DSP

Digital Signal Processing

Introduction

Looking at the digital products in sound system engineering, the products on the market today have surpassed all expectations. I would like to point out some of the options that is on the market and how they define the design the system. The Digital Signal Processing in these products provided for a tremendous amount of optimization for the system. I will first start at the end with the amplifier with embedded DSP, next will be the management of speaker systems and last covering the DSP features provided in the new digital mixers.

  • Powered speakers have embedded DSP in the plate amps
    • At least they have some type of limiter for speaker protection and some form of equalization where other have a full blown DSP capabilities.
  • Power amplifiers with most being Class “D” have embedded DSP. This includes:
    • A delay, phase selection and peak limiter
    • Also you see a crossover with high and low pass filters per channel
    • Parametric equalization with five to eight filters to be used is several different ways
    • Dynamic equalization with compressor functions.
  • For those system with legacy amplifiers with no DSP, there is the speaker management system that provides all of these DSP features in one package. Details include:
    • Three inputs each with gain, delay, parametric and dynamic equalization.
    • A fourth section with the same above features is used for summing (Sub Output).
    • There are six outputs with seven function for each
      • First there is the source selection, gain and crossover (high and low pass filters)
      • Next is parametric and dynamic equalization with a limiter
      • Last is phase selection, short and long delay functions

Filters/Crossover

This function offers adjustable hi-pass and lo-pass filters. Used in stereo or dual mono, these filter can provide for the required window for the type of speaker this is providing power for. When running in a bi-amp mode, using these filters in junction provides a crossover function. In some cases, several types of filter algorithms are provided with multiple slopes. Example of these algorithms include:

  • Linkwitz-Riley with 12, 24 and 48 dB slope selections
    • Linkwitz-Riley (LR12, LR24, LR48).
  • Butterworth with 6, 12, 18, 24 and 48 dB slope selections
    • Butterworth (BUT6, BUT12, BUT18, BUT24, BUT48)
  • Bessel with 12, 24 dB slope selections
    • Bessel (BES12, BES24),

In most cases for a professional application a 24 dB slope is used.

Parametric Equalization

The PEQ module can deploys up to eight EQ bands for precise sound sculpting. Provides for the control the frequency range from 20 Hz to 20 kHz, with 15 dB of cut/boost. Each band is selectable for LS6 (low shelving EQ, with a 6 dB slope), LS12 (low shelving EQ, with a 12 dB slope), HS6 (high shelving EQ, with a 6 dB slope), HS12 (high shelving EQ, with a 12 dB slope) or PEQ (parametric modes). The selected frequency represents the cutoff frequency for low and high shelving modes or the center frequency for parametric mode. An additional parameters in parametric mode is Q or quality parameter. The parameter defines the width of the curve where a high Q values produce a narrow band or steep curve and a low Q values create a wide band with a gentle slope. The DSP function in the amplifier can provide for adjustment in the minor flaws in the speaker response.

Dynamic Equalization

A dynamic EQ is a combination a parametric equalizer and the function of a compressor. The parameters used in this function include: frequency, gain, and bandwidth of a filter, with compression parameters that include threshold, attack, and release. This provides adjustment for 2 bands of level-dependent, dynamic equalization per channel.

  • Gain
    • Defines the amount of cut or boost for this function.
  • Frequency with a range from 20 Hz to 20 kHz,
    • The programmed frequency can represent either the center frequency for band-pass mode, or the threshold frequency for low- and high-pass modes.
  • The Q parameter or frequency width
    • High Q values produce a narrow, steep curve, while low Q values create a wide curve with a gentle slope.
  • The Type of filters
    • BP (bandpass filter)
    • LP6 (lo-pass filter, with a 6 dB slope)
    • LP12 (lo-pass filter, with a 12 dB slope)
    • HP6 (hi-pass filter, with a 6 dB slope)
    • HP12 (hi-pass filter, with a 12 dB slope)
  • Threshold
    • The dynamic EQ trigger to provide the compression or expansion function.
  • Attack
    • Amount of time before compression starts after the threshold is exceeded.
  • Release
    • Amount of time to stop compressing after the signal crosses below the threshold.
  • Ratio – gain adjustment for reduction/boost ratio (Ratio).
    • Controls the amount of compression, once the input signal crosses the threshold level,
    • The Ratio Selection includes 1:2.0, 1:3.0, 1:5.0 and 1:10
    • The higher the Ratio setting, the more the equalizer will cut or boost the signal levels.

Delay

The delay function, expressed as either distance or time, allow for adjustment of the output to compensate between speaker arrays or the distance between a stage and distant speakers. This function, in most cases, can be defined in milliseconds (ms), meters (m), or feet (ft), which is useful working with speaker distances. The milliseconds would be useful if working with speaker alignment with a speaker cabinet for alignment of multiple speaker in a bi-amp mode application. Also included in this function is the phase function that is useful for subwoofer alignment. The combination of these two function is used when doing a cardioid subwoofer application.

Peak Limiter

Limiter function is provided for loudspeakers protection by setting the threshold of the limiter to match the recommended power ratings of the driver(s). These ratings are only a guide but a little knowledge you can derive the required settings needed to customize your speaker protection to maximum reliability.

Some of this is discussed in the “Power Relationship” page on this website.

