parsons expo 2011 dsp platform oddities ... expo 2011 dsp platform oddities unpleasant surprises in...

http://www.bennettprescott.com DSP Platform Differences

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


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


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


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


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


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.


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


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.

http://www.bennettprescott.com DSP Platform Differences32

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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.


BONUS TOPIC:LIMITERS

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LIMITER PARAMETERSTHRESHOLD

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• dBu

• dBFS

• VU

• Volts

•Watts

•Off, -3, -6, -12dB


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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