LA Audio Electronic BCL20 User manual

Dual/Stereo Broadcast Compressor/Limiter
Operation Manual
August 2004

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CONTENTSCONTENTS
1.1. IntroductionIntroduction Page 2
2.2. InstallationInstallation Page 3
3.3. WarrantyWarranty Page 5
4.4. Description of controlsDescription of controls Page 6
5.5. ApplicationsApplications Page 8
6.6. SpecificationsSpecifications Page 14
7.7. Front panel blanksFront panel blanks Page 16

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1.0 INTRODUCTION1.0 INTRODUCTION
The BCL20 is a stereo compressor-limiter with a de-essing
capability which allows the end user to process two different
signals or one stereo program.
Applications include :Applications include : FM transmitter protection, disc
mastering, SPL limiting in PA and discotheque applications,
tightening of drums and sustain control on guitars and bass.
Main parametersMain parameters, including threshold, gain, attack and release
times and compression ratio are user controllable.
A unique feature of the BCL20 is the continuously variable
detection characteristics. A potentiometer allows mixing of RMS
and Peak detectors to tailor the timing parameters to the
program being processed. The BCL20 takes advantage of a
unique transformer distortion cancelling circuit reducing it an
order of magnitude compared to conventional designs.
2 bargraphs display the amount of compression and the
input/output level. The de-essing functions involves a slew-rate
sensitive network which, while keeping operation very simple,
allows frequency-conscious compression, in accordance with
FM transmission or disc mastering pre-emphasis.
A rear panel jack allows insertion of an external equaliser to
achieve more specific frequency-conscious processing.
RFI protection has been a main concern in the design of the
BCL20 allowing its use in broadcast installations.

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2.0 INSTALLATION2.0 INSTALLATION
IMPORTANT: PLEASE READ THIS SECTION BEFORE USING THE BCL20IMPORTANT: PLEASE READ THIS SECTION BEFORE USING THE BCL20
2.1 INSPECTION AND UNPACKING2.1 INSPECTION AND UNPACKING
The BCL20 has been carefully packed at our factory in a carton
designed to withstand handling in transit. Should the unit appear
to have been damaged in transit notify your dealer immediately
and do not discard any of the packing. The carton should
contain -
•The BCL20 unit
•Power cord
•Operator Manual (this book)
2.2 OPERATING ENVIRONMENT2.2 OPERATING ENVIRONMENT
The BCL20 is designed to operate between 0 and 50°C (32-
122°F) with relative humidity no more than 80%. Should the unit
be installed in an equipment rack, ensure that the ambient
temperature conforms to these levels.
2.3 CE STANDARDS AND THE LOW VOLTAGE DIRECTIVE (LVD)2.3 CE STANDARDS AND THE LOW VOLTAGE DIRECTIVE (LVD)
The BCL20 has been designed to meet the latest
Electromagnetic Compatibility (EMC) regulations. However we
recommend you do not operate the unit close to strong emitters
of electromagnetic radiation such as power transformers,
motors, mobile telephones or radio transmitters.
The unit should only be connected to a power supply of the type
described in 2.4 POWER REQUIREMENTS or as marked on the
unit. The unit must be earthed for safe operation. Disconnect the
mains supply before removing the cover.
2.4 POWER REQUIREMENTS2.4 POWER REQUIREMENTS
There is a mains fuse located inside the unit which is accessed
by removing the cover. This fuse is for protection against unit