Please note that the midrange and high frequency drivers will not be able to handle as much power as a low-frequency drivers. This is why for medium to large systems, it is recommended to use separate amplifiers for each application. Because of the different ratings, these will require different limiter setting.

Parameters for these limiter include:

  • Threshold
    • Setting at which the limiter will be engaged
  • Hold
    • Amount of hold time after the threshold is exceeded
  • Release
    • Amount of time to stop limiting after the signal is below the threshold and the hold time has expired.
  • dBfs
    • Decibels relative to full scale
    • Difficult to relate to so make note as setting others to acquire usable values.
  • Vp
    • Voltage peak of speaker(s)
    • Using the peak parameter, define this by Sqrt(ohms*watts)
    • This is power that can be applied to stay within the Xmax parameter.
  • Watts (option on amps that I use)
    • Wattage of output
  • Ohm (option on amps that I use)
    • Load on amp
    • Select for total combined load of all speakers connected to amplifier’s outputs.
    • Once a Load setting is selected, the watt rating appears and adjustment can be made to match the total speaker power used.

Speaker Management

This addresses the amplifiers that does not have embedded DSP. This package provides all of the DSP function previously discussed and is all in one package for two to three inputs and six outputs. A two input device would allow for bi-amp application where a three input device would allow for tri-amp application.

Another option with the three input device is to provide the left, right and Aux Sub in the third inputs.

Inputs

  • Gain – Two or three XLR inputs each with a gain to maintain an optimum gain structure
  • Delay – A delay for compensation depending on the configuration
  • PEQ – Parametric Equalization for fine tuning as a group before feed to all six outputs
  • DEQ – Dynamic equalization to address on a group basis
  • Summing – A fourth section is provided same above features is used for summing (Sub Output).
  • Each of the inputs are feed to a 4ch bus for the selection of each output.

Outputs

  • Gain – Six XLR outputs each with a gain to maintain an optimum gain structure
  • Crossover – This function offers adjustable hi-pass and lo-pass filters used this function.
  • PEQ – Parametric Equalization for each output for fine tuning for all six outputs
  • DEQ – Dynamic equalization for each output for optimization for that output
  • Limiter – Primarily providing speaker protection to address clipping of the amplifier.
  • Phase – Used for subwoofer alignment and for cardioid subwoofer application.
  • Delay – A delay for compensation depending on the configuration

Crossover Function

  • This function offers adjustable hi-pass and lo-pass filters used this function.
  • There several types of filter algorithms are provided with multiple slopes.
  • Example of these algorithms include:
    • Linkwitz-Riley with 12, 24 and 48 dB slope selections
      • Linkwitz-Riley (LR12, LR24, LR48).
    • Butterworth with 18, 24 and 48 dB slope selections
      • Butterworth (BUT18, BUT24, BUT48)
    • Bessel with 12, 24, 48 dB slope selections
      • Bessel (BES12, BES24, BES48),

PEQ – Parametric Equalization

  • Parametric Equalization for fine tuning for all six outputs
  • This PEQ module provides up to nine bands for precise sound sculpting.
  • Frequency range from 20 Hz to 20 kHz with 15 dB of cut/boost.
  • Each band is selectable for low shelving, high shelving or parametric modes.
  • Selected frequency represents the cutoff frequency for low and high shelving.
  • Selected frequency represents the center frequency for parametric mode.
  • An additional parameters in parametric mode is Q or quality parameter.
    • The parameter defines the width of the curve where a high Q values produce a narrow band or steep curve and a low Q values create a wide band with a gentle slope.

DEQ – Dynamic Equalization

  • Combines a parametric equalizer and the function of a compressor.
  • Gain – Defines the amount of cut or boost for this function.
  • Frequency with a range from 20 Hz to 20 kHz,
  • The selected frequency represents the center frequency for band-pass mode
  • The Q parameter or frequency width
    • High Q values produce a narrow, steep curve, while low Q values create a wide curve with a gentle slope.
  • Shelving frequency represents the threshold with a slope of 6 or 12 dB/octave
  • Threshold – defines the trigger to provide the compression or expansion function.
  • Ratio – gain adjustment for reduction/boost ratio (Ratio).
    • Ratio Selection from includes 1.1:1 to infinity for a total of 16 selections
    • The higher the Ratio, the more cut or boost of the signal.
  • Attack – Amount of time before compression starts after the threshold is exceeded.
    • Selection provided for 1 to 200 ms
  • Release – Amount of time to stop compressing after the signal crosses below the threshold.
    • Selection provided for 20 to 4000 ms

Limiter

  • The function provides for loudspeakers protection by setting the threshold of the limiter.
  • Parameters for these limiter include:
    • Threshold – Setting at which the limiter will be engaged (0 to -24 dB).
    • Release – Amount of time before the deactivation of the limiting after signal drops below the threshold (20 to 4000 ms).

Delay (Short and Long)

  • Short delay function, expressed in milliseconds for adjustment of the output to compensate between speaker arrays (0 to 11.64 ms)
  • Also provided with the short delay is the phase function
    • Useful for subwoofer alignment or cardioid subwoofer application.
    • Provide for the inverter of the phase as well as phasing from 0 to 180 degrees.
  • Long Delay function for larger distance between a stage and distant speakers with an adjustment of 0 to 582.24 ms.
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