Bbe Section; Compression Effect; The Limiting Effect - BBE MAXCOM Owner's Manual

OPERATION

bbE SECTION

In order to address problems inherent in basic loudspeaker design, BBE Sound, Inc. has developed a circuit that
has two primary functions. The first adjusts the phase relationship of the low, mid and high frequencies. Since a
loudspeaker¹s natural tendency is to add progressively longer delay times to higher frequencies, the BBE circuit
adds progressively longer delay times to lower frequencies. This creates a kind of "mirror" curve to the time delay
curve in the speaker and neutralizes its distorting effect.
The second major element in the BBE system is the augmentation of the higher and lower frequencies. Loud-
speakers tend to be less efficient in their extreme treble and bass ranges. Most sound-reproducing systems
include a circuit for boosting high and low frequencies, showing an accepted awareness of the loudspeaker¹s ef-
ficiency problem. The BBE system, however, provides a dynamic, program-driven augmentation which combines
with the phase compensation feature to restore the brilliance and clarity of the original live sound. The result is, as
one professional journal phrased it, "The most hearable advance in audio technology since high fidelity itself!".

COMPRESSION EFFECT

At the beginning of this chapter, we dealt briefly with the basic functions of a compressor/limiter. Now we will
discuss this point in more detail, with reference to the threshold setting, input level and compression: consider
an input signal which is applied to the inputs of two compressors. The threshold of the second compressor is
set 10 dB higher than the threshold of the first compressor. Since a compressor only affects signals that exceed
the threshold level, it is fairly obvious that the signal of the first unit, will be compressed more because as the
threshold is lower, it is exceeded to a greater exceeded before.
Furthermore, it is interesting to note that by comparing the input and output waveforms for the compressed
mode, the quietest sections of the input signal have been effectively raised in level, whereas the loudest sec-
tions have been effectively decreased in level. The overall effect is that both ends of the dynamics range have
been pushed towards the middle. The squashing effect of compression is important to remember and highlights
the major difference between compressing and limiting.
Compressing and limiting differ in one more aspect: the dynamic setting for attack and release times. For com-
pression purposes, a preferably longer attack and release time is generally the best in order to keep the overall
output signal within a specified dynamic range.
For limiting applications, considerably shorter times are necessary to control transient signals or to increase
headroom. To achieve inaudible compression, it is imperative to work with program dependent attack and re-
lease times and a Soft Knee response. The advantage of program dependent compression is most apparent
when processing musical material that is varied. The MAXCOM is suitable for all applications because of its
various time settings and the AUTO processor. With the AUTO switch engaged, the unit uses program depen-
dent attack and release times. When not engaged, the control times can be set manually. Please note, while the
AUTO switch is depressed the MAXCOM does not provide a limiter function even if the RATIO control is turned
fully clockwise because signal peaks are NOT radically cut off above the threshold point.
If you want to use the MAXCOM as an ultimate means of protection against overload, it is recommended to
study the following chapter.

THE LIMITING EFFECT

The attack time is defined as the time taken for a compressor to respond to program levels which have exceed
the threshold point. Because of the physical relationship between frequency and slew rate, the resulting factor is
that for relatively low frequencies a longer attack time was required than for higher frequencies: any unpleasant
dynamic distortion would be avoided. When compressing a program mix that includes a wide range of frequen-
cies, a compromise must be made when setting the attack time. This setting would generally suit the lowest
frequency components present. For general dynamic-range control using the compressor mode, this is of no
serious consequence. However, in a limiting mode, we are restricting the peaks of our signals to a maximum
operating level to avoid distortion in any subsequent devices. If we are using a conventional compressor in a
limiter mode, the result will be in very fast high frequency signal transients passing through unaffected by gain
reduction. It is therefore necessary to choose the limiter control times precisely in order to govern any kind of
transients. In order to use the MAXCOM as a peak limiter, it is necessary to operate the unit in limiter mode
(RATIO control turned fully clockwise) and to switch out the AUTO function. This enables the limiter function to
exclusively and instantaneously respond to signals which exceed the threshold level.
It is important that the AUTO switch is switched off and that the attack time is as short as possible to avoid any
audible distortion. The RELEASE control should be set to a value whereby side effects such as "flutter" and
"pumping" etc. do not occur.
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