Digital crossover design example using minidsp and Leap VCL Eidolon

Transcription

1 Digital crossover design example using minidsp and Leap VCL Eidolon Date: 5 th of November 2016 Author: Paul Vancluysen Version: final Introduction An example of a digital crossover design using minidsp is presented in this document. After the successful redesign by us of an Usher BE20 speaker with a digital Xover using the minidsp 4x10HD with very good listening results, we got the idea to design also a minidsp digital Xover for the VCL Eidolon. A digital version of the Linkwitz-Riley 4 th order crossover is designed. The VCL Eidolon design is published earlier on our website. The design of the VCL Eidolon was completely done in Leap EnclosureShop. So the SPL curves of all the transducers in the enclosure are already available for the crossover design. For other speaker designs the individual transducer in the enclosure responses have to be acoustically measured or simulated. Design The digital crossover design can completely be done in Leap CrossoverShop. The transducer in enclosure response curves are imported and the Linkwitz Riley filter targets are generated. Using the functional analog blocks, the crossover filter can be designed. The transfer of these functional blocks are bilinear transformed to coëfficiënts for a digital Xover built with biquads. At first a basic crossover filter with functional blocks is presented with already a good SPL response of the total speaker with that crossover filter. Then a second more enhanced crossover filter with functional blocks is designed to improve the overall SPL response. Small imperfections of the basic SPL response can be improved by adding local frequency response corrections. All corrections are checked off axis to maintain a good polar response. As a general rule transducer related imperfections can be corrected. Enclosure diffractions mostly cannot be corrected because their SPL is angle dependent off axis. In minidsp the transfer curves of the crossover filter can be programmed with biquads. All the coëfficiënts of these biquads and other settings can be imported in the minidsp as a configuration file. To define this configuration the biquad coefficients, calculated in Leap EnclosureShop, are used. For this design example of the VCL Eidolon the configuration file MiniDSP-2x8_VCL Eidolon LR4 V27.xml is created and available to use for free. 1

2 Contents 1 Import in Leap CrossoverShop of the SPL responses of the transducers in the enclosure Creating the crossover filter targets in Leap CrossoverShop Design of the speaker crossover filter in Leap CrossoverShop Acoustic centers of the transducers Basic Crossover filter with functional blocks Basic schematic with functional blocks Voltage transfer of the individual crossover filters On axis SPL and phase response of the filtered transducers and the sum Comments Enhanced Crossover filter with functional blocks Enhanced Crossover schematic with functional blocks Voltage transfer of the individual Xover filters On axis SPL and phase response of the filtered transducers and the sum Horizontal off axis response Calculation of the digital Xover for minidsp Woofer Xover programming in minidsp Txt file woofer Xover biquad coëfficiënts for import in minidsp Description woofer Xover biquads Woofer Xover response in minidsp Woofer Xover delay setting Midrange Xover programming in minidsp Txt file midrange Xover biquad coëfficiënts for import in minidsp Description midrange Xover biquads Midrange Xover response in minidsp Midrange Xover delay setting Tweeter Xover settings in minidsp Txt file tweeter Xover biquad coëfficiënts for import in minidsp Description tweeter Xover biquads Tweeter Xover response in minidsp Tweeter Xover delay setting

3 4.4 Configuration file for minidsp Conclusion Abbreviations DI FR FS H HS Imp IB SPL TSP V Directivity Index Frequency Response Full Space Horizontal Half Space Impedance Infinite Baffle Sound Pressure Level Thiele Small Parameters Vertical 3

4 1 Import in Leap CrossoverShop of the SPL responses of the transducers in the enclosure SPL all transducers, 2.83Vrms blue curve = Leap simulation FS 1m 2.83Vrms green curve = Leap simulation FS 1m 2.83Vrms red curve = Leap simulation FS 1m 2.83Vrms These curves have been simulated in Leap EnclosureShop and will be imported in Leap CrossoverShop for the digital crossover design 2 Creating the crossover filter targets in Leap CrossoverShop The targets can easily be created in Leap CrossoverShop using the Analog Active filter blocks. Linkwitz - Riley 4 th order filter Crossover frequencies: 300Hz and 3000Hz Low frequency target chosen at 38Hz, Q=0.60 4

5 3 Design of the speaker crossover filter in Leap CrossoverShop 3.1 Acoustic centers of the transducers The acoustic centers of the Accuton tweeter C25 and midrange C90 are the same, because they both are Accuton Cell driver concepts. At this moment the Accuton woofer S280 acoustic center is assumed to be 20mm behind the tweeter. Out of the enclosure dimensions and the acoustic centers of the transducers: - Tweeter at zero position - Midrange 13.8 mm forward w.r.t. tweeter - Woofer 10.6 mm forward w.r.t. tweeter These position values are used for the transducers position settings in Leap CrossoverShop. Later on these position values are also used for the delay settings in minidsp for the different transducer Xovers. 5

