Ashly LIMITER/COMPRESSORS CL52E User's Manual
Contents
1. 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 N 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 clipping turn down the CL 50E s OUTPUT volume control by 5 dB 10 GB or whatever safety margin you desire You can turn the music down now 9 Start with a 1 ms 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 loud bursts of energy above 0 VU will cause little or no clipping in the system U THEORY The Need For Gain Control The human ear excels in its ability to detect an extremely wide range of loudness levels from 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 dyn2. groundwork for many of the sounds which are now considered indispensable in contemporary music In this role the compressor is used because you can hear it working and control of dynamic range is only a secondary consideration The Ashly CL models with their wide range of control parameters are well suited to both of these applications The Compressor Limiter As A Protective Device The CL SERIES provides fast and accurate gain control for the prevention of sound system overload due to unexpected transients Sound system distortion is usually a result of amplifiers running out of power in which case nice round waveforms turn into harsh sounding squared off waveforms Looking at it from the perspective of a speaker diaphragm this means that whereas in normal operation the diaphragm is required to accelerate slow down smoothly change direction and accelerate again distorted operation requires an instant acceleration instant stop a change of direction and instant acceleration again Installing the CL 50E for use in a sound system with a passive crossover Since speaker diaphragms are subject to the laws of physics they won t take this kind of punishment for long The diaphragm may shatter or its voice coil may overheat In addition to the damage caused by sustained overload the speaker may also be damaged by occasional one shot high level overload for example the sound of a microphone falling face first onto a hard floor Ev
3. leave ourselves some headroom helps prevent distortion but at the same time it moves our average program level closer to the noise floor thereby compromising signal to noise performance It becomes apparent that to get the most out of an audio system you have to keep your standard operating level as high as possible without risking distortion CLIPPING nt Distortion Occurs Above This Level The useful dynamic range of any piece of audio equipment 15 lmited by noise ard distortion ey Gain Riding One solution to the noise vs distortion trade off is to keep your hand on the level control and manually adjust gain to suit the program Indeed there are times when this approach is entirely satisfactory However in most types of music there are instantaneous short duration volume peaks or transients which would be difficult to anticipate and impossible to respond to with manual gain riding you simply could not bring the level down fast enough In many situations this can present real problems For example in recording an extra burst of enthusiasm from a lead singer might overload the capabilities of your recording tape causing ragged distortion and necessitating another take In sound reinforcement a sudden burst of energy through the system can blow fuses or even damage loudspeakers In addition to the problem of response time with manual gain riding it also requires your constant attention which takes you away from more important
4. the output control is in the out position Output Level Meter Display A unique feature of the CL 52E is an output level meter for each channel These peak responsive meters give a good indication of the actual output level up to and including clipping 20dBV The output level meters are functional at all times whether gain reduction is occurring or not Gain Reduction In Out Switches When either of these switches are in the OUT position the corresponding channel does not affect the level of the audio signal it acts as a simple linear amplifier The GAIN and OUTPUT LEVEL controls still function but no gain reduction will occur When either switch is pushed in LED illuminated gain reduction will occur for any input signal exceeding the threshold Stereo Tie Switch This switch replaces the STEREO TIE jacks on the CL 50E and links the two channels for accurate stereo tracking when pushed in LED illuminated The concept of stereo operation is sometimes confusing but it is less difficult to understand if you keep a few basic rules in mind When the switch is in 1 Gain reduction is identical for both channels Any gain reduction that occurs on channel 1 will also occur on channel 2 and vice versa 2 The TIE switch overrides an individual channel s IN OUT switch For example if Ch 1 is switched OUT but Ch 2 and the TIE switch are both IN then any limiting action which occurs on Ch 2 will also occur on Ch 1 even though Ch 1 i
5. volts or decibels Generally corresponds to the volume or intensity of an audio signal ATTACK TIME The amount of time that elapses before a compressor limiter begins to attenuate the output level after threshold has been reached BALANCED A 3 wire circuit arrangement in which two conductors are designated as signal lines and and the third is a shield and chassis ground The signal lines are of opposite polarity at any given moment and are of equal potential with respect to ground Balanced input amplifiers are used in all Ashly products to improve hum and noise rejection Jumpering signal minus to ground provides an unbalanced input BREATHING A usually undesirable fluctuation of background noise resulting from compressor action Also called Pumping CENTER FREQUENCY The frequency or pitch at which a filter is most effective In a parametric equalizer it refers to the frequency where a particular boost cut control has maximum effect COMPRESSOR An amplifier which reduces its gain as its input is increased beyond a predetermined threshold dB A unit by which audio levels can be COMPARED Often thoroughly misunderstood are the concepts that decibels represent the level of a signal compared to some reference level 15 dB cut means a certain level less than a previous level the absolute level of the signal need not be known and that decibels are a logarithmic unit Some handy number