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faults and as such will not normally need replacing. If you think
you have a power related problem please refer to your dealer.
The voltage and fuse rating have been factory set for -
230V 100mAUK
220V 100mARepublic of Korea
115V 200mANorth America
•• The correct voltage and fuse rating is indicated on theThe correct voltage and fuse rating is indicated on the
rear panel.rear panel.
•• Please check that yourPlease check that your unit is correctly rated for theunit is correctly rated for the
voltage of the country of operation.voltage of the country of operation.
If the fuse requires changing at any time please ensure the
correct fuse is fitted. An incorrect fuse could cause damage to
the unit and may constitute a fire hazard.
If you carry your BCL20 from one country to another it is very
important to ensure the voltage is compatible.
For your information the unit can be internally rewired for use in
230V, or 110-125V countries - contact your dealer for
information. A special 100V version is available for Japan and a
220V, 60Hz version for Republic of Korea.
The mains lead connections to the appliance are coloured in
accordance with the following code:
Green-and-YellowGreen-and-Yellow EarthEarth
BlueBlue NeutralNeutral
BrownBrown LiveLive
•WARNING:WARNING: THIS APPLIANCE MUSTTHIS APPLIANCE MUST BE EARTHEDBE EARTHED
2.5 EXTERNAL CONNECTIONS2.5 EXTERNAL CONNECTIONS

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BCL20 rear view showing Channel 2 and Mains connections
The input and outputs of the BCL20 are on XLR connectors. For
balanced input and outputs, connections to the unit should be
wired as follows:
XLR +4dBuPin 1 Screen (Signal ground)
Pin 2 Hot (+ve)
Pin 3 Cold (-ve)
For unbalanced operation join pins 1 and 3 on either input or
output.
The 'Side chain' connectors are on ¼ inch TRS jacks and are
wired as follows
TRS jack Tip Send
Ring Return
Sleeve Screen (signal ground)
3 WARRANTY3 WARRANTY
Your LA Audio BCL20 has been manufactured to a high
standard using quality components. If correctly installed and
operated the unit should give years of problem free operation.
However in the event of a defect in material or workmanship
causing failure of the unit within one year of the date of original
purchase we will agree to repair, or at our discretion replace,
any defective item without charge for labour or parts. To receive
service under this warranty it is necessary to return the unit to
an LA Audio authorised service centre or to the factory with a
dated receipt as proof of purchase. After repair the unit will be
returned to you free of charge.
Limitations:Limitations:
This warranty does not cover damage resulting from accident or
misuse. The warranty is void unless repairs are carried out by
an authorised service centre. The warranty is void if the unit has
been modified other than at the manufacturers instruction. The
warranty does not cover components which have a limited life,
and which are expected to be periodically replaced for optimal
performance. We do not warrant that the unit shall operate in
any way other than as described in this manual.

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44 DESCRIPTION OF CONTROLSDESCRIPTION OF CONTROLS
GAIN REDUCTION DISPLAYGAIN REDUCTION DISPLAY
This 12 Led bargraph indicates the amount of compression (in
dB). The law is expanded at low values.
LEVEL DISPLAYLEVEL DISPLAY
This 12 Led bargraph indicates either the input level or the
output level as selected by the input/output toggle switch.
The bargraph can also be switched between peak and VU
BYPASSBYPASS
This switches the compressor in and out of circuit, so that
comparisons may be made between the compressed and
uncompressed signal.
IN / OUTIN / OUT
This switches the second bargraph display between input and
output level.
DS IN / OUTDS IN / OUT
This button instantly adds de-essing capability to the BCL20 by
adding a filter into the side-chain. This makes the compressor
"frequency conscious", which means that it compresses more on
the frequency chosen, in this case the frequency where
sibilance occurs.
RATIORATIO
This determines the ratio of compression applied. This may be
varied from 1:1 to LIMIT (20:1). The ratio of compression may
be defined as the ratio which determines how much the output is
allowed to increase by in comparison to an increase of the input.
For example, with a 10:1 compression ratio, once the input
signal is above the threshold level, the output will only rise by
1dB for every 10 dB that the input rises. When the ratio is in
excess of 10:1 the action of such large amounts of compression
is described as "limiting", because the output is unable to
exceed the limit imposed by the compressor.

