Handbook for Sound Engineers

Consoles 923

employed here is very largely due to the individual
summing amplifiers, scaled by the gain asked of them.
Noise resulting from them is defined to low
(100 dBu or better) floor levels—fairly meaningless
under the stampede of typical front-end or machine
noise.

Distortion is primarily due to the automodulation of
the CMOS transmission gates; that is, the path resis-
tance varies with the instantaneous signal voltage. This,
at zero level, is typically a nonsensical value. Both the
harmonic and intermodulation products are almost
unmeasurably low principally because of the near
virtual-ground operation of the active CMOS elements.
There is no voltage swing, no automodulation.

25.15.2 Function Modes

Reference should be made to Figs. 25-95 through 25-98

during this discussion of the channel system. These

illustrations show the overall channel in block diagram

form and the various ways the circuit blocks are config-

ured for the different functions expected of the channel

in use. Fig. 25-95 has all the reconfiguration represented

by diagrammatically accurate but forbiddingly incom-

prehensible mechanical switching. Figs. 25-96 and

25-97 replace those in the main signal paths with elec-

tronic switching elements, which may seem more or

less of a jungle, dependent on whether you were

brought up on hard-gold contacts or silicon.

Certainly there are fewer electronic switchpoints

than there were mechanical. This rationalization is

primarily due to yet another incursion of esoteric (for

audio) digital things.

A simplified representation of the four basic channel

operating modes is given in Fig. 25-98A for recording,

Fig. 25-98B for mixdown/direct to stereo, and Fig.

25-98C for overdubbing. The Xs show the switching

points. Briefly, main multitrack operating modes and

their implementation in this system are outlined here.

25.15.3 Recording Mode

In the recording mode, the object is to get a live source

(e.g., microphone) through the signal modification chain

(i.e., limiting, equalization) and on to a track or tracks of

the multitrack machine. Level control on this path is by

the main fader (or VCA fader if automation is appli-

cable). Before and after monitoring of the tape track

dedicated to the channel is routed onto the main stereo

monitoring/mix-bus via the secondary level control.

25.15.4 Mixdown Mode

The machine return is brought through the modification

chain and mixed onto the main stereo moni-

toring/mix-bus via the main/VCA fader. The machine

monitoring chain is disabled.

Since a major justification for keeping the multitrack

routing open during mixdown is to provide additional

effects feeds, this is best served if the secondary level

control is fed post main fader and post mute/solo

switching. To enable this, a crossfeed electronic routing

is included in Fig. 25-98B. However, independent

control is restored when required if a fader reverse is

called.

Another mode, direct to stereo, is a derivative of

mixdown. It enables live sources to be mixed straight on

to the main stereo bus, obviating the need to use multi-

track routing.

25.15.5 Overdub Mode

A halfway house between record and mixdown, the

overdub mode is intended for use when most of the

console is in mixdown but individual channels are being

laid or touched up. The signal flow is the same as in the

record mode, only with the main/VCA and secondary

level controls interchanged. The main/VCA fader in this

mode, therefore, controls the monitor feed into the main

stereo mix-bus, which ties in with the operation of this

fader on all the other channels that are in mixdown.

A handy interlock exists in this mode to facilitate

single button drop in. When the channel system func-

tion is selected to overdub and the monitoring path is set

to A check (machine input), a relay closing pair is made

that may be plumbed into the remote control access of

the machine. Provided the track is armed ready, hitting

A check automatically drops the machine into record

simultaneously.

25.15.6 Logic Control

This particular console design was intended to be

capable of running with or without control by a micro-

processor. Much of the localized logic dealing with

switches, control, and indication encompasses the

necessity to stand alone and in some instances to

provide an optional microprocessor with a means of

reading back actual console status. Using conventional

switch-matrix sensing and latched control/indicator

driving by a microcontroller could make redundant

much of this hardware, if permanent processor control

were envisaged. Similarly, much of the discrete logic