6. 1 Creating a fatter kick drum or snare sound 2 Thickening acoustic guitars and electric pianos 3 Adding punch and sustain to electric bass 4 Lengthening the sustain of an electric guitar In general slow attack times fast release times and large ratios will work well for these techniques Experimentation is highly recommended Using The CL Series On Stage To use the Ashly compressor limiter with for example a guitar accompany it with an instrument preamp such as the Ashly BP 41 The compressor limiter is placed between the preamp and power amp or in the effects loop of the preamp e Send kd Return L INSTRUMENT PREAMP 666 The CL 50E inserted into the effects loop of a musical instrument preamplifier Voice Over Compression Ducking The CL SERIES can be used to automatically reduce music to a background level when an announcer is speaking In this scheme only the music signal is actually gain reduced by the Ashly limiter However the Detector is connected to respond to an announcer s voice instead of the music s peaks The music output and the announcer s voice are then combined in an external mix circuit A variable gain control on the announcer s voice level is helpful in matching the announcer signal level to the nominal 0 dBV threshold level of the detector The CL 50E as a Voice Over Compressor Stereo Operation Two or more Ashly mono compressor
7. AIN control A very significant difference in dynamic range is achieved simply by changing the relationship between nominal signal level and threshold as a result of varying the GAIN control The most interesting effect to be noted however is seen by comparing the original Input signal with the Output signal The quietest portions of the original signal will be effectively increased in volume while the volume of the loudest portions of the original l signal will be decreased In effect both ends of the dynamic spectrum will be pushed toward the middle This is quite different from simple limiting where only loud peaks are subjected to gain reduction More than anything else it is this double ended effect which distinguishes compression Remember in all of these examples the only control which was changed was the GAIN control The RATIO ATTACK TIME and RELEASE TIME were assumed to be identical in all cases The only factor which turned limiting into compression was a change in the relationship between input level and threshold Compression is further differentiated from limiting by careful selection of Attack and Release times When limiting is employed to protect an audio system against transient volume peaks and possible overload Attack time is usually set as fast as possible consistent with distortion free performance Release time would also be relatively short so that the output signal would be restored to normal as quickly a
8. ASHLY owner s manual CL 100 CL 5OE GL52E LIMITER COMPRESSORS ASHLY AUDIO INC TABLE OF CONTENTS BRP OCU CH ON essenza A If You Are In A Hurry Mechanical Installation sisaan OAE Gain Ratio Attack Time Release Time Output Level Bypass Switch Threshold Gain Reduction Indicators Initial Setup And Checkout TREOLY ccceeresareee and Limiters Do Compressor Limiter Circuitry The Ashly VCA Voltage Controlled Amplifier Detectors Applications ssesnenzznenzznznnna PIERSI 19 The Compressor Limiter asa Protective Device Alternatives Voice Over Compression Ducking Stereo Operation Gain Reduction Meter Display Output Level Meter Display In Case of Trouble ssenuzzznnnnnznennnnenanse 28 The Ashlv CL SERIES of Peak Compressor Limiters was designed in response to the need for a universal peak sensitive automatic gain control AGC device with audio performance comparable to that of a professional mixing console It took over five years of research to realize this goal with Ti much of that time spent in developing a wide bandwidth ultra low distortion low noise VCA voltage controlled amplifier The resulting product is a versatile and highly listenable compressor limiter suitable for use in professional sound reinforcement recording and broadcasting The Ashly CL 52E Stereo Peak Compressor Limiter was developed to meet the need for a stereo compressor limiter that
9. ATION The CL 50E and CL 52E Compressor Limiters mount in a standard 19 inch equipment rack The mounting screw threads vary with different rack manufacturers and you should refer to your rack instructions for proper hardware An oval head or flat head screw with a plastic countersink washer is preferred to protect the finish of the compressor limiter under the screw The CL 100 is housed in an industry standard half rack enclosure which provides a variety of mounting options It can be used free standing in a half rack sized enclosure mounted in a vertical 19 inch adaptor with up to eight other half rack products or side by side with another half rack product in a standard 19 inch rack Mounting accessories are available from Ashly to accomplish this Your Ashly compressor limiter is housed in a rugged steel case and will tolerate moderate abuse However for road systems which may be dropped or otherwise subjected to extreme forces we recommend some rear support for the chassis to prevent bending the front panel when these forces occur Q WIRING The CL 50E and CL 52E Compressor Limiter should be connected to a 3 wire grounded outlet supplying 120 Volts 50 60 Hz Power consumption is 12 watts The CL 100 uses a standard RAP 6 remote AC power supply The INPUT is a 10k ohm active balanced type on a standard stereo phone plug that will accept levels of up to 20 dBV The or in phase connection is on the tip and the or out of phase conn