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THRESHOLDTHRESHOLD
This adjust the signal level above which compression occurs.
The lower it is set, the greater is the compression. The threshold
range is from -40 dBu to +20 dBu.
ATTACKATTACK
This determines how fast the compressor will act upon the
incoming signal. This time may be varied from between 0.1 milli-
seconds and 70 milliseconds. (Caution : Very fast attack times
introduce distortion at low frequencies with ANY form of
compressor. This is caused by a low frequency signal being
longer than the attack time. The low frequency waveform will be
distorted by the initial gain reduction because the action is
almost certain to occur mid-waveform.)
RELEASERELEASE
This determines how fast the action of the compressor will take
to be turned off as the incoming signal falls below the threshold.
This time may be varied from between 100 milliseconds and 2
seconds.
(Caution : Very fast release times introduce distortion at low
frequencies with ANY form of compressor. This is caused by a
low frequency signal being longer than the release time. The low
frequency waveform will be distorted by the initial release of gain
reduction because the action is almost certain to occur mid-
waveform.)
RECTIFIERRECTIFIER
This controls the respective percentage of RMS detection and
Peak detection that will be processed by the side-chain. In the
extreme positions, it uses only one of the rectifiers. In the
intermediate position, it uses a blend of both.
A very basic rule-of-thumb would be to use RMS detection on
individual tracks and Peak detection on mixed program (see
applications). As always this is very much open to
experimentation.
The unique continuously-variable detection characteristic of the
BCL20 makes experimentation as easy as turning a knob.
NB : The Attack and Release controls relate to the Peak rectifier
only. Hence, when the Rectifier control is in the full ccw position
(RMS), these controls are inoperative.
GAINGAIN
This controls the output gain after compression and is used to
match the output with the level of the incoming signal. This may
be varied from -20 dB to +18 dB.
STEREO/DUALSTEREO/DUAL
The unit may be switched between stereo mode and two
separate mono channels.

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POWERPOWER
(On rear panel)
This switches the unit ON or OFF and the adjacent led lights
when the unit is ON
5 APPLICATIONS5 APPLICATIONS
DEFINITIONS OF TERMS USEDDEFINITIONS OF TERMS USED
Compressor :Compressor :
A variable gain amplifier whose gain decreases as the input
increases beyond a threshold point. A compressor uses ratios of
up to 8:1
Limiter :Limiter :
A limiter is a compressor with very high compression ratios,
usually above 10:1. A high ratio maintains an almost constant
output level despite large changes in the input level above the
threshold point.
Compression ratio :Compression ratio :
The ratio in dB of input level change to output level change
above the threshold point. A 4:1 compression ratio implies that
the output level will only change by 1dB for every 4dB that the
input level changes above the threshold point.
Threshold :Threshold :
This is the point at which compression starts to be applied as
the signal rises, or starts to be removed as the signal falls. The
Gain Reduction display will indicate when compression
commences, and then the threshold can be varied to obtain the
desired effect. Reducing the threshold point will give more
compression effect than leaving it just below the input level.
Peak detection :Peak detection :
It is closely related to the maximum electric amplitude of the
signal.
This characteristic is ideal for compressor-limiters used in
protection of transmission lines, amplifiers, broadcast
transmitters etc...
RMS Detection :RMS Detection :
It is related to the psycho-acoustic perceived "loudness". This
characteristic is more suitable for "creative" applications such as
drums fattening, bass sustain, vocal processing, etc...
De-essing :De-essing :
Reducing or removing the sibilance effect produced by some
microphones, some people, and certain sound sources. This is
achieved by making a compressor "frequency conscious". This
means that a pre-emphasis filter is placed in the side-chain,
which controls the compressor, making it work harder upon
those particular frequencies. In true de-ess mode, the

