Technical Notes Volume 1, Number 27 Signal
Contents
1. produce surprisingly large quantities of low frequency energy Compensation for this phenomenon has been factored into DSP templates provided by JBL Professional for use with VT4889 arrays File Naming Convention for VT4889 DSP Templates We have discussed the primary mechanical array segment types We have also looked at the importance of understanding low frequency directivity in systems of this type due to the line array summation effect JBL Professional has developed a suite of DSP templates specifically designed to provide system users with the necessary tools for achieving optimum system performance under varying array setup conditions Each DSP template consists of an 8 character alphanumeric name Knowing the naming convention is key to correct selection of DSP templates for use with various VT4889 arrays The eight characters in each DSP template name designate size shape and signal processing drive configuration of the array that file has been created for The first two numeric characters are reserved for the number of boxes in an array This ranges from 4 to 18 The next three alphabetical characters describe the shape of the array These described shapes follow the primary mechanical array types previously discussed The next characters 6 7 describe the number of signal processors used for a given array and where in the array top or bottom the processor is assigned if multiple processors spli2. JBL PROFESSIONAL Technical Notes Volume 1 Number 27 Signal Processing Guidelines for VT4889 Introduction VT4889 line array elements can be used in a wide variety of ways offering system users a very flexible mechanical coupling and suspension system This flexibility while providing the system user with many array setup options requires careful selection of DSP Digital Signal Processing system drive files to achieve optimum acoustical results This document describes various array shapes that will be encountered during system use the low frequency characteristics of these arrays and considerations that must be taken into account when using JBL Professionals factory recommended DSP templates on supported system controllers To achieve the best results when using VT4889 s in arrays knowledgeable system users will rely on careful system gain stage calibrations and a thorough VERTEC Line Array Systems understanding of the chosen Digital System Controller and its user configurable operating parameters The VERTEC Line Array Calculator software application and practical experience gained from using the system will increase confidence The number of VT4889 s used in an array and the splay angles chosen between adjacent array elements will determine which DSP template should be selected for use in the system controller s Characteristics of Digital System Controllers used with VT4889 Lin
3. OX CONSTANT FAR CONSTANT NEAR 2 DSP BOTTOM 4 BOXES 12 BOX CONSTANT FAR CONSTANT NEAR PROGRESSIVE 2 DSP TOP 4 BOXES 12 BOX CONSTANT FAR CONSTANT NEAR PROGRESSIVE 2 DSP BOTTOM 8 BOXES 12 BOX CONSTANT FAR PROGRESSIVE 1 DSP 18 BOX CONSTANT FAR PROGRESSIVE 1 DSP S Straight Array 0 1 degrees C Constant Curvature Near Coverage 7 degrees or greater F Constant Curvature Far Coverage 2 6 degrees P Progressive Spiral e g 1 2 3 4 5 degrees 12 PROFESSIONAL H A Harman International Company TN VOL 1NO27 9 01 JBL Professional 8500 Balboa Boulevard P O Box 2200 Northridge California 91329 U S A
4. SP template than a group of constant curvature boxes set for far coverage 2 7 Because of the overall system design including the VT4889 s unique rigging system that allows splay angles greater than 5 users have the ability to set up a broad range of array designs To fully take advantage of the design features of the VT4889 system JBL recommends that users employ split processing in complex non constant curvature arrays larger than 8 boxes or those having two different families of baffle splay angles J ARRAY Split array processing may be advisable Type Variations CONSTANT CURVATURE ARRAY NO OF BOXES IN ARRAY 4 5 6 7 8 9 10 11 12 13 14 15 16 1 Size Variations Realizing that split processing is often not possible or desirable JBL has provided single processor settings for 8 12 and 18 box arrays Considerations When Ground Stacking VT4889 Arrays DSP Templates that are supplied also work well with ground stacked arrays However boundary surface effects should be taken into account When ground stacking a 4 box array for example users may benefit from selecting the next larger DSP template 8 box array files due to the reflected groundplane image at longer wavelengths Technical Note Summary The information contained herein has been compiled with the benefit of measurements taken of a wide variety of VERTEC arrays set up under cont
