Handbook for Sound Engineers

Consoles 975

exactly the same audio data originally sampled at the
lower inadequate sample rate, peaks are captured accu-
rately enough for all practical purposes. The missing
peaks of Fig. 25-145 are actually being filled in by the
reconstruction effect of the low-pass filter employed by
the upsampler, in exactly the same way as a D/A’s
reconstruction filter would.

As such, there is a very strong argument for over-
sampling in peak-limiter and other fast dynamics
processing. Since it has one tightly defined purpose, the
impact of doing the sample-rate conversions does not
have too great an impact on DSP processing cycle
budgets.
Unfortunately, for the live circumstance of that
example snare drum, actually initially converting at the
higher sample rate is unavoidable if a more analog
behavior is required; the greater number of samples
affords fewer gaps for those nasty transients to escape
through.

25.22.4 PreDelays, or Look-Ahead

Applied only in rare cases in analog because of the diffi-
culties in providing for good audio delay, predelaying is
eminently achievable in digital dynamics sections.
Predelay is the technique where the main signal path
through the dynamics is delayed for a short period (1 ms,
2 ms or so) to allow the side-chain processing to deter-
mine the right amount of gain reduction to be applied;
this value is then applied to the main signal path in a
gain-control element discrete from that used in the side-
chain, Fig. 25-146. The prime use is in peak limiters
(which are nearly always feedback style, even where the
other sections may be feed forward), where overshoot,
which can occur during this onset settling period, can be
completely avoided. An improvement in sound results,
too, since very hard and brutally short attack times can
be mellowed out knowing that overshoot is not going to
increase as a result. A relatively soft attack time (for a
peak limiter) of 1 ms combined with a comparable
predelay captures the peaks without the need for subsid-
iary clipping and yet is sufficiently aggressive that it
retains its loud characteristic but without the usual tell-
tale ripping hard edge.”

Look-ahead limiting is extensively used in broad-
cast air-chain processing, and especially on feeds to
streaming compression codecs (AAC, MP3, or HD
radio, for example), which generally do not react well to
the artifacts generated by more conventional clipping or
unavoidable transient escapee overloads from ordinary
limiters.

Only occasionally would such processing be done in

a console channel, but when it is, it should be remem-

bered to apply equal delay (whether limiting or not) to

other contributory channels in the mix.

25.23 Digital Mixer Architectures

Two distinct approaches seem to be taken to the

signal-processing architecture in mixing consoles, prob-

ably stemming from how deeply steeped in traditional

computer science the designer is.

25.23.1 The Sea of DSPs approach (sometimes

known as a Sharc Tank).

In this, a large enough array of DSPs for all envis-

aged processing is closely coupled to enable the rapid

transfer and sharing of data between them. The “tank” is

fed with all sources, and all destinations are taken from

it. In a telephone-exchange kind of approach, signals

requiring processing are farmed out to other processors

in the tank and the results returned; it could be regarded

from the outside as one big processor. The main advan-

tages of this approach are that not being physically

constrained to a particular organization, reconfiguration

is straightforward; any signal can go anywhere at any

time for any purpose. If more processing is needed it is

merely attached to the busing system, growing as

required. The major downside is that all the flexibility

makes programming such a beast very difficult (the

word nightmare has been bandied around).

The “one big processor,” is of course a reality for

many modest-sized applications, in the form of the

humble PC. These have increasingly faster and more

capable processing cores, and multiples of those, too.

Although they have their own issues as far as

processing audio (e.g., they generally don’t do DSP

terribly efficiently), brute force, speed, and might make

Figure 25-146. Dynamics section pre-delay system.

3

Input

'Normal'

feedbacklimited

sidechain

1

Delay line

Threshold

sensing

Attack,

release

Main (delay)path gain control

Output