6 3.2 Basic Crossover filter with functional blocks Abbreviations of used functional blocks: LP1 = first order Low Pass filter; HP1 = first order High Pass filter LP2 = second order Low Pass filter; HP2 = second order High Pass filter Basic schematic with functional blocks Woofer filter: LP2 at 170Hz, Q=0.55, gain = 0dB ; LP2 at 270Hz, Q=0.80, gain = 0dB ; LP2 at 3000Hz, Q=0.70, gain = 0dB ; LP1 at 3000Hz, gain = 0dB Midrange filter: HP2 at 170Hz, Q=0.55, gain = -5.5dB ; HP1 at 200Hz, gain = 0dB; LP2 at 2800Hz, Q=1.0, gain = 0dB ; LP1 at 3000Hz; gain = 0dB Tweeter filter: HP2 at 2700Hz, Q=1.30, gain = -6.0dB ; HP2 at 2600Hz, Q=0.25, gain = 0dB 6

7 3.2.2 Voltage transfer of the individual crossover filters On axis SPL and phase response of the filtered transducers and the sum free field, 1m on axis, 2.83Vrms targets in light grey 7

8 Phase targets in light grey Comments - With this basic crossover filter design the SPL and phase response are already close on the targets. The same result can be obtained with a passive filter. The sound performance with such a filter would be already very good. - Some SPL ripple of the midrange can be observed around 1kHz and a resonance peak at 9.5kHz. The filtered transducer mapping on target is good, but might be better. Using a more enhanced crossover filter will improve these limitations of the basic crossover filter. 8

9 3.3 Enhanced Crossover filter with functional blocks Abbreviations of used functional blocks: LP1 = first order Low Pass filter; HP1 = first order High Pass filter LP2 = second order Low Pass filter; HP2 = second order High Pass filter BEQ = Bandpass Equalization filter; HEQ = Highpass Equalization filter Enhanced Crossover schematic with functional blocks Woofer filter: LP2 at 170Hz, Q=0.55, gain = -1.5dB ; LP2 at 270Hz, Q=1.0, gain = 0dB ; LP2 at 3000Hz, Q=0.70, gain = 0dB ; LP1 at 3000Hz, gain = 0dB; BEQ at 60Hz, Q=1.0, gain = 1.5dB; BEQ at 700Hz, Q=0.5, gain = -2.5dB; BEQ at 1500Hz, Q=1, gain = 4dB Midrange filter: HP2 at 260Hz, Q=0.9, gain = -6.5dB ; HP1 at 310Hz, gain = 0dB; LP2 at 2900Hz, Q=1.0, gain = 0dB ; LP1 at 3000Hz; gain = 0dB; BEQ at 980Hz, Q=2.5, gain = 2.0dB; BEQ at 1750Hz, Q=2.5, gain = 1.0dB; BEQ at 9500Hz, Q=8, gain = -14dB Tweeter filter: HP2 at 2700Hz, Q=1.30, gain = -6.5dB ; HP2 at 2600Hz, Q=0.25, gain = 0dB; BEQ at 2000Hz, Q=1.2, gain = -1.5dB; BEQ at 4500Hz, Q=3.0, gain = 1.0dB; BEQ at 9000Hz, Q=3, gain = -1.5dB; HEQ at 12000Hz, gain = 1.5dB Comment: For the enhanced filter some Band Equalization filters are added to make the overall SPL response flatter. These BEQ filters are only used when both on and off axis can be improved. 9

10 3.3.2 Voltage transfer of the individual Xover filters Comment: the BEQ filtering can be observed in the voltage transfer of the individual transducer Xovers On axis SPL and phase response of the filtered transducers and the sum free field, 1m on axis, 2.83Vrms targets in light grey 10

11 Phase targets in light grey Comments: - With an enhanced Xover design the SPL and phase response mapping on the targets is much better. Such a good result cannot be realized with a passive filter. - Our experience with such enhanced digital Xovers is a more balanced and more quite and natural sound Horizontal off axis response SPL Horizontal off axis 0, 15, 30, 45, 60,75 and 90 degrees not normalized 11

12 Horizontal polar Hz not normalized Horizontal polar khz not normalized 12

13 4 Calculation of the digital Xover for minidsp The digital filter coëfficiënts for the biquads in minidsp can be calculated in Leap CrossoverShop. For each transducer, a bilinear transform is executed on the analog functional blocks of the Xover filter and the biquad coëfficiënts are stored in a txt-file conform the minidsp format. 4.1 Woofer Xover programming in minidsp Txt file woofer Xover biquad coëfficiënts for import in minidsp biquad1, b0= , b1= , b2= , a1= , a2= , biquad2, b0= , b1= , b2= , a1= , a2= , biquad3, b0= , b1= , b2= , a1= , a2= , biquad4, b0= , b1= , b2= , a1= , a2= , biquad5, b0= , b1= , b2= , a1= , a2= , biquad6, b0= , b1= , b2= , a1= , a2= , biquad7, b0= , b1= , b2= , a1= , a2= , 13