10. N This indicator displays the gain control action of the compressor limiter Gain Reduction dB ___ 10 3 oo o Thr o Two important features emerged from this research 1 We designed the detector to let the attack and release times speed up as more and more limiting occurs The compression ratio also increases This lets us maintain peaks fairly close to a constant ceiling level but allows the illusion of increasing loudness as input level increases thereby preventing complete loss of dynamics when limiting 2 We incorporated a double release time constant When release time was set slow with a single time constant transients such as mic pops and record scratches caused a quick reduction in gain and a slow fade up making the action of the limiter very obvious With the double time constant release from gain reduction after a brief transient is always fast with a slower release after a sustained overdrive When choosing a compressor limiter you can see that it is very important to listen to it in your particular application and see that it sounds the way you want There are lots of these devices with seemingly excellent specs which sound very different with real program material applied to them Peak Or RMS There are several ways of looking at a signal to determine its level A peak detector looks at the maximum voltage a signal reaches regardless of its waveform while an RMS root mean square detector looks
11. a waveform producing a distorted sound The Ashly CL Series provides continuously variable attack times from 200 microseconds to 20 milliseconds Release Time Another parameter which affects compressor limiter performance is Release time or the time required for the limiter to restore system gain to normal after the input signal has fallen below threshold level Again proper Release time will depend on the type of program material being processed and the way in which the limiter is being used When subtle limiting is desired slow release times are often chosen to avoid a condition referred to as pumping or breathing This occurs when overall gain is modulated up and down by repeated peaks which are followed by quieter intervals If the release time is set too fast then the overall level will jump up and down producing an objectionable and unsettling effect Note that in some cases an individual track or channel which seems to be pumping may sound acceptable when heard in context of a complete mix A unique feature of the Ashly Compressor Limiters is the incorporation of a double release time constant When a conventional compressor limiter is adjusted for slow release times transients such as mic pops and record scratches may cause a severe reduction in gain followed by a slow fade up making the action of the limiter very obvious With the double time constant release from gain reduction after a brief transient is always
12. amic range What is it that compromises the dynamic range of this equipment The useful 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 Comparison of the dvnamic range of several common audio svstems At the other end of the loudness spectrum the limitation on dynamic range is usually distortion either in the form of amplifier overload or tape saturation In most transistorized equipment the transition from clean undistorted operation to severe distortion is very abrupt Therefore it is common practice to operate a piece of equipment at a level that is somewhat below the distortion point leaving a margin of safety for unexpected transient volume peaks in the music This safety margin is known as headroom and may range from 10 to 25 dB Lowering our standard operating level to
13. at the energy in a signal regardless of the short term voltage levels This makes a peak detector the correct choice for preventing clipping overmodulation or tape saturation while an RMS detector can be used to restrict material to a given loudness When an RMS limiter is used to prevent clipping the result is unpredictable For instance a flute and a snare drum which are limited to the same RMS level might have peak levels as much as 30 dB apart Use peak limiters to prevent clipping Q The applications of the Ashly CL SERIES Compressor Limiters can be divided into two basic categories it may be used as a protective device to prevent audio levels from overloading associated systems such as tape recorders amplifiers speakers or transmitters or it may be used to create special effects and unusual sounds for recording and musical performance These two different approaches to using the compressor limiter impose vastly different and contradictory demands on the unit s performance When used in a protective mode the unit is usually required to control the dynamic range of an incoming signal and to do so without audible side effects The listener should be unaware of the limiter s presence In the early 1960 s when musicians began looking at the recording process as a way to create new sounds the pumping effect which had been avoided like the plague by earlier engineers was suddenly seized upon and utilized as a creative tool laying the
14. ate the dB increase in input above threshold required to produce a 1 dB increase in output This can be expressed conveniently as a ratio If the output remains constant no matter how high the input level we have an infinite cc input output ratio It should be remembered that the Ratio control has no effect on signals which are below threshold since the CL SERIES products do not alter signals at below threshold levels There is a common but incorrect notion that limiting always implies the use of an infinite ratio Although there are times when an infinite ratio is desirable there will be situations where infinite or hard limiting action is neither appropriate nor necessary In fact it should be noted that an infinite ratio setting is likely to cause noticeable side effects in the sound and may not be usable on programs where subtle control is desired Attack Time The response of the compressor limiter to signal levels above threshold is further defined by the ATTACK TIME control Attack time is the amount of time that the unit takes to attenuate the output level after threshold has been reached For verv fast transients such as hand claps snare drums or other percussive sounds a very fast attack time is usually desirable so that the limiter can respond in time to control the peak level On other types of program material a slower attack time may be preferred A too fast attack may on some material square off the top of
15. bandwidth reattacking the frequency control and making the cut shallower if possible 5 As soon as the first feedback frequency has been removed the Ashly CL unit will automatically bring up system gain until another feedback point is induced Repeat the equalization procedure until it becomes impossible to distinguish individual predominant feedback frequencies 6 Return all mixer EQ overall gain and compressor limiter gain controls to normal settings Recording The Ashly limiter can be used to prevent saturation of magnetic recording tape and to control tape hiss In professional recording studios the saturation level of the tape system headroom and the output level of the console are all known quantities making the application of limiting and compression very easy The CL 50E used to prevent tape saturation and improve S N ratio An example of the use of limiting to prevent tape saturation and improve signal to noise performance was given in the earlier discussion of the RATIO control By shifting the input GAIN upward further compression and improved signal to noise can be achieved at the expense of dynamic range De Essing A special type of saturation problem often encountered in recording is the sibilant Ssss sound of the human voice High frequency sibilant sounds can reach very high energy levels so that a voice that is otherwise undistorted breaks up on the esses producing a raspy undesirable sound Wi
16. e THRESHOLD LED lighting up If not increase the GAIN control Do not expect to hear any effect when the input level to the CL 50E is below threshold since the unit is simply a linear amplifier at those levels WHEN USING HEAVY COMPRESSION BACKGROUND NOISE IS NOTICEABLE DURING QUIET SECTIONS OF THE PROGRAM As defined in the section on compression quiet program material is effectively made louder while loud peaks are made quieter When the program source is thus raised in volume its noise floor is also raised in volume by a proportionate amount This is not a defect in the compressor limiter but an unavoidable side effect of the gain altering process If the noise becomes a problem the solutions are to either clean up the program source or use less compression EXCESSIVE HUM OR NOISE Hum is often caused by a ground loop between components Try using the suggested balanced input and output hookups if the other pieces of equipment used in conjunction with the CL 50E have balanced inputs and outputs Noise can also be caused by insufficient drive levels Make sure you are sending a nominal 0 dBV line level signal to the unit Q ACTIVE Electronic circuits which use devices such as transistors and integrated circuits and which are capable of voltage and power gain as well as loss Circuits using only resistors capacitors transformers etc are referred to as passive AMPLITUDE The voltage level of a signal May be measured in
17. ection is on the ring To use the input as a common unbalanced type simply use a mono phone plug in the usual way The OUTPUT connection is a standard 1 4 phone jack and mates with a standard phone plug such as a Switchcraft 280 Output impedance is 200 ohms and full headroom is realized with any load of 600 ohms or greater For rack mounted unbalanced audio systems the output ground may be separated from the case ground by using a stereo phone plug for the output a connection The output ground is then wired to the ring of the stereo plugs rather than the sleeve In this manner ground loops in the rack may be eliminated This output can be fed to a balanced input by wiring the input to the tip the input to the ring and the shield to ground Stereo Tie This patch point is used to tie together two or more CL 50E s or CL 100 s for stereo or multi channel operation To connect place a patch cord between the STEREO TIE connections of both units If signal ground isolation is being used be sure to use a stereo patch cord NOTE If more than two units are to be tied together use a common Y type adaptor to split the signal as necessary The Stereo Tie point actually represents the DC output of the detector circuit When tied in parallel for stereo operation the VCA s of the two units will both respond to the most negative control voltage present at the Stereo Tie point thus insuring that the stereo image never shifts le
18. en if this type of transient doesn t destroy a speaker outright it may damage the speaker surround in such a way as to cause mechanical abrasion and future failure Alternatives For Sound System Installations To install the CL unit in a sound system using a passive crossover insert it between your mixing console output and the power amplifier input For systems using electronic crossovers there are two ways to use an Ashly limiter It may be inserted between the console output and the crossover input in which case it will act on the entire audio frequency spectrum Alternately if the limiter is inserted between an output of the electronic crossover and the input of a power amp it will only affect a specific band of frequencies The CL 50E in a biamplified system Compression For Feedback Control A common ritual in sound system set up is equalizing the room to remove feedback This is generally accomplished by turning up system gain to purposely induce feedback searching for the center frequency of the feedback and then equalizing at that frequency to remove the feedback Once this frequency has been cut system gain is again increased to induce another feedback point and the whole procedure is repeated until the engineer is satisfied that the significant problem frequencies have been corrected The major problem with this approach is that the feedback can easily get out of control and the engineer ends up dashing back and f
19. er level controls down 2 Switch the unit s LIMIT switch to the OUT position 3 Set both the GAIN and OUTPUT level controls to 0 dB 4 L 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 CL 50E and adjust the GAIN control so that the yellow THRESHOLD LED lights only on peaks 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 now 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 for Ashly crossovers this is 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 sound system just begins to sound a little distorted