5. e Arrays It is useful to understand the characteristics of contemporary digital system controllers and how the design parameters vary between different models This issue affects the acoustical performance of any multi way sound reinforcement system including VT4889 line arrays Line array systems require careful setup and adjustment of system drive files to achieve satisfactory coverage and tonal quality The VT4889 s enclosure geometry high density of sound radiating components and complex array summation effects which vary with different mechanical array setups require that close attention be given to the correct selection and implementation of JBL s recommended DSP templates An important pre requisite for understanding the concepts discussed here will include careful reading of the following documents e VT4889 System User Manual e Vertical Technology White Paper The appropriate Digital System Controller manual will familiarize the system user with both general operating features and with user configurable options In the VT4889 System User Manual carefully read Chapter 5 Digital Signal Processing The VT4889 System User Manual provides important information related to overall system and array setup Prior to using these arrays the system user should understand how the chosen digital signal processors are set up to properly match the input sensitivities of the chosen power amplif
6. ering has provided DSP templates for various popular supported processing platforms This allows system operators to confidently work with any of the supported DSP platforms while maintaining correct electro acoustical performance from the system Explanation of Mechanical Array Configurations The directional factor of an array of identical sources is the product of the directional factor of an array with identical geometry but simple sources and the directional factor of a single element of the array Kinsler and Frey 3 edition Therefore a line array system made of identical elements all enclosures with the same coverage characteristics will offer certain benefits In addition to offering the advantages of a single enclosure inventory the VERTEC VT4889 system with its flexible coupling and suspension system allows the creation of a wide variety of array types The wide range of vertical baffle splay angles from 0 to 10 afforded by the VT4889 s flexible rigging system enables the construction of arrays from a single type of enclosure that can be classified into several basic types These configurations can be viewed as building blocks when building large or complex arrays The four primary array shapes include l Straight Arrays Arrays or array segments in which the splay angles between boxes are 1 or less 0 Il Constant Curvature Far Coverage Arrays or array segment
7. ge information for potential array sizes and shapes intended to cover a specific audience area There is a particular sequence of events that when followed will prepare the system user to successfully evaluate anticipated coverage characteristics Once array size and shape has been selected system users will be equipped with knowledge that can be reliably applied to selection of correct DSP templates to use as guidelines with chosen array formats The appropriate steps to follow include l Use the Line Array Calculator software to determine an acceptable array size and shape for the venue based on seating area geometry Il Match the shape of the chosen array with the DSP template that most closely matches the array IIl Perform a preliminary listening evaluation of audience coverage and system tonality to validate DSP template selection and system bandpass levels IV With the selected DSP template as a starting point the system user can now begin to further adjust bandpass levels and system program e q to suit taste To preserve system predictability the user is cautioned not to alter internal DSP parameters Considerations for Amplitude Shading and Split Processing of VT4889 Line Arrays The DSP templates include accommodations for split processing of arrays In some instances this means system users may energize different parts of the system for example the upper 8 boxes and the lower 4 b
8. iers and how the limiter section of the controller is adjusted to properly protect system components Initial System Drive Path Setup Before selecting DSP templates for use with specific VT4889 arrays it is assumed that the system operator will verify that the signal drive path through the system controller and power amplifier channels is correctly set up An appropriate way to do this includes the following sequence of activities 1 Amp Gain Calibration Be sure that the chosen system controller s bandpass gain settings are correct based on the specific power amplifiers that will be used with the system JBL s factory supplied DSP templates have been created with the assumption that all power amplifier channels used in the system have identical gains and input sensitivities 2 Limiter Settings Prior to using the system controller to drive the loudspeaker system properly set output bandpass limiter thresholds 3 Verify Controller Signal Paths Check all inputs outputs for all channels and ensure that each band s limiters are engaging properly Connect Controller to Amps Complete connection of the system s drive electronics to the power amps and visually check each channel for signal present overload indication and limiter threshold levels without speakers connected Low Level Test Connecting VT4889 arrays to the power amps supply low level audio program signals for testing pu
