Introduction to ARM A15 Linux DSP Platform Software Apps Team 04/19/2013
PARSONS EXPO 2011 DSP PLATFORM ODDITIES ... EXPO 2011 DSP PLATFORM ODDITIES Unpleasant Surprises in...
Transcript of PARSONS EXPO 2011 DSP PLATFORM ODDITIES ... EXPO 2011 DSP PLATFORM ODDITIES Unpleasant Surprises in...
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PARSONS EXPO 2011DSP PLATFORM ODDITIES
Unpleasant Surprises in Loudspeaker Processing
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DSP PLATFORMS DIFFER
• Everyone seems to agree processors are not identical
• For what reason?
• How can “good sounding” processors sound bad with some loudspeakers?
•Other differences ascribed to sample rate, converter quality, country of origin, price tag, phase response, etc.
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QUICK & DIRTY DSP STUDY
• Came up with imaginary “loudspeaker preset”
• Designed to show variations in filter definition
• Asked LAB & SoundForums.net members to measure their DSPs
• Got nearly two dozen results
• Everything from DSPs to digital mixing consoles
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RESULTS FROM MY STUDY4
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HOW CAN THIS BE?
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• Three Effects at work:
• Bad DSP Behavior
• Unit Conversion
• Filter Definition
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BAD DSP BEHAVIOR
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BAD DSP BEHAVIOR
• BZT Filter Warping near Nyquist
• Differing behavior within same family
• Or processors branded for different companies by same OEM
• Errors with Certain Filter Inputs
•Math gets FUBARed, DSP doesn’t tell you
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BAD DSP BEHAVIORFilters narrow as they approach Nyquist
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BAD DSP BEHAVIOR
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Filters narrow as they approach Nyquist
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BAD DSP BEHAVIOR
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Filters narrow as they approach Nyquist
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BAD DSP BEHAVIORPolarity Reversal
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UNIT CONVERSION
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UNIT CONVERSION
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•Octaves vs. Q for Bandwidth
• Different bell filter types... Constant Q, Adaptive Q, Raised Cosine?
•What HP/LP filters and slopes are available?
•What Shelf Filter slopes are available?
• Coarseness of available input values
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UNIT CONVERSIONOCTAVES VS. Q
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•Octaves are how you and I probably think about bandwidth
•Q is from the electronics folks
• Represents the “Quality” factor of a filter... how well damped it is
• Inverse of octaves, higher Q number is narrower filter
• Conversions widely available
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UNIT CONVERSIONEntering Q into an Octaves Device
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UNIT CONVERSIONSHELF FILTERS
• 6dB Slope
• 12dB Slope
• Bandwidth in Q
• Bandwidth in Octaves
• Bandwidth in “Slope”
• Simply labeled “Shelf ”
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UNIT CONVERSIONCROSSOVER FILTERS
• Everything seems to have:
• Linkwitz-Riley & Butterworth
• 12 & 24dB/octave
•Many simply do not have Bessel
•Or odd-order filters (6dB, 18dB/octave and so on)
• God forbid you need a Chebyshev, NTM, or FIR “brickwall” filter.
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FILTER DEFINITION
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Bell Filters
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FILTER DEFINITIONBELL FILTERS
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•What does Bandwidth mean?
• 3dB from peak gain?
• 3dB from zero gain?
• 3dB at midpoint?
• How is a 2dB boost defined?
Image Courtesy Rane Corp.
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FILTER DEFINITIONBell Filters: “Constant Q”
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FILTER DEFINITIONBell Filters: “Adaptive Q”
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FILTER DEFINITIONBell Filters: Constant Bandwidth Midpoint
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FILTER DEFINITIONLAKE BELL FILTERS
• Input filters are Raised Cosine
• Only in use by Lake and Powersoft
•Output filters are “normal”
• Bandwidth at midpoint
•Where to input settings?
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FILTER DEFINITIONShelf Filters
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FILTER DEFINITIONShelf Filters
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FILTER DEFINITIONBessel Filters
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BESSEL FILTERS
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-3dB Normalization
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BESSEL FILTERS-6dB Normalization
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BESSEL FILTERSPhase Match Normalization
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BESSEL FILTERSNo Normalization
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CONCLUSION
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Must Measure to Transfer Settings Between Processors.
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QUESTIONS?
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Many thanks to Rich Frembes of Fulcrum Acoustic for a large number of the measurements used in this presentation.
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BONUS TOPIC:LIMITERS
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LIMITER PARAMETERSTHRESHOLD
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• dBu
• dBFS
• VU
• Volts
•Watts
•Off, -3, -6, -12dB
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LIMITER PARAMETERSATTACK / RELEASE
•Milliseconds / Microseconds
• Seconds
• dB / Second
• ms / dB
• ms / 20dB
• Slow / Medium / Fast
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LIMITER PARAMETERSOTHER
• Ratio (1.2:1 - INF)
•Over Easy (0 - 10)
• Knee (Soft, Medium, Hard)
•Overshoot (1 - 6dB)
• Peak Stop (On / Off, Threshold)
• Corner (0 - -100)
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LIMITER PARAMETERSLIMITER TYPE
• RMS or Peak detection?
• Neither strictly defined.
• Power limiter (RMS or measured power detection)
• How do you measure the behavior of a limiter?
• Limiter settings that behave predictably in one processor may not do so in another.
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THE END
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