20. fast with a slower release after a sustained overdrive Output Level Because compression and limiting are both gain reduction processes the output of a compression limiter is frequently at a lower level than the nominal system level To make up for this loss an OUTPUT level control is provided to restore up to 18 dB of system gain NOTE When the unit is in the BYPASS mode the OUTPUT control still functions Bypass Switch This switch enables you to quickly switch the CL compressor limiter in or out of the audio chain making comparisons between processed and unprocessed signals easy When the switch is in the OUT position all limiting and compression controls and functions are bypassed with the exception of the output control which continues to function as a straightforward level control For all normal compress and limit functions this switch should be depressed to the IN position Threshold Gain Reduction Indicators Five LED s on the CL 50E and CL 100 front panels provide a convenient visual indication of the amount of gain reduction that is taking place at any time As soon as the 0dBV threshold level is reached the yellow LED illuminates Depending on how far the input level rises above threshold and the settings of the RATIO ATTACK and RELEASE controls successive red LED s will illuminate indicating increasing gain reduction Gain reduction is a useful way of expressing compressor limiter action We have seen that the
21. ft or right The Stereo Tie points of CL 50E s and CL 100 s may be directly interconnected Detector Patch The CL 50E and CL 100 Compressor Limiters have a DETECTOR PATCH point which can be used in conjunction with an equalizer to produce discussed later Alternatives For Compressor Limiter Sound System Installations To install the CL unit in a sound system using a passive crossover insert it between your mixing console output and the power amplifier input For systems using electronic crossovers there are two ways to use an Ashly compressor limiter If it is inserted between the console output and the crossover input it will act on the entire audio frequency spectrum Alternately the compressor limiter may be inserted between an output of the electronic crossover and the input of a power amp in which case it will only affect a specific band of frequencies 0 CONTROLS AND INDICATORS Gain The gain control is used to match the level of the signal source to the fixed 0 dBV threshold of your Compressor Limiter It should be adjusted so the yellow threshold indicator illuminates only when the signal reaches the maximum level desired Then above this level the Compressor Limiter will tend to minimize further increases in level Ratio This control determines the ratio of change in output level to changes in input level for all signals above threshold The numbers printed around the Ratio control are calibrated in dB and indic
22. han each other Since they are all restricted with respect to maximum audio level modulation their best tactic is to squeeze the dynamic range of their programs to just a few dB The audio output level of the station virtually never changes and the listener perceives this continuous high level sound as being louder than the same material in an uncompressed form Although both compressed and uncompressed programs reach the same peak levels the compressed signal stays near peak level more of the time and thus appears louder This technique makes the broadcast more intelligible over the road noise in your car and increases the geographical area over which the broadcast is audible to the home listener A similar if less pronounced effect can be used in sound reinforcement and recording applications In general use a gentle compression RATIO say 4 1 with a 10ms ATTACK time 0 1 sec RELEASE time and enough GAIN to cause 6 to 10 dB of GAIN REDUCTION Try using this effect to help bring out a lead vocal or instrumental solo in a cluttered mix The compressor is also a great corrective tool when working with singers whose own dynamic control is less than adequate A little compression helps to keep their quieter lines from becoming buried in the mix Altering The Texture Of Musical Instruments It would be impossible to mention here all of the ways that compression is used to create new sounds with familiar instruments Some typical uses are
23. in to react If the source feeding the limiter is increased in overall level then more of the peaks will now be above OdBV those peaks which were previously just below threshold level will now be at or above threshold in addition to the volume peaks which had already been above threshold Thus varying the input level to the limiter is inversely proportional to changing the threshold level On the Ashly CL SERIES this effect is easily accomplished by changing the input GAIN control Increasing the GAIN control is equivalent to lowering the threshold point A gain control range of plus or minus 30 dB is provided What the limiter does once a peak above threshold is detected is determined by the RATIO ATTACK TIME and RELEASE TIME controls Compression There is a good deal of disagreement and confusion in the audio industry over the definitions of limiting and compression and the terms are frequently and incorrectly swapped back and forth The fact is compression and limiting are closely related effects and the differences between them are largely a matter of degree Indeed it could be said that the difference between compression and limiting is more often than not a matter of what you intend the device to accomplish Returning to our original definition of limiting we stated that gain reduction occurred when input signals rose above a predetermined threshold level of 0 dBV and that the effective threshold could be varied by changing the G
24. increase in output level of a compressor limiter is less than the increase in input level by some amount Using a limit ratio output level will remain nearly constant as input levels increase above threshold With a gentle ratio say 2 1 input signals above threshold will be gain reduced at the output by exactly 1 2 Thus gain reduction can be expressed as input level increase divided by output level increase For example a 12 dBV input signal that is 3 1 compressed will produce 4 dB of change at the output and 8 dB of gain reduction has occurred 12 dB input minus 4 dB output 8 dB gain reduction The 6 dB red LED on the CL 50E or CL 100 front panel would be seen to light up because we are past 6dB reduction but not yet to 10dB Q 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 dBV level of 778 Volts Also note that input sensitivities of power amplifiers also vary 1 Begin by turning the crossov