9. oxes in the same 12 box array using more than one processor However the issue of amplitude shading should first be understood and considered prior to implementing a more complex system drive scheme When working with large arrays or speaker arrays that cover distinctly different seating planes it is often necessary to use amplitude shading to ensure uniform sound pressure level throughout the audience area Simple amplitude shading can be done by turning specific amplifier channels down to taper the sound level of an array It is also possible to change the frequency response of the array by turning down specific bandpass amplifier channels down in the front rows An example of this is turning the high frequency amplifiers of the bottom enclosures down to balance the sound levels A more advanced method of frequency and amplitude shading involves using split processing Split processing is defined as using more than one DSP system processor to control the amplitude and frequency response of an array The use of split processing amplitude or frequency shading of 10 any system enables more even coverage of the audience area One specific reason for utilizing split processing with VT4889 arrays is to be able to use more than one family of vertical splays within an array JBL Engineering research has shown that for optimum results adjacent boxes set at straight splay angles 0 1 will require a different D
10. reness of how low frequencies behave differently in arrays of varying lengths that may be used in different setups There is a direct correlation between the height of the array and the wavelength of the frequency under consideration For example as shown in the follow ing chart an 8 box array has a height of 13 feet 3 96 meters At 87Hz this array will have a directivity index of 3 5 dB When that array size doubles to 16 boxes a height of 26 feet or 7 92 meters the larger array will have the same directivity at 43 Hz than the smaller array had at 87 Hz of Array 4 2 31 2 Boxes Height 4 6 5 43 85 170 227 Hz Hz Hz Hz 6 9 8 28 57 113 170 8 13 0 22 43 87 131 10 16 3 17 35 69 104 12 19 5 15 29 58 87 14 22 8 13 25 50 87 16 26 0 11 22 43 65 18 29 3 10 19 39 59 Low Frequency Array Dimensions Where A wavelength What this means for the system user is that in typical field conditions the power response will be the inverse of the directivity For flat frequency response on axis the power response will be the inverse of the low frequency array s directivity factor Directivity increases monotonically with increasing frequency it decreases monotonically with decreasing frequency Simply stated the longer the array the greater directivity will be at lower frequencies Because of the line array summation effect longer arrays can
11. rolled conditions and actual use field observations by JBL Applications Engineers Input has also been incorporated from sound system operators who have used JBL VERTEC line array systems under a variety of conditions for musical concerts playback events public address and other sound reinforcement applications for various special events This information will be most useful to VT4889 line array system users who become thoroughly familiar with initial system setup procedures and who take care to calibrate their overall system signal drive paths in accordance with User Manual instructions Upon ensuring that electronic system drive configurations fully match all factory recommendations system operators may then choose to carefully optimize the system bandpass level parameters and program equalization of their VERTEC system to match the unique needs of varying musical program styles specific venue setups and regional tastes The factory pre programmed controller settings for VT4889 system array configurations found on VERTEC System User Information CDROM s dated from September 2001 should be used as the default settings when these systems are set up and deployed Any modifications or additional equalization should be evaluated by system users through the use of critical listening skills in accordance with professional audio industry best practices and particular customer preferences System users will al
12. rposes Confirm proper signal present in all components High Level Test Verify proper system operation under full load conditions only after above steps have been completed Custom Configurations With growing familiarity users can program additional custom memory settings in the system controller s for different VT4889 Line Array system applications using the factory recommended templates as the baseline reference Certain parameters such as crossover points and bandpass signal delays must never be altered by the system user if optimum system performance and coverage characteristics are to be maintained Controller Filter Topologies And Their Effect on JBL s DSP Templates for VT4889 Arrays Digital signal processors designed for use with multi way sound reinforcement systems can have different characteristics which affect the audio signal throughput Differences are typically centered around group delay latency and user selectable delay settings Filter topology frequency and slope directly influence group delay Due to these facts it is seldom possible to merely match the specific settings on one brand of controller with another by entering identical crossover delay settings An electrical transfer function must typically be mapped for each specific parameter crossover points filter slopes bandpass delay etc and then compared to the reference setting s For these reasons JBL Engine
