17
The Dynamics
of Model 261
Processing
The Limiting and Compression functions of the 261 utilize an
equivalent common signal path and control loop for the digital audio
signal. Limiting and Compression are separated by a floating gain
platform that establishes static gain and the processor s attack and
recovery timing.
Limiting is a very quick peak-control function. Attack timing of the
Model 261 is negatively instantaneous, as the look-ahead technol-
ogy is able to act on a program peak actually before it happens! Re-
lease is similarly quite fast, the object being to control the level of
isolated program peaks without poking a hole or causing audible
ducking in the program envelope.
The term Compression is subject to various interpretations and de-
finitions. Historically, a studio level compressor had a specified
compression
threshold
and certain compression
ratio
. Beginning at
a specified input level, the 1:1 input vs. output relationship would
take a different slope; maybe 1.5:1, 2:1 or even steeper. But com-
pression, as offered by the 261 and many other fine Inovonics prod-
ucts, means just that:
compression of the program dynamic range
.
There is, of course, still a threshold, below which the program is
treated on a 1:1 basis. But in our products this tends to be quite
near the output final-limited ceiling value, leaving all dynamics of
the program untouched before a certain critical amplitude, at which
all action starts to take place.
The
Compression
Platform
Beginning with the introduction of our first audio-dynamics proces-
sor for broadcasting in 1974, Inovonics has utilized the floating
platform principle of dynamic range compression. A certain
amount of static gain is added to the signal, forcing it well into final
limiting, with attack and release timing of the complex dynamic con-
trol defining the difference between Compression and Limiting.
In short, a dual time constant applies to processing dynamics: a
quick release of peak reduction to a certain platform level, and
then a slower release of the platform toward the restoration of full
circuit gain. The platform level is established by an
average
re-
sponse to program dynamics, and typically rides about 6dB below
the normal value of peak reduction.
This 6dB target figure depends very much on the dynamics of the
program material. Speech, with its high average-to-peak ratio, will
result in a greater amount of peak limiting, while contemporary pop-
music (generally already highly compressed!) will show lighter peak
control. In this manner, the dynamics of program control action
complement the dynamics and requirements of the source material.
The peak limiter works harder on peaky sources and the compres-
