Biquad

  • 5 mins read

This object computes bi-quadratic filter coefficients and calculates the output of the input audio buffers
processed through the filters that users specify.

From SFD, the user can select what type of filtering to carry out via the “Biquad Topology field”:

  • Direct Form I
  • Direct Form II
  • Transposed Direct Form II

This object operates in one of the six modes:

  • Parameter Mode – This mode generates a set of filters per channel, so each channel has its own set of unique filters.
  • Crossover Mode – This mode generates a set of filters in the order of one through four per channel.
  • Coefficient Mode – In this mode, the object does not generate any filter coefficients – it simply receives them from the tuning tool.
  • Parameter One Mode – This mode generates a set of filter coefficients for all audio channels. The only difference between this mode and the Parameter mode is that this mode generates one set of coefficients for all channels as opposed to one set per channel.
  • Parameter One Mode Set Control Frequency – This mode only uses one filter and applies the filter characteristics to all channels as does the Parameter One Mode above. It also exposes one control (accessible from the HU) variable to manipulate the cut-off frequency.
  • Coefficient One Mode – This mode generates a single set of filter coefficients for all audio channels. The only difference between this mode and the Coefficient mode is that this mode generates one set of coefficients for all channels as opposed to one set per channel.
  • Equalizer Mode – This mode generates a unique set of filters for every channel.
  • Equalizer One Mode – This mode generates a single set of filters for all audio channels. The difference between this mode and Equalizer mode is that this mode generates a unique set of filters for all channels as opposed to one set per channel.

Equalizer modes support ramping functionality whenever filter type or parameters change at run-time.
Ramping variables are based on sampling rate, block length and ramp time. Implemented ramping strategy is
based on first-order low-pass interpolation scheme.

In the SFD, users can set the number of channels (the number of input and output channels is the same) as
well as the number of filters. Users can also select the object corresponding to the required mode.

In the GTT, the object exposes several of its variables based on mode:

Parameter Mode – For each filter in each channel, the object exposes:

  • Frequency (cut-off)
  • Gain (in dB)
  •  Q-value
  •  Type of filter:
    • All pass 1
    • All pass 2
    • High pass 1
    • High pass 2
    • Low pass 1
    • Low pass 2
    • High shelf 1
    • High shelf 2
    • Low shelf 1
    • Low shelf 2
    • Parametric (Peaking)

If using the GTT (specifically) for tuning, these additional types of filters are available:

    • High pass Butterworth 3
    • High pass Bessel 3
    • High pass Butterworth 4
    • High pass Bessel 4
    • High pass Linkwitz-Riley 4
    • Low pass Butterworth 3
    • Low pass Bessel 3
    • Low pass Butterworth 4
    • Low pass Bessel 4
    • Low pass Linkwitz-Riley 4
  • Bypass (on/off)

Crossover Mode – For each filter in each channel, the object exposes:

  • Frequency (Cut-off)
  • Type of filter
    • Bypass
    • High pass 1
    • High pass Butterworth 2
    • High pass Bessel 2
    • High pass Linkwitz Riley 2
    • High pass Butterworth 3
    • High pass Bessel 3
    • High pass Butterworth 4
    • High pass Bessel 4
    • Low pass Linkwitz-Riley 4
    • Low pass 1
    • Low pass Butterworth 2
    • Low pass Bessel 2
    • Low pass Linkwitz Riley 2
    • Low pass Butterworth 3
    • Low pass Bessel 3
    • Low pass Butterworth 4
    • Low pass Bessel 4
    • Low pass Linkwitz-Riley 4
  • Bypass (on/off)

Coefficient Mode – For each filter per channel, these five values representing filter coefficients are exposed:

  • b0
  • b1
  • b2
  • a1
  • a2

Parameter One Mode – This is identical to the Parameter Mode, except, instead of presenting the variables for each filter per channel, this mode only presents a set of coefficients per filter, regardless of the number of
channels.

Parameter One Mode Set Control Frequency – This is identical to Parameter One Mode but it has only one filter.
The Biquad object does not expose any variables for control except for the Parameter One Mode Set Control Frequency, where it exposes the cut-off frequency of the filter.

Coefficient One Mode – For each filter regardless of the number of channels, below five values representing
filter coefficients are exposed:

  • b0
  • b1
  • b2
  • a1
  • a2

Equalizer Mode – For each filter in each channel, the object exposes the following parameters for both tuning and state-tuning:

  • Frequency (in Hz)
  • Gain (in dB)
  • Q-value
  • Type of filter:
    • Bypass
    • Bypass
    • All pass 1
    • All pass 2
    • High pass 1
    • High pass 2
    • Low pass 1
    • Low pass 2
    • High shelf 1
    • High shelf 2
    • Low shelf 1
    • Low shelf 2
    • Parameteric
    • Bandpass
    • Bandstop
    • Amplifier
    • Resonant
    • Bell
    • Low shelf variable-slope
    • High shelf variable-slope
    • Peaking GEQ
    • Peaking HPro
    • Low shelf HPro
    • High shelf HPro
    • Peaking AFS
    • Peaking PA Plus
    • Peaking Crown
    • Low shelf PA Plus
    • High shelf PA Plus
  • Bypass (on/off)
  • Width (in octaves- only for Bell filter)
  • Asymmetry (on/ off- only for Bell filter)
  • Slope (in dB per octave- only for variable-slope low/ high shelving filters)

Equalizer One Mode – This is identical to the Equalizer Mode. Except, instead of specifying the parameters for each filter per channel, this mode specifies the same set of coefficients per filter, regardless of the number of
channels.

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