DSP Crossover Implementation & Design. Al Clark Danville Signal Processing, Inc. Paul Beckmann - DSP Concepts, Inc.

Transcription

1 DSP Crossover Implementation & Design Al Clark Danville Signal Processing, Inc. Paul Beckmann - DSP Concepts, Inc.

2 Outline Introduction and motivation Typical loudspeaker algorithms Audio Weaver design tools Live system design and tuning Conclusion

3 Introduction About Al Clark 1976: Founded a high end audio company specializing in amplifiers and preamplifiers 1976: Invented the split passive phono preamp (patent : US A) 1977: B.Electrical Engineering, University of Minnesota About Danville Signal Manufacturer of SHARC based DSP boards specializing in high performance audio products targeted at OEMs/ODMs Analog Devices & XMOS partner Automated Assembly in Cannon Falls, MN : Bruel & Kjaer : Timewave Technology - founder/cto 1998-present: Danville Signal - founder/ceo

4 Introduction About Paul Beckmann : M.I.T., SB, SM and PhD in EE specializing in DSP : Bose Corp., developed algorithms for home theater, pro audio, 3D audio : Enuvis Corp., VP Engineering, developed high-sensitivity GPS algorithms About DSP Concepts Provides audio processing solutions Licenses Audio Weaver: versatile, modular DSP software that requires no coding Develop audio processing IP Audio DSP consulting/engineering services 2003-present: DSP Concepts founder/ceo

5 Why DSP Crossovers? 1. Freedom of Design 2. Delay compensation 3. Improved power efficiency and headroom 4. Relaxed amplifier loading 5. Distortion Reduction 6. Loudspeaker Protection 7. Stability of Performance

6 Economic Considerations Complexity is expensive with a passive crossover and free with a DSP crossover DSPs follow Moore s law Data Converters have improved Most Source Material is digital Higher quality switching amplifiers are readily available

7 Freedom of Design 1. Arbitrary Frequency or Phase Response 2. Non mechanical delay compensation 3. Dynamic reconfiguration 4. Real time tuning

8 Delay Compensation Passive Time Alignment (mechanical - a.k.a. the hard way) DSP Time Alignment (delay line and maybe a little math) Bowers & Wilkins DM6

9 Headroom Illustration Two Simple Inputs, Assume FS = 1

10 Sum of Inputs Headroom Illustration - 2

11 Sum of Inputs, Assume FS = 1 Headroom Illustration - 3

12 Distortion DSP Crossovers do not correct non-linearities (distortion), but they can avoid creating new ones. Power amplifiers are not ideal. If the load is easier to drive, the amplifier will do better Loudspeaker impedances can change with temperature and current. Passive components are also non-ideal. This causes signal and temperature dependent distortion in passive systems. Individual amplifiers in a DSP crossover are bandlimited. This reduces intermodulation products and keeps many distortion components out of band. Damping factor is near zero in a DSP crossover since there is a direct connection to a driver. Appropriate crossover design keeps drivers from operating outside their linear regions.

13 Loudspeaker Algorithms Equalization Crossovers Frequency response correction Loudness compensation (Fletcher Munson) Time correction Time delay Gain Phase compensation Speaker protection Compressors / limiters / Soft clipper IV sense Other Noise gates

14 Audio Weaver Design Tools Audio Weaver Features Graphical interface Large library of optimized modules Real-time tuning MIPs and memory profiling Regression testing Multirate processing Frequency domain processing Advanced features using MATLAB DSPC IP and 3 rd party IP

15 Crossovers Split signals into different frequency bands Overall response (the sum) should sum to 0 db Two main families: Butterworth (odd order) Linkwitz Riley (even order) Higher order crossovers require allpass phase compensation Two-way Crossover Low High Allpass Two-way Crossover Low High Woofer Midrange Tweeter Source:

16 Butterworth Response

17 Linkwitz-Riley Response

18 Advanced Processing High precision filters Loudness compensation Limiters Noise gate

19 When to use High Precision Filters Your product has a high sample rate (96 khz or above) You need to make very low frequency changes (below 80 Hz) You are concerned about providing the absolutely best fidelity to your customers

20 Example 1 Peak Filter Peak EQ freq = 40 Hz gain = 20 db Q = 1 Sample Rate = 48kHz, 20 db peak at 40Hz. The SNR of the standard Biquad at 40Hz is 60dB while the high precision Biquad is 120dB. Implemented using SecondOrderFilterSmoothed module -160 Frequency Response -180 THD+N Standard THD+N HP

21 Example 2 High Pass Filter Butterworth HPF - 2nd order freq = 40 Hz Sample Rate = 48kHz, cutoff frequency = 40Hz. Equivalent to sample rate of 96kHz and cutoff frequency of 80Hz Implemented using ButterFilter module -160 Frequency Response -180 THD+N Standard THD+N HP

22 Example 3 Tweeter EQ Tweeter Speakers exhibited a crunching sound during low frequency kick drum beats. Implemented using a BiquadCascade module. Actual speaker EQ from an automotive system Frequency Response -180 THD+N Standard THD+N HP

23 Loudness Compensation Fletcher Munson loudness curves Sensitivity of human hearing depends upon the intensity of the sound Nonlinear behavior Perceptual effect: low and high frequencies disappear as the volume is reduced Can be compensated by volume dependent equalization Source:

24 Audio Weaver Loudness Compensation Low Frequency Table High Frequency Table

25 Speaker Protection The goal is to maximize the speaker output while reducing distortion Critical for small speakers! Peak limiter to reduce peak excursion and limit distortion RMS limiter to reduce thermal overload

26 Conclusion DSP Crossovers can turn a good loudspeaker design into a better loudspeaker design. Loudspeakers need careful processing to maximize sound quality The right tools make the job much easier and demystify the whole process Danville Signal will be in Suite at the Venetian