Parametric Equalizer

Overview

Media Center's DSP Studio includes a powerful and flexible parametric equalizer (PEQ) that can be configured with any number of filters. PEQ can be used for simple tasks like equalization and channel routing as well as advanced tasks such as creating a crossover for active speakers, performing bass management and applying a compressor.

This page provides an explanation of how each option works as well as some guidance on typical uses.

Verifying Your DSP Studio Configuration

See Verifying DSP Studio for details of how to measure your configuration.

All graphs in this section were created using the method described in that page.

Use Cases

Use Case	Description	Link
Custom Mixing	Allows the user to perform their own custom mixing, e.g. for bass management	Mixing

Adjust a Frequency

This option creates a "Peaking EQ" filter, such a filter may also be known as:

• a parametric EQ filter
• a notch filter

Such a filter provides a cut or boost which is centred on the specified centre frequency

4 values must be entered by the user:

• Frequency: the centre frequency of the filter
• Bandwidth (Q): specifies the frequency range over which the filter will boost or cut, the higher the Q, the narrower the range over which the filter operates
• Gain: the maximum boost or cut applied by the filter, i.e. the boost or cut applied at the centre frequency
• Channels: the channels to apply the filter to

The maximum Q supported is 25.

What type of peaking filter is used?

MC implements a type of filter known as an RBJ filter after the author Robert Bristow-Johnson who published the EQ Cookbook. This type of filter is also known as a proportional Q filter in rePhase

Understanding Q

Q is defined relative to the centre frequency of the filter, it is the centre frequency divided by the bandwidth of the filter where the bandwidth is defined by the +/-3dB points.

For example, if the centre frequency is 500Hz and the -3dB points are at 450 and 550Hz then we have a bandwidth of 100Hz. Therefore Q = 500/100 = 5

Another example, if the centre frequency is 100Hz but we maintain the same 100Hz bandwidth then Q = 100 / 100 = 1

Examples

Varying Q

This example shows a series of filters centred at 500Hz with Q of 1, 2 and 5 and a +6dB gain

Stacking Filters

Filters can be stacked to produce complex shapes

We can break this down visually using REW

• Red: +3dB at 100Hz with Q=1
• Green: +3dB at 500Hz with Q=1
• Blue: +6dB at 500Hz with Q=5
• Yellow: +2dB at 5000Hz with Q=3
• Cyan: the total filter

Adjust high/low frequencies

This option creates a high or low shelf filter.

This type of filter, as the name suggests, has a response that steps ("shelves") the response up or down.

The shape of the shelf is determined by the user specified Q and, as with peaking filters, the higher the Q, the steeper the slope of the shelf.

4 values must be entered by the user:

• Frequency: the centre frequency of the filter, i.e. the point by which half of the shelf gain has been achieved.
• Bandwidth (Q): generally speaking determines the rate at which the change specified by the gain is achieved however see #Q or S for full details
• Gain: the maximum boost or cut applied by the filter, i.e. the level at which the top or bottom of the shelf resides
• Channels: the channels to apply the filter to

As with the Peaking filter, this filter is defined in the RBJ Cookbook.

Q or S

DSP Studio asks the user to enter a value for Q when entering a shelf filter however it is important to realise this label is incorrect. This value is actually S as per the RBJ cookbook.

It is extremely important (for correct results) to realise this fact as most software that generates such filters will use Q as per its correct definition. You will need to manually calculate S from the given Q instead.

This is straightforward albeit the equation is somewhat intimidating as you need to solve

1/((((1/Q)^2-2)/((10^(d/40))+1/(10^(d/40))))+1)

where

• Q = the actual Q
• d = the required gain

One easy way to solve this is to enter the above equation into an online equation solver such as https://www.mathpapa.com/algebra-calculator.html and then enter the required values

For example, if we want a shelf filter with Q = 0.707 and a gain of +5dB then we need to enter Q = 0.99971 and gain = +5 into DSP Studio

Effect of Changing S

This example shows a series of filters centred at 1000Hz with S of 0.5, 1, 2 and 5 and a +5dB gain

Notice two things

• how each curve crosses the same point at 1000Hz and that this frequency is the centre point of the shelf, i.e. +2.5dB in this example
• how S>1 overshoots

Peaking Shelf Filters

When S > 1 (which typically means a real Q > 0.707), the filter is somewhat unstable in that the shelf will overshoot at both ends of the shelf. This means the maximum boost/cut applied by the shelf filter will be greater than the entered gain.

Ignoring this can lead to clipping in your signal chain due to the (unexpected) excess gain.

This example compares a +5dB high shelf at 1000Hz with S=1 vs S=5, the S=5 filter overshoots by ~1.9dB at each end

One option to avoid this problem is to use PEQ to correct the overshoot, in this example 2 filters are used to roughly correct each peak

Low vs High Shelf

A positive low shelf is a filter whose (magnitude) response increases as frequency decreases. A positive high shelf is a filter whose (magnitude) response increases as frequency increases.

Negative High Shelf Filters

Now compare the positive low shelf to a negative high shelf, the 2 filters have exactly the same shape but the negative high shelf has a maximum value 5dB lower than the positive high shelf.

Put another way, the a negative high shelf achieves the same end result as a positive low shelf with a negative gain (volume) adjustment. This is important when digital signal headroom is limited (which is invariably is).

High Pass Filter

Attenuates all frequencies below the set frequency by a continuously increasing amount (as frequency decreases).  The slope is the steepness of that attenuation (6dB per octave, 12 dB per octave, etc.).  A Low Pass Filter is the exact opposite, it attenuates all frequencies above the set frequency.  Here's an illustration of five different butterworth low-pass filters with different slopes (6dB, 12dB, 18dB, 24dB, 30dB) http://en.wikipedia.org/wiki/File:Butterworth_Filter_Orders.svg.  The 6dB, 12dB, and 24dB filters in Jriver should look exactly like the illustrated ones (1,2, and 4).  Correct use of low pass and high pass filters is kind of complicated, but they are typically used to create speaker crossovers, for protection of vented (ported) loudspeakers, or to interface a subwoofer with existing speakers.  

Adjust Volume

Lets you adjust the volume (up or down) of an entire channel by the amount set in the Gain parameter.  This is useful if you, for example, have multiple speakers of different varieties in a surround setup and you need to level match them; the Room Correction DSP module also has level matching tools.  

Delay

Lets you delay channels so that they output later than other channels.  This is useful if you have multiple speakers that are different distances from you, or if you're using an active crossover.  I'd recommend against setting delay in Parametric Equalizer unless you have a measuring microphone handy. The Room Correction DSP module also has tools to help address speakers that are different distances from you, and might be easier to use (without a mic) if you just need distance adjustment.

Reverse Polarity

Reverses the absolute polarity of the selected channel(s).  This is useful if your speakers are wired backwards, if you're using an active crossover, or for better subwoofer integration in some room setups.  The Room Correction DSP module also has an option to reverse polarity on a speaker by speaker basis.  More info about what "polarity" means in this context here: http://en.wikipedia.org/wiki/Absolute_phase#Sound_reproduction

Linkwitz Transform

A filter that boosts low bass performance of a speaker.  It is not typically a good idea to use a Linkwitz transform with vented speakers (speakers with ports in them), and isn't necessarily a good idea with sealed speakers (depending on your speakers and the settings you choose).  It's generally a more "advanced topic."  Here's a very good article on what it does with illustrations: http://sound.westhost.com/linkwitz-transform.htm

Limiters

Prevents sounds that are louder than the limit from playing on the selected channels.  They're essentially a method of protecting your speakers if you're concerned that they might get a damagingly loud signal sent to them.  This can usually be achieved in other, simpler ways (like turning down the volume on your amp), but there are circumstances where PEQ is the best place to handle that kind of thing.  

More

• http://yabb.jriver.com/interact/index.php?topic=75303.msg510479#msg510479
• http://yabb.jriver.com/interact/index.php?topic=85606.msg584954#msg584954