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
Unique to one family of DSPs (so far?)11
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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
(or vice versa)15
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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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