25. istortion and noise are very low Unfortunately the repsonse time of photocells is slow and unpredictable so their use in a fast peak limiter is really not feasible Also the matching between units is very poor so that stereo tracking is not possible without tedious hand matching of photocells Another approach uses a field effect transistor FET as a variable resistor Here at least the response time is fast in the nanosecond range but matching between units is still poor requiring hand matching for stereo An additional problem is that a FET will only act as a pure resistor with very small signals applied so it is necessary to attenuate an input signal before the gain control FET and then amplify it again Of course this results in less than ideal noise performance and imposes a frustrating tradeoff less noise more distortion A number of VCA s based on the exponential voltage current characteristic of a bipolar junction transistor have been used One of the most common is called a transconductance amplifier Using the inherent matching obtained by integrated circuit technology these devices have very predictable control characteristics Tracking within 1 dB over a 40 dB range is common Not only do the control characteristics match well from unit to unit but they can easily be made exponential logarithmic so that even increments of control voltage produce even increments of gain change in decibels The response time is also ve
26. jobs The need for a fast acting reliable automatic gain control is answered by limiters and compressors What Compressors and Limiters Do Limiting and Compression are closely related effects their differences sometimes being very subtle Since limiting is generally an easier concept to deal with than compression the following sections will talk about limiting first followed by a discussion of compression The use of the front panel controls is discussed in terms of limiting also Bear in mind that these controls will have the same function when the CL SERIES unit is used as a compressor Limiti In any musical program there are constant changes in loudness It is the jobofa limiter to detect when the volume has exceeded a predetermined level and to then turn down the volume When the incoming signal returns to its original safe level the limiter should respond by restoring the gain to normal Thus when the level is within a specified safe range the limiter has no effect When an occasional peak occurs the limiter responds This situation is completely analogous to manual gain riding except that it occurs faster and more consistently To determine which peaks are acceptable and which are too loud the limiter establishes a reference level known as the threshold and then responds to those peaks which exceed this point In the Ashly CL SERIES this threshold is fixed at OdBV When the input level reaches 0dBV the limiter will beg
27. l scale the higher its frequency FREQUENCY RESPONSE Refers to relative gain and loss at various frequencies across the audio band May be illustrated by a graph called a frequency response plot usually graphing decibels vs hertz or octaves GAIN REDUCTION The amount expressed in dB by which a compressor limiter soutput has been reduced ia fost with respect to its uncompressed vel HEADROOM Refers to the increase in level above normal operating level that can be obtained without clipping Usually expressed in dB IMPEDANCE Essentially the AC equivalent of resistance It describes the drive capability of an output or the amount of drive required for an input at any given signal level KHz Kilohertz 1 000 Hertz LEVEL The magnitude of a signal expressed in decibels or volts LINE LEVEL Meaning somewhere around OdBV as opposed to MIC level of around 40dBV LIMITER An amplifier which reduces its gain as its input is increased beyond a predetermined threshold Usually used to protect audio systems against sudden high level signals and possible overload OCTAVE A logarithmic unit to compare frequencies 1 Octave means double frequency 1 Octave means half frequency OHM The unit of electrical resistance or impedance PREAMPLIFIER The first stage of amplification designed to boost very low level signals to line level RATIO The relationship between change in input level and resultan
28. limiters may be tied together to provide accurate stereo limiting and compression This is simply a matter of inserting a single patch cord between the STEREO TIE points of the two units The GAIN REDUCTION LED s will still read independently with each indicator corresponding to half of the total gain reduction action The actual gain change is always identical for each limiter and is determined by the louder channel In this way there is never any shifting of the stereo perspective In permanent rack mount installations the Stereo Tie points of two CL 50E s or CL 100 s may be brought out to a convenient front panel switch SPST inserted into the hot line between the two patch points This switch will then select either stereo or independent tracking of the two units Q SPECIAL NOTES ON THE CL 52E STEREO COMPRESSOR LIMITER The CL 52E Compressor Limiter is closely related to the model CL 50E model This section covers the minor differences and new features of the CL 52E Gain Reduction Meter Display The CL 52E features a threshold gain reduction display which is very similar to that used on the CL 50E but with an expanded scale for greater resolution Each channel has its own display and they operate independently except when the unit is in its stereo tie mode The gain reduction display will not be illuminated when 1 The input signal is below the threshold and no limiting is taking place 2 The limit in out switch near