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compressor ONLY acts upon those frequencies and passes all
other ones.
GENERALGENERAL
The usual purpose of Limiting and Compression is to increase
loudness and to provide overload protection and generally to
control level.
LimitingLimiting
Limiting is used mainly to provide overload protection ; i.e. to
limit the output to some desired output level.
The protection required is usually against transients which
exceed the peak input level required for a tape recorder or a
transmitter.These high energy transients are usually of very
short duration (micro-seconds) but may wreck a transmitter or
ruin a good recording by introducing distortion.
Control of these transients by limiting does not noticeably affect
the dynamic range, since the gain reduction, when it does occur,
is momentary and of a relatively low order of magnitude. In most
cases, a fast release time will be employed so that the transients
are controlled quickly without any apparent change in
programme content.This form of compression, using ratios of
10:1 and above allows the engineer to reduce his system
headroom, and operate with a higher recording or broadcast
level without fear of overload and thus increase the useable
dynamic range of a recording or transmission system.
CompressionCompression
Compression describes conditions of gain reduction that are
more or less continuous, so that the original dynamics are
compressed. A low compression ratio is used when it is
desirable to preserve a relationship with the original dynamics,
but higher ratios provide useful effects, particularly in "pop"
recording and the transmission of speech, where the main
sound source needs to be brought out clearly. This is effectively
increasing the loudness of the signal, and this enhances its
prescence in a recording or over a background.
Attack and Release timeAttack and Release time
Attack and release times are important when using
compression, since they determine the moment to moment gain
change in a system, and therefore the apparent loudness. A fast
release time combined with a high ratio will bring more of the
lower level signal content up towards peak level, and a fast
attack time will really bring out the prescence. However, this
kind of setting can sometimes be heard to be "breathing" or
"pumping". This is the effect where the main sound source is
intermittent and you notice the compressor working. (Suddenly
the main voice comes through and sounds veryfull, and then as
it pauses, the compressor turns off, allowing the back-
ground noise to be heard, and then the voice cuts in again
immediately obscuring the background). In these cases it may

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be necessary to either reduce the times used or reduce the ratio
depending upon the effect required.
ModulationModulation
If more than about 6db of compression is used, it is desirable to
treat individual sound sources or groups of similar sound
sources separately in order to avoid modulation effects, when a
dominant sound modulates the rest of the signal. It is particularly
difficult to limit low frequency sounds in a final balance, because
the effect on the remaining signal and ambience will be most
noticeable. Separate compression will increase the overall
compression effect.
THE BCL20 AS A LOUDNESS LIMITERTHE BCL20 AS A LOUDNESS LIMITER
The perceived loudness (SPL) is not linear as a function of
frequency. The human ear is more sensitive to the mid-
frequencies than to very low or very high frequencies. This
phenomenon must be taken into account when using a limiter to
control the maximum loudness in certain areas such as
discotheques, clubs and concert halls. To emulate the typical
loss of high and low frequency sensitivity of the human ear, a
weighting filter must be inserted in the side-chain, according to
figureÿ1. This weighting filter is available from a number of
companies manufacturing test equipment. It must be set at "C"
weighting (high levels).
Fig. 1

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As an alternative, one can emulate such a filter, using a stereo
one-octave equalizer set in such a way that the frequency
response approximates that of figure 2.
Fig. 2
THE BCL20 AS AN ACOUSTIC NUISANCETHE BCL20 AS AN ACOUSTIC NUISANCE LIMITER :LIMITER :
This is a complementary aspect of loudness limiting, but in this
instance, the sound pressure level which must be limited is one
that remains after attenuation and conduction outside the room
or building where the sound is produced.
Very often, only certain low frequencies are transmitted, in
particular those which excite resonant modes of cavities, walls
or structures.
To achieve efficient limiting of those frequencies, it is necessary
to measure the noise transmission, using a real-time spectrum
analyser (preferrably 1/3ÿoctave).
When this is done, one has to copy the frequency response to a
graphic equalizer which is inserted in the side-chain.
In operation, limiting will occure predominantly at those
frequencies which create a nuisance.
NB : To improve this scheme, it may be necessary to use a
parametric equalizer, tuned precisely at the frequencies which
excite resonances.

