LA Audio BCL2 User manual

PRO-SERIES
BCL2
OPERATION MANUAL

BCL2 1
CONTENTS
1. Introduction 2
2. Specifications 3
3. Installation 5
4. Description of the controls 6
5. Applications 8
6. Technical description 14
7. Maintenance 16
Appendices:
Circuit Diagrams
PCB Diagrams
Outiline and dimensional drawings

BCL2 2
1. INTRODUCTION
The BCL2 Is a stereo compressor-limiter with a de-essing capability which allows the
professionnal user to process two different signals of one stereo program.
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 parameters, Including threshold, gain, attack and release times and
compression ratio are user controllable.
A unique feature of the BCL2 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 BCL2 takes advantage of a unique
transformer distortion cancelling scheme reducing it an order of magnitude compared
to conventional designs.
2 bargraphs control the compression amount and the input/output level. The de-
essing functions Involves a slew-rate sensitive network which, 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 equalizer to achieve more specific
frequency-conscious processing.
RFI protection has been a main concern in the design of the BCL2 allowing its use in
broadcast installations.
A ground lift switch allows the elimination of hum loops.

BCL2 3
2. SPECIFICATIONS
2.1. ELECTRICAL
Frequency Response
20 Hz to 20 kHz +0, -0.25 dB
Signal to Noise Ratio
+ 4 dBu Input, + 4 dBu Output 94 dB
Input Impedance 20 kOhms balanced
Output Source Impedance 20 Ohms
Output Load Impedance 600 Ohms to infinity
Maximum Input Level + 20 dBu
Maximum output Level
600 Ohms Load, 1 kHz 20 dBu
Total Harmonic Distortion (THD)
+ 4 dBu Output,
1 kHz, 10 dB compression 0.07 %
1 kHz, by-pass 0.01 %
Intermodulatlon Distortion 0.01 %
Compression Ratio
Continously variable from 2 : 1 to 20 : 1
Attack Time (Peak)
Continously variable from 0.1 to 70 millisec.
Release Time (Peak)
Continously variable from 100 millisec to 3 sec.
Attack Time (RMS)
Program dependant Typ. 12 ms for 12 dB
Typ. 4 ms for 24 dB.
Release Time (RMS)
Program dependant Typ. 120 ms for 12 dB
Typ. 240 ms for 24 dB
Mains power
Consumption 6 Volts-amps
(115/220 VAC, 50-60 Hz)
Meter Calibration 0 Vu = + 4 dBu
Maximum Gain Reduction more than 50 dB
DS Action ** 12 dB at 8 kHz

BCL2 4
2. SPECIFICATIONS
2.2. PHYSICAL
Input Connector XLR/QG 3 pin balanced
Output Connector XLR/QG 3 Pin balanced
Dimensions 483mm W x 190mm D x 44.5mm H
19"W x 7.5"D x 1.75”H
Weight 3.9 kg, 8.6 lbs
Shipping Weight 4.8 kg, 10.6 lbs
Temperature Range
Operating 0oC to +50oC
Storage -30oC to +75oC
Finish Light Grey epoxy
Note : In these specifications, 0 dBu is referenced to 0.775 V RMS.
* SMPTE Method : 60 Hz + 7 kHz mixed 4:1, + 20 dBu output.
** DS action Is defined as the Increase of compression at the specified
frequency, referenced to compression at 1 kHz.
Product Information and specifications are subject to change without notice.

BCL2 5
3. INSTALLATION
IT IS IMPORTANT TO READ THIS SECTION BEFORE USING THE UNIT.
3.1 INSPECTION & UNPACKING
The Model BCL2 is carefully packed at the factory and the container designed to
withstand handling in transit.
However, if transit damage Is evident, DO NOT discard any of the packaging material
and notify the carrier immediately, as it is he who is responsible for such claims.
The shipment should contain the following :
•The unit BCL2
•Power cable
•Technical Manual (this book)
If any of the above Items are missing, notify your supplier Immediately, and DO NOT
discard any of the packaging materials.
The power cable may be supplied without a connector at the supply end. This is
because there is no world-wide standard, and therefore the user should supply a
mains connector for use with the unit.
3.2. OPERATING ENVIRONMENT
The unit is designed to operate between 0 and +50 degrees Celcius (32 -122 F) and
in an atmosphere of relative humidity up to 80%.
Should the unit be installed in an equipment rack, it is important to ensure that the
operating temperature inside the rack does not exceed the upper limit. This may often
be the case where the rack contains power amplifiers and a cooling fan may be
necessary in such installations.
Although designed to withstand hum pick-up, and it is VERY important that the unit is
not installed close to any electro-magnetic fields which may be generated from power
transformers or motors.
3.3. POWER REQUIREMENTS & LINE VOLTAGE SWITCH
The unit may be operated from either 115-125 Volts AC 60Hz single phase or 200-
250 Volts AC 50Hz single phase. (A 100 Volt special unit is available for Japan)
The voltage is normally pre-selected at the factory for the country of destination, and
a sticker will indicate the voltage the unit is set for. IF IN DOUBT CHECK THE
VOLTAGE SELECTOR SWITCH INSIDE THE UNIT AND ENSURE THAT THE
CORRECT FUSE IS USED. (0.1 amp for 200-250 & 0.2 amp for 100-125 ; both slow
blow).

BCL2 6
4. DESCRIPTION OF THE CONTROLS
The following controls are the same for both left and right channels, each side
containing the following :
GAIN REDUCTION DISPLAY:
This 12 Led bargraph Indicates the amount of compression (In dB). The law Is
expanded at low values.
LEVEL DISPLAY :
This 12 Led bargraph indicates either the input level or the output level as selected by
the Input/output push-button.
COMP/BYPASS :
This switches the compressor in and out of circuit, so that comparisons may be made
between the compressed and un-compressed signal.
INPUT/OUTPUT:
This switches the second bargraph display between input and output level.
DS/FLAT:
This button instantly adds de-essing capability to the BCL2 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.
GAIN:
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 +20 dB.
RATIO :
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.
THRESHOLD :
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.
ATTACK :
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.)
RELEASE :
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 3 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

BCL2 7
4. DESCRIPTION OF THE CONTROLS
RECTIFIER:
This controls the respective percentage of RMS detection and Peak detection that will
be processed by the sloe-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 BCL2 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.
STEREO/DUAL :
The unit may be switched between stereo mode and two separate mono channels.
POWER:
This switches the unit ON or OFF and the adjacent led lights when the unit is ON.

BCL2 8
5. APPLICATIONS
5.1. DEFINITIONS OF TERMS USED
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
LImlter:
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 :
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 1 dB for every 4dB
that the input level changes above the threshold point.
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 :
It is closely related to the maximum electric amplitude of the signal. This characteristic is
ideal for compressor-llmiters used in protection of transmission lines, amplifiers,
broadcast transmitters etc...
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 :
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 compressor
ONLY acts upon those frequencies and passes all other ones.
5.2. GENERAL
The usual purpose of Limiting and Compression is to increase loudness and to provide
overload protection and generally to control level.
Limiting
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..

BCL2 9
5. APPLICATIONS
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.
Compression
Compression describes conditions of gain reduction that are more of 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 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 background noise to be heard, and then the voice cuts in again immediately
obscuring the background). In these cases it may be necessary to either reduce the
times used or reduce the ratio depending upon the effect required.
Modulation
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.