, as provided

by nearly any mechanical switch, sends flip-flops frantic.

Retarding the switch with time constants is nearly

foolproof, but the arrangement in Fig. 25-99 is practi-

cally faultless. The 4098 contains two monostables,

which are handy since the 4013 contains two flip-flops.

It can sense either positive or negative transitions, posi-

Figure 25-97. Channel function modes.

Mic

Return (B)

Tape

Send (A)

Direct

Mic

Tape (B)

return

Mixdown

Mic

(B)

(A)

Tape

C. Overdub.

B. Mixdown/direct.

A. Recording.

Multitrack

matrix

Main/monitor

mix stereo

Main

stereo mix

Crossfeed

Multitrack

matrix

Multitrack

matrix

Monitor

stereo mix