29. maintains correct stereo imaging at all times The CL 100 is the half rack version of the CL 50E The greater part of the text and illustrations in this manual therefore refers to the CL 50E Special features of the CL 52E are explained separately We ask that you please read this instruction manual thoroughly before operation so that you may realize all the features and benefits that the Ashly Compressor Limiter has to offer Q IF YOU ARE IN A HURRY We will assume that you are familiar with compressor limiters in general Please keep the following points in mind 1 For safety your CL 50E or CL 52E should be connected to a standard ground AC outlet supplying 120 volts 60Hz The CL 100 uses standard remote RAP 6 power pack This should also be connected to a grounded 120 volt 60Hz outlet 2 The inputs and outputs of Ashly compressor limiters can be used as either balanced or unbalanced depending on the connector For unbalanced connections use a standard mono Phone plug For connections use a stereo phone plug with the signal on the tip the signal on the ring and the ground on the sleeve 3 For a good starting point set the input and output levels at 0 and T the ratio attack and release controls at mid rotation Apply signal and adjust from there to suit your application 4 If you have problems please refer to the more detailed sections and to the troubleshooting guide in this manual Q MECHANICAL INSTALL
30. nly provides it with a control voltage Experience showed us that both are crucial to the overall sound and that if anything the detectors performance is the harder to judge by conventional measurement techniques While the VCA is doing its job if it has low noise and distortion the detector must constantly adjust the gain of the audio path in a manner which keeps the level under control while sounding acceptable to the listener This constantly changing gain is a DYNAMIC action and conventional audio measurements like noise and distortion checks are STATIC at a constant level We became painfully aware of this problem with some of our earlier limiter prototypes which measured fine and sounded terrible This led us to use a purely subjective approach in the design of the detector we did a lot of listening to determine what sounded good and what didn t The Ashiv CL 50E Compressor Limiter GAIN Adjusts the input sensitivitv so gain control action starts at the proper signal level 393 e s E oux e in p 30 dB J ATTACK Adjusts the speed at which the gain is reduced in response to a signal above the threshold level IN OUT This switch enables or defeats the action of the compressor limiter RATIO Adjusts the rigidity of the ceiling placed on the signal leval J RELEASE Adjusts the speed at which the gain is increased as the signal falls below the threshold level GAIN REDUCTIO
31. orth between the mixer volume controls and the equalizer controls while everyone in the room plugs their ears and prays it will end soon Products in the Ashly CL SERIES can turn this procedure into a fast painless job eliminating loud feedback levels and the possibility of speaker or ear damage Procedure 1 Set up the CL unit s controls as follows a OUTPUT level control to 20 dB 6 Input GAIN control to maximum c RATIO control to infinity 09 d ATTACK time to 5 ms e RELEASE time to 1 sec f LIMIT IN OUT switch IN 2 Adjust equalizer controls to a flat setting and if the equalizer has an overall volume control boost it by 10 to 15 dB 3 Open up several microphone input channels to a normal operating level with typical EQ settings and turn the console master fader up to a louder than normal setting At this point the system should be well into feedback but the room volume will remain constant due to the action of the limiter You can listen to the feedback at any level you like by simply varying the CL 50E s OUTPUT volume control although below a certain monitoring level the feedback will stop 4 Try to determine the feedback frequency and then equalize it by adjusting the center frequency bandwidth and boost cut controls of your parametric equalizer Note a graphic equalizer can also be used with less accuracy After eliminating the problem frequency try to further define it by sharpening up the
32. ry fast The problem with simple transconductance amplifiers is that like FET VCA s they can handle only very small signals so the noise performance is poor A number of linearizing circuits have been devised to minimize this problem but even the best transconductance amplifiers have an equivalent input noise of about 80dBV which compares poorly to straight linear amplifiers The best compromise to date is the class AB current ratio multiplier Early implementations of this circuit used two matched pairs of transistors one pair of NPN s and one pair of PNP s The problem here is that excellent matched integrated NPN pairs are available but integrated PNP s are not The PNP s must be hand tested and matched Careful trimming is necessary for low distortion and even minor temperature changes make re trimming necessary because of differing characteristics between the two types The Ashly VCA Voltage Controlled Amplifier The Ashly VCA is an integrated current ratio multiplier circuit It has low noise 90 dBV low distortion 05 excellent response time and tracking and does not suffer from thermal drift The noise and distortion are at state of the art levels and the circuit is consistent in mass production with minimal trimming and no hand selection of transistors Detectors It would seem that of the two components in a compressor limiter the VCA is the more critical since the audio passes through it and the detector o
