woofer, midrange and tweeter must be the same for most
passive, high level crossover systems). If possible, choose
the crossover frequency and slope by the criteria
described in the previous paragraphs. Also, choose a
passive, high level crossover with adequate power
handling (for reliability), good quality components (for
low loss and low distortion) and with rugged physical
construction.
The designer of a biamplified (or triamplified)
system must choose an electronic crossover from an
expanded set of criteria. A professional electronic cross-
over should meet the professional criteria described on
Page FIVE 1 for balanced inputs and outputs, and for
input and output levels and impedances. In addition, in
order to be usable in a variety of professional systems, an
electronic crossover should give the designer a choice of
crossover frequencies and slopes. Some electronic cross-
overs restrict the choices, or require hardwired changes
or plug in cards to choose different frequencies or slopes.
Yamaha's F1030 is a two way or three way electronic
crossover which gives the designer a wide choice of
crossover frequencies selectable for each of three bands
by means of front panel controls. The controls are
recessed to avoid accidental setting changes. Either
12dB/octave or 18dB/octave slope rates can be selected
by internal switches. The F-1030 meets all the criteria
for a professional unit, and, in addition, has both XLR
and phone jack input and output connectors.
ECHO, REVERB, AND DELAY
Artifical echo is usually obtained in either of two
ways, with a tape delay (similar to a standard tape
recorder) or with a digital delay unit. Repeated echoes
are obtained by feeding some portion of the delayed
output back to the echo input (regeneration), or by using
multiple output taps along a tape or digital delay path.
In a tape recorder, the delay results from the time it
takes for the tape to travel from the record head to the
playback head (or heads). In a digital delay unit, the
audio is converted to a computer-like digital code (using
an analog-to-digital converter), delayed by shift
registers, and then reconverted to audio (using a digital-
to-analog converter). Other methods of obtaining time
delay are available, from "bucket brigade" (analog) time
delay units to a simple, effective technique where a
microphone is inserted in one end of a length of tubing
and a speaker at the other end.
Besides its use as an effect, a time delay device can be
a very useful tool in commercial sound systems. If two
speaker systems, which are fed by the same signal, are
separated by more than about 30 feet, a listener can
hear a distinct echo. By slightly delaying the signal to
the speaker system nearest the listener, such echoes are
avoided. This situation is presented in the Applications
section, in the diagram for a typical system in a theatre
with a primary speaker system at the stage, and a
secondary system under a balcony (which cannot be
covered directly by the stage speaker system).
COMPRESSION AND LIMITING
Dynamic range (also see Page FIVE 2) is the differ-
ence in dB, between the highest and the lowest volume
levels in any audio program. A compressor is a device
that shrinks that dynamic range. The "threshold" of a
compressor is the level above which compression begins.
The "compression ratio" is the ratio of output level
change, to input level change, in dB, for any program
material above the threshold. A limiter is a compressor
with a high compression ratio (usually 1 0 : 1 or higher).
Often a single device can be used for either compression
or limiting, since the distinction depends mainly on the
threshold and ratio settings.
Radio stations use compressors and limiters. Limiters
keep audio peaks from overmodulating and distorting
their broadcast signal (an FCC requirement), and com-
pressors keep their average modulation levels high (in
order to reach the maximum audience).
The dynamic range of better quality magnetic tape
recorders is about 65dB. Since much live program
material has a dynamic range of 90dB or greater, a
recording studio can use a compressor/limiter to restrict
the dynamic range of a program to fit the dynamic
range of the tape medium. Special "noise reduction"
devices are available for tape recording that make use of
complementary compression and expansion to lower
the noise levels on a tape recording and to retain the
original dynamic range of the program.
In a paging system, a compressor can keep the
average level of different announcers' voices more con-
stant so that paging can reach noisy areas of a factory
or airport more consistently. In addition, because of
reduced dynamic range, peaks are lowered, reducing the
chance of clipping distortion.
In concert sound reinforcement, or other large sound
reinforcement systems, a compressor/limiter can reduce
the chance of peak clipping, and can thus help avoid
amplifier or speaker damage from large turn-on/turn-off
transients, or from sudden, loud feedback. These uses of
compressor/limiters are valid for recording studio
monitoring as well as for sound reinforcement (although
feedback should not be a problem in studio monitoring).
Fig. 66 - Biamplified System showing Placement of
Optional Limiters.
While useful, compressors (compressor/limiters) are
not cure-all devices. The compressor "makes its
decision" to begin compressing by continuously
monitoring the program level. Unfortunately, the
highest levels are usually low bass notes. Thus the com-
pressor/limiter may compress the high frequencies
needlessly when it detects a bass note that is too loud.
One solution to this problem is to use a compressor on
each output of an electronic crossover on a biamplified
or triamplified system so that the compressor acts only
on the frequencies in each band. This method requires
two or three devices and is probably not applicable to
broadcast. Another solution is to use a separate com-
pressor on each mixer input that receives excessive
program levels.
Another problem with a compressor is that if it is
over-used, it can reduce the quality of sound in a musical
performance. Reduced dynamic range is often audible
and a poor quality compressor can add appreciable
distortion to a program, especially at high compression
ratios.
