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Core Concepts

Signals and Filters

BEQDesigner is an application that allows you to see the effects of filters on signals.

What is a Signal?

A signal is a mono audio track. This typically means a signal is one of the following: sm * a single channel in a multichannel audio track * a combination of all channels from a multichannel audio track mixed into a mono audio file with channel levels adjusted to account for the LFE channel offset

The Extract Audio utility is an easy to use interface for extracting such content from standard movie file formats.

What is a Filter?

A filter is a way of making particular sounds louder or quieter, i.e. it is a change in the frequency response of a signal.

What types of filters are supported?

BEQDesigner is designed to be used with commonly available and used DSP platforms. Such platforms typically allow the user to apply various types of parametric EQ filters which can change the frequency response of the signal in various ways.

The most commonly used device for this purpose is the MiniDSP 2x4 HD and BEQDesigner supports all the (IIR) filter types supported by this device. These are:

  • Low and High Shelf filters
  • Parametric EQ
  • High and Low Pass Filters of the following types:
  • Butterworth (1st to 24th order)
  • Linkwitz-Riley (2nd to 24th order)
  • User defined Q

Shelf filters and PEQ are implemented using the formulae shared in the RBJ Cookbook

Warning

All filters are calculated using 64 bit floating point arithmetic. This ensures there are no biquad precision issues regardless of centre frequency or Q. However most commonly available DSP platforms do not operate at this precision and hence can suffer from inaccuracies, particularly at very low frequencies (i.e. <20Hz). Always be aware of the accuracy of your own hardware when applying such filters.

Linking Signals

BEQDesigner allows certain signals to be linked together. This means a filter can be defined once but applied to many signals. When signals are linked, one signal becomes the master signal and all linked signals become slaves.

This terminology can be ignored for most practical purposes, it simply means that the filter applied is owned by the master signal.

Refer to Link Signals for how to link signals in BEQDesigner.

Why is this useful?

There are 2 common scenarios where this is relevant

  1. Applying a post bass management BEQ filter to the input channels (either when remuxing or when using a DSP device that sits before bass management in the signal chain)
  2. When designing a set of pre bass management BEQ filters and some subset of channels rolloff with a very similar shape. Common groups in this case are the LCR or the surrounds.

Signal Analysis

All analysis methods are based on some form of STFT analysis as provided by the scipy library.

As the analysis is STFT based, the time vs frequency tradeoff is ever present, i.e. increasing the frequency resolution of the analysis reduces the temporal resolution and vice versa.

This is controlled via the relevant Preferences.

Avg Frequency Response

The avg frequency response is a measure of the average level by frequency of the entire track. It is calculated using Welch's method with spectrum scