Consoles 933some very odd things happening inside the channel
signal routing. Similarly, a ground follow-through
lockout arrangement is used on the master function
mode selection. Otherwise, the consequences of more
than one button being pushed simultaneously would be
to select a virtually random mode.
Note that all the switching is to ground from the logic
5 V supply. This interfaces with the majority of the
channel logic as described. An important feature is the
master reset bus and its control. Ordinarily, an array of
random logic circuitry dependent on flip-flops and
latches (of which this design is an example) would, on
power up, tend to settle into whatever state these regis-
ters felt like at the time. The result would depend on
device symmetry, temperature, and humidity, but worse
still, the results are not usually repeatable. An intriguing
exception to this is the knack of CMOS flip-flops to
come back up in their previous state after a short power
disablement, probably a function of small charge storage.25.15.15 Power on ResetWisdom and common sense dictate that on power up the
console should come on neutral, with all channels
muted and with monitoring functions such as PFL and
solo disabled. The one exception to this is if the console
control surface is totally under the control of a computer
(all functions), in which case it may be arranged to
come up in exactly the state in which it was last turned
off, or otherwise lost power. Even in this case, it would
be wise to bring the monitoring up muted. As well as
providing a frame of reference from which to start
reusing the console, it saves all the aggravation of
finding the one function that’s killing the monitoring.
There are few things scarier or more frustrating than a
large console that is mysteriously and totally silent.
Console mode and basic monitoring conditions can be
set up just by pushing the relevant master controls.
Processor control of course would afford the console
being reinstated to its last used configuration.
TR 1 in Fig. 25-102 grounds the master reset bus
(M.R.B.) for as long as the 22μF capacitor takes to
charge up—around a quarter of a second. This charging
takes place when the 5 V logic supply appears. Should
the supply collapse, the capacitor is rapidly discharged
via D 1 ready to reinitialize the M.R.B. signal as soon as
power is reestablished.
Although it would be extremely simple to do, no
top-panel master reset control is made available because
sooner or later someone would hit that button at exactly
the wrong moment. This follows the same philosophy
that frowns upon a top-panel ac line switch.
25.15.16 Meters and Head roomThere are plenty of proprietary meters of the popular
standards and types, plus quite a few strange ones, too.
It’s all a matter of personal preference and the informa-
tion hopefully gleaned from the assorted needles, lights,
and cathode rays dancing before the eyes.
Without jumping into the argument of average
versus peak-reading instruments, it is relevant to state
that the choice will directly affect the operational levels,
the level architecture, the machine lineups, and the
various tweaks, notably the input stage limiter threshold
in this design. Out of habit, this console was designed
with standard PPMs in mind, where the peak opera-
tional level throughout the system is expected to be
PPM 6, or +8 Bu. Lineup level (i.e., the system and
output level for which the front-end gain stage is cali-
brated) is 0 dBu, PPM 4. This will suit any current or
expected PPMs.
VUs are very good for giving an idea of subjective
loudness and not worrying you about transients that can
often be anything up to 20 dB above the indicated value.25.15.17 System Level ArchitectureNonunity-level architectures are regrettably necessary
under some conditions—detailed here are ways (quite
typical fixes within most console designs) that are
directly applicable to this described console.
Given standard +4 dBm referred VU meters, under
normal operational circumstances, head room in any
console is perilously skinny. Various ways of dealing
with potentially inadequate head room are in use, Fig.
25-103. A favorite is to run the entire console system at
a depressed level, usually 4 dB, the necessary 4 dB
makeup at the end being done passively by an output
transformer ratio stepup. This is a poor choice for two
reasons. The transformer stepup arrangement is overly
critical to termination impedance, and the frequency
response could suffer with a heavily reactive load such
as a long line.
A more modern solution on a similar theme is to
adopt a depressed level of 6 dBu and make up the level
at the output in a quasi-balanced electronic output
stage—in this way head room is not compromised at
any point along the way.
Head room is mostly a problem in input channels,
before the channel gain-controlling element, the fader.
Both ragged unpredictable input sources and equalizer
gain gobble up the nonmargin. Hopefully, beyond that
point the levels and, hence, the mix are easily and well
regulated by the faders. Dropping the channel operating