33. s possible after the transient Compression is frequently used to keep overall signal level within a specific dynamic range and for this application slower Attack and Release times are usually chosen This approach is analogous to our manual gain riding example where our operator is fading the music up and down to keep it fairly constant but is doing it slowly enough so that the listener is unaware that the gain is being altered It should now be clear that the Ashly CL 50E CL 100 and CL52 can function as either limiters or compressors due to the wide range of control parameters which are available to the user Compressor Limiter Circuitry Volt Controlled ifiers Early VCA s were based on vacuum tubes with a remote cutoff characteristic The tube would simply change its gain in response to a changing bias voltage Tubes developed for this purpose did an excellent job in fact they could exceed the noise and distortion performance of today s best solid state VCA s Unfortunately they also had some serious disadvantages peculiar to tubes change of gain and matching as aging took place heat microphonics high cost and the need for both high voltage and filament power supplies Over the vears the need for a good low cost solid state VCA brought about many innovative approaches A good example is the electro optical attenuator where a photocell is used as one leg of a potentiometer Since the photocell behaves as a true resistor d
34. s switched OUT The Ch 1 GAIN REDUCTION meter will verify that this is true 3 When both channels are switched IN and the STEREO TIE switch is IN either channel can cause gain reduction on both channels In other words whichever channel exceeds the threshold first will cause an equal gain reduction on both channels 4 When in the STEREO TIE mode the attack and decay characteristics of both channels will be determined by whichever channel actually caused the gain reduction If both channels simultaneously receive an above threshold signal then the attack time will be determined by the channel with the fastest ATTACK TIME setting and the release time will be determined by the channel with the slowest RELEASE TIME setting 5 If you want to deliberately make one channel a slave to the other simply switch OUT the channel that is to be the slave switch IN the master channel and switch IN the STEREO TIE Q IN CASE OF TROUBLE NOTE Unshielded cables improperly wired connectors and cable with broken strands of wire rattling around are very common problems Use good quality cables with good quality correctly wired connectors NO OUTPUT Check AC Power Is the power switch on Check input and output connections are they reversed Are you sure you have an input signal CONTROLS HAVE NO EFFECT Is the LIMIT IN OUT switch IN Perhaps the RATIO control is set too low to produce an audible effect or the input level is below threshold Is th
35. s to remember when dealing with decibels 3 dB Double Power 6 dB Double Amplitude Quadruple 6 dB Power 10 dB 10X Power 20 dB 10X Amplitude 100X Power dBm A unit of measurement in decibels where 0 dBm a power level of 1 milliwatt into a 600 ohm load Originally defined by the telephone company to measure line levels dBV Decibel Volts an update of the dBm definition where 0 dBV the same voltage level as 0 dBm but with no regard to power or impedance 0 dBV 0 778 Volts This unit is much more appropriate for modern audio equipment with high impedance inputs and low impedance outputs DISTORTION Generally refers to ANY modification of an audio signal which produces new frequencies which were not in the original Examples are harmonic distortion where a circuit adds overtones to a fundamental signal and intermodulation or IM distortion where two frequencies beat together to produce sum and difference frequencies EQUALIZATION Modification of the frequency response of an audio system for either corrective or enhancement purposes FEEDBACK Generally refers to any process where an output is in some form routed back to an input to establish a loop Negative feedback tends to be be self stabilizing while positive feedback causes instability FREQUENCY The repetition rate of a waveform Frequency is measured in Hertz One cycle per second cps is one Hertz Hz The higher a note ona musica
36. t change in output level RELEASE TIME The time required for a compressor limiter to restore system gain to normal after the input signal has fallen below threshold SATURATION The point at which the magnetic storage capability of a piece of recording tape is exceeded SIBILANCE The distortion caused by loud high frequency signals such as the Ssss soundsin human speech THRESHOLD The level above which a compressor limiter begins to reduce gain TRANSIENT A sudden burst of energy in an audio signal such as a breath blast in a microphone the sound of a snare drum or a deep scratch in a record Transients frequently reach peak levels of 10 to 30dB above standard operating level and may cause distortion oreven damage to equipment UNITY GAIN Output level Input level We thank you for your expression of confidence in Ashly products The unit you have purchased is protected by a five year warranty To establish the warranty be sure to fill out and mail the warranty card attached to your product Fill out the information below for your records
37. th the current trend toward crisp bright equalization of vocal tracks the problem is magnified Add to that the inherent tendency of magnetic tape to saturate earlier at high frequencies and the internal high frequency boost record pre emphasis of a standard tape recorder and the need to control sibilants becomes apparent The solution is frequency dependent limiting which is easily accomplished with the CL SERIES By inserting an equalizer into the Detector Patch points and boosting the equalizer at high frequencies in the vicinity of the sibilant the limiter s detector circuit becomes more sensitive to this particular range of frequencies and so will limit the bothersome sibilants more than other frequencies Realize that this technique is very different from simple equalization Equalizing a sibilant vocal by cutting high frequencies would result in a loss of important high frequency information at all times whereas de essing has no effect whatsoever on the signal except at the instant of the sibilant At that moment the Ashly limiter will reduce overall gain Frequency response is unaffected and the sibilant is controlled De Essing with the CL 50E The Compressor Limiter As A Special Effects Device Loudness Enhancement Compression has long been used as a tool to make an audio signal appear louder A good example is in broadcasting where competing stations with identical transmitters and power attempt to sound louder t
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