13. s in which the splay angles between boxes are constant and from 2 to 6 Example Constant Curvature Array Far Coverage 4 boxes set at 5 baffle splay angles IIl Constant Curvature Near Coverage Arrays or array segments in which the splay angles between boxes are constant and 7 or greater Example Constant Curvature Array Near Coverage 4 boxes set at 9 baffle splay angles IV Progressive Spiral Arrays or array segments in which the splay angles between boxes change incrementally e g 1 2 3 or 2 4 6 etc Example Progressive Array 8 boxes set to baffle splay angles of 1 2 3 4 5 6 7 8 There is also a more complex array type For example a Constant Curvature Far array segment and a Progressive Spiral array segment will often be combined into a single array This results in a hybrid mechanical format we shall call the J Array V J Shape Arrays made of a combination of two or more of the primary array types I IV Low Frequency Characteristics of Line Arrays The Correlation Of Frequency and Array Length With all line array systems the directivity at any given frequency will be proportional to the length of the array The array beamwidth will be inversely proportional to the product of the square of the array s length and the frequency of interest It is particularly important for system users to have an awa
14. so benefit from the correct application of software test and measurement tools like SmaartLive For additional technical information and helpful tips it is suggested that system users investigate the VERTEC section of the JBL Pro Website accessible electronically over the World Wide Web at lt www jbipro com gt K kk 11 VT4889 DSP Templates August 2001 Source Code B dbx BSS XTA DSP Filename FILE DESCRIPTION 4S1B 4F1B 4C1B 4P1B 8S1B 8F1B 8C1B 8P1B 8S2BT 8S2BB 8F2BT 8F2BB 8C2BT 8C2BB 8P2BT 8P2BB 12S2BT 12S2BB 12SFP2BT 12SFP2BB 12FC2BT 12FC2BB 12FCP2BT 12FCP2BB 12FP1B 18FP1B Legend 4 BOX STRAIGHT 1 DSP 4 BOX CONSTANT FAR 1 DSP 4 BOX CONSTANT NEAR 1 DSP 4 BOX PROGRESSIVE 1 DSP 8 BOX STRAIGHT 1 DSP 8 BOX CONSTANT FAR 1 DSP 8 BOX CONSTANT NEAR 1 DSP 8 BOX PROGRESSIVE 1 DSP 8 BOX STRAIGHT 2 DSP TOP 4 BOXES 8 BOX STRAIGHT 2 DSP BOTTOM 4 BOXES 8 BOX CONSTANT FAR 2 DSP TOP 4 BOXES 8 BOX CONSTANT FAR 2 DSP BOTTOM 4 BOXES 8 BOX CONSTANT NEAR 2 DSP TOP 4 BOXES 8 BOX CONSTANT NEAR 2 DSP BOTTOM 4 BOXES 8 BOX PROGRESSIVE 2 DSP TOP 4 BOXES 8 BOX PROGRESSIVE 2 DSP BOTTOM 4 BOXES 12 BOX STRAIGHT 2 DSP TOP 4 8 BOXES 12 BOX STRAIGHT 2 DSP BOTTOM REMAINING BOXES 12 BOX STRAIGHT CONSTANT FAR PROGRESSIVE 2 DSP TOP 8 BOXES 12 BOX STRAIGHT CONSTANT FAR PROGRESSIVE 2 DSP BOTTOM 4 BOXES 12 BOX CONSTANT FAR CONSTANT NEAR 2 DSP TOP 8 BOXES 12 B
15. t processing are used For example a DSP template with the name XXXXX2XT describes 2 processors used for a single array top and bottom split and that this DSP template is for use on the top T portion of the array The seventh character at the end of the file name B for instance refers to the specific batch file code that has been developed from rigorous array measurement sessions under controlled conditions Following is the VT4889 file naming format Format for VT4889 DSP file naming The first five places describe the array PEE Number of Boxes in Array Top or Second Bottom Number of Source T Top Overall Array Array Process Code for B Bottom Array Shape Shape Channels DSP File If Split Shape If Used If Used Processed Legend Array Types Shapes Straight Array 0 1 degrees Constant Curvature Near Coverage 7 degrees or greater Constant Curvature Far Coverage 2 6 degrees Progressive Spiral e g 1 2 3 4 5 degrees As can be seen in the above sample the name of each DSP template includes a specific array characterization a guide as to whether it is best used as a split processed array a marker noting which part of a split processed array this DSP template is meant for and an archival note detailing what source code based on rigorous array measurement sessions was used to develop this DSP template The suite of DSP templates for VT4889 arrays includes a variet
16. y of files for arrays of different sizes shapes and types A total of 26 separate DSP templates are available on the VERTEC CDROM Version 2 0 September 2001 VT4889 System User Information It is prudent to carefully study the master list of templates provided as a spreadsheet at the end of this document Selecting the Correct DSP Template We have discussed various array types and the suite of DSP templates for arrays of different sizes and shapes Each DSP template serves as a general guideline for the different array setups that will be encountered A DSP template in the family of files is a starting point it is not intended to replace final system optimization or creative program equalization It should also be noted that a thorough examination of the DSP templates would show that files for general array sizes are provided For instance files are provided for array sizes of 4 8 12 or 18 VT4889 array elements A DSP template with the 4 prefix is not intended for use only with 4 box arrays It is applicable to arrays or array segments that range from 4 to 6 boxes A DSP template with the 8 prefix is applicable to arrays or array segments that range from 7 to 10 boxes A DSP template with the 12 prefix will serve arrays or array segments ranging in size from 11 to 16 When preparing to select DSP templates for use with a VT4889 array the system user will first want to review predictive covera
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