14 4.1.2 Description woofer Xover biquads Biquad 1 = LP2 at 270Hz, Q=1.0, gain = 0dB Biquad 2 = LP2 at 170Hz, Q=0.55, gain = -1.5dB Biquad 3 = LP2 at 3000Hz, Q=0.70, gain = 0dB Biquad 4 = LP1 at 3000Hz, gain = 0dB Biquad 5 = BEQ at 700Hz, Q=0.5, gain = -2.5dB Biquad 6 = BEQ at 60Hz, Q=1.0, gain = 1.5dB Biquad 7 = BEQ at 1500Hz, Q=1, gain = 4dB Woofer Xover response in minidsp Woofer Xover delay setting The woofer Xover delay setting is 0.03ms. 14

15 4.2 Midrange Xover programming in minidsp Txt file midrange Xover biquad coëfficiënts for import in minidsp biquad1, b0= , b1= , b2= , a1= , a2= , biquad2, b0= , b1= , b2= , a1= , a2= , biquad3, b0= , b1= , b2= , a1= , a2= , biquad4, b0= , b1= , b2= , a1= , a2= , biquad5, b0= , b1= , b2= , a1= , a2= , biquad6, b0= , b1= , b2= , a1= , a2= , biquad7, b0= , b1= , b2= , a1= , a2= , 15

16 4.2.2 Description midrange Xover biquads Biquad 1 = HP2 at 260Hz, Q=0.9, gain = -6.5dB Biquad 2 = HP1 at 310Hz, gain = 0dB Biquad 3 = LP2 at 2900Hz, Q=1.0, gain = 0dB Biquad 4 = LP1 at 3000Hz; gain = 0dB Biquad 5 = BEQ at 980Hz, Q=2.5, gain = 2.0dB Biquad 6 = BEQ at 1750Hz, Q=2.5, gain = 1.0dB Biquad 7 = BEQ at 9500Hz, Q=8, gain = -14dB Midrange Xover response in minidsp Midrange Xover delay setting The midrange filter delay setting is 0.04ms. 16

17 4.3 Tweeter Xover settings in minidsp Txt file tweeter Xover biquad coëfficiënts for import in minidsp biquad1, b0= , b1= , b2= , a1= , a2= , biquad2, b0= , b1= , b2= , a1= , a2= , biquad3, b0= , b1= , b2= , a1= , a2= , biquad4, b0= , b1= , b2= , a1= , a2= , biquad5, b0= , b1= , b2= , a1= , a2= , biquad6, b0= , b1= , b2= , a1= , a2= , 17

18 4.3.2 Description tweeter Xover biquads Biquad 1 = HP2 at 2700Hz, Q=1.30, gain = -6.5dB Biquad 2 = HP2 at 2600Hz, Q=0.25, gain = 0dB Biquad 3 = BEQ at 2000Hz, Q=1.2, gain = -1.5dB Biquad 4 = BEQ at 9000Hz, Q=3, gain = -1.5dB Biquad 5 = BEQ at 4500Hz, Q=3.0, gain = 1.0dB Biquad 6 = HEQ at 12000Hz, gain = 1.5dB Tweeter Xover response in minidsp Tweeter Xover delay setting The tweeter Xover delay setting is 0ms. 4.4 Configuration file for minidsp Once the biquad coefficients and delay settings for all Xovers are programmed in minidsp, the complete setting is saved in a xml minidsp configuration file. For this design example a configuration file named MiniDSP-2x8_VCL Eidolon LR4 V27.xml is created. This file is available for free on demand. 18

19 5 Conclusion Using minidsp as the digital Xover platform for speakers is performing very well. We have that experience after listening to the Usher BE20 speaker redesign with a minidsp digital Xover. The Xover design is very critical with respect to the accuracy of the individual SPL responses (magnitude and phase) of the different transducers in the enclosure. Measuring all the transducers on a IEC baffle and simulate their responses in the enclosure in Leap EnclosureShop guarantees a good and correct Xover design and the correct settings for minidsp. Digital Xover design can be done by VCLLabs on demand. - In the case correct SPL responses (magnitude and phase) of the transducers in their enclosure, including the position of the acoustical centers if possible, can be delivered to us, we can do the complete digital X-over design and deliver a configuration file that can be loaded in the minidsp. We can give support also how to measure the acoustical responses. - In the case SPL responses of the transducers in their enclosure cannot delivered to us, we can use the supplier specification of the transducers and the dimensions of the enclosure to simulate all the responses in the enclosure in Leap EnclosureShop. These responses can be used for the digital Xover design. There is a big risk that there is a difference between the used transducers in the project and the transducer supplier specifications. For a good sound the SPL has to be correct within 0.5dB at least. - In the case the used transducers are measured in our lab on the IEC baffle and the responses in the enclosure are simulated in Leap EnclosureShop, the digital Xover can be designed by us correctly. We can do this on demand only for some projects. 19