Ashly LIMITER/COMPRESSORS CL-100 - Manual (Page 6)

INITIAL SETUP AND CHECKOUT
Exact control of system headroom requires defining the clipping point of
the amplifiers in relation to the nominal zero VU of the mixing console. A
signal generator and oscilloscope would be helpful in determining this point
Since many people do not have the facilities or time to set up their systems in
this way, an alternate approach is suggested.
* First, realize that the sound system may have several different “0 dB"
levels, especially if equipment of different manufacturers are being mixed.
0 VU on the mixer’s meters may or may not correspond to a 0 dB V level of
.778 Volts. Also, note that input sensitivities of power amplifiers also vary.
1. Begin by turning the crossover level controls down.
2. Switch the unites LIMIT switch to the OUT position.
3. Set both the GAIN and OUTPUT level controls to 0 dB.
4. Set the RATIO control to 10 and adjust the ATTACK and RELEASE
controls to a relatively fast setting.
5. Plug a good quality music source into the mixing console and adjust
the console so that its meter peaks at around 0 VU.
6. Look at the LED display on the CLr50E and adjust the GAIN control
so that the yellow THRESHOLD LED lights only on peaks.
7. Note the new position of the GAIN control and adjust the OUTPUT
control inversely. That is, if you increased the Gain control to +10 dB
in order to light the Threshold LED, then decrease the Output control
to-10 dB. Or, if you had to decrease the Gain control to light the
Threshold LED, then increase the Output by an equal amount. You
have rww gotten the console and the compressor/limiter into relative
agreement over what is a maximum permissible signal level; as long
as the console’s output meter stays out of the red, little or no limiting
will occur. At or above console 0 VU, limiting will take place.
8. Turn the input volume control of your electronic crossover up to its
nominal zero point fforAshly crossooers this is it 7). Turn the
OUTPUT level controls of the crossover up until you achieve a good
musical balance of low and high frequencies, and continue to increase
the output volume controls until the soimd system just begins to sound
a little distorted. Since audible distortion may already be around 5%,
its a good idea to back off the output volume controls just a touch after
you first begin to hear any distortion. All elements of the sound system
ABHty
are now in agreement. Console OVU - Limiter Threshold = onset of
clipping in your power amplifiers. If you wish to allow extra
headroom in the system between console OVU and the onset of
dipping, turn down the ClrSOHs OUTPUT volume control by 5 dB,
10 dB, or whatever saftty margin you desire. (You can turn the music
down now.)
9. Start with alms. ATTACK time, a.5 sec, RELEASE time, and a
RATIO of 10. Adjust these parameters according to the type of
program material, and firmness of control desired.
10. Activate the compressor/limiter by depressing the LIMIT IN/OUT
switch. Now, bud bursts of energy above 0 VU will cause little or no
dipping in the system. □
THEORY
77ie Need For Gain Control
The human ear excels in its ability to detect an extremely wide range of
loudness levels, fix}m the quietest whisper to the roar of a jumbo jet. When
we attempt to reproduce this dynamic range, by means of amplifiers, tape
recorders, records, or radio transmitters, we run into one of the fundamental
limitations of these electronic media: limited dynamic range. Amplifier
dynamic range is quite good, and is adequate for most musical program
material. However, some types of audio equipment, such as cassette tape
recorders, have a very narrow useful dynamic range.
What is it that compromises the dynamic range of this equipment? The
useftil operating region of a piece of audio equipment is squeezed in between
noise and distortion. As program level decreases, it approaches what is
known as the “noise floor”, and if the volume of the program material goes
lower still, it is engulfed by the noise. The noise floor, or minimum constant
noise level, will consist of hiss, hum, transistor noise, record scratches, tape
hiss, buzz and whatever noises are inherent in the medium. When the
program level is considerably higher than the noise floor, our hearing masks
the noise, and it is not a problem. However, when listening to very quiet
sections of a program for example, a pause between movements of a string
quartet the noise can become very bothersome.
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