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When this is done, one has to copy the frquency response to a
graphic equaliser which is inserted in the side-chain.
THE BCL20 AS AN AUTOMATIC VOICE-OVER (DUCKER) :THE BCL20 AS AN AUTOMATIC VOICE-OVER (DUCKER) :
This application is useful in discotheques and radio where it
provides an "automatic fader" which reacts to the voice of the
disc-jokeys, reducing the background music accordingly.
To do this, the music channels must be routed to separate
groups which will be processed through the BCL20 and then
reinjected to the Mix busses (sub-group).
The voice channels must be routed directly to Mix and to an
auxiliary output whichs feeds the Side-Chain insert Return.
The BCL20 must be in the Stereo mode.
The active front panel controls are those of the channel
receiving the auxiliary send.
The other controls must be set as to be inoperative ; it is
sufficient to set the Threshold at its maximum value.

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The following two applications show the BCL20 inserted into an
individual channel. After processing, the signal is mixed with the
other channels. This is done because the BCL20 is used to
modify a sound, not to control its amplitude.
Bass DrumBass Drum
VocalVocal
The next two applications show the BCL20 connected at the
output of the mixer. This is because if the BCL20 was inserted in
the Mix inserts the level would vary with the setting Of the
Master fader.
Cassette MasteringCassette Mastering
FM TransmissionFM Transmission

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6 SPECIFICATIONS6 SPECIFICATIONS
Frequency ResponseFrequency Response
20 Hz to 20 kHz +/- 0.5dB
Signal to Noise RatioSignal to Noise Ratio
+ 4 dBu Input, + 4 dBu Output -94 dB
Input ImpedanceInput Impedance 20k Ohms balanced
Output Source ImpedanceOutput Source Impedance 20 Ohms
Output Load ImpedanceOutput Load Impedance 600 Ohms to infinity
Maximum Input LevelMaximum Input Level + 20 dBu
Maximum output LevelMaximum output Level 600 Ohms Load, 1 kHz + 20 dBu
Total Harmonic DistortionTotal Harmonic Distortion (THD)
+ 4 dBu Output,
1 kHz, 10 dB compression 0.07 %
1 kHz, by-pass 0.01 %
Intermodulation DistortionIntermodulation Distortion 0.01 %
Compression RatioCompression Ratio Continously variable
from 2 : 1 to 20 : 1
Attack Time (Peak)Attack Time (Peak)
Continously variable from 0.1 to 70 millisec.
Release Time (Peak)Release Time (Peak)
Continously variable from 100 millisec to 3 sec.
Attack Time (RMS)Attack Time (RMS)
Program dependant Typ. 12 ms for 12 dB.
Typ. 4 ms for 24 dB.
Release Time (RMS)Release Time (RMS)
Program dependant Typ. 120 ms for 12 dB
Typ. 240 ms for 24 dB
Mains power ConsumptionMains power Consumption
(115/220 VAC, 50-60 Hz) 6 Volts-amps
Meter CalibrationMeter Calibration 0 Vu = + 4 dBu
Maximum Gain ReductionMaximum Gain Reduction more than 50 dB

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DS ActionDS Action 12 dB at 8 kHz
Input ConnectorInput Connector XLR 3 pin balanced
Output ConnectorOutput Connector XLR 3 Pin balanced
DimensionsDimensions 483mm W x 190mm D x 44.5mm H
19"W x 7.5"D x 1.75"H
WeightWeight 3.9kg, 8.6lbs
Shipping WeightShipping Weight 4.8kg, 10.6lbs
Temperature RangeTemperature Range
OperatingOperating 0C to +50C
StorageStorage -30C to +75198>C
Note:
In these specifications, 0 dBu is referenced to 0.775 V RMS
LA Audio reserves the right to alter any feature or specification without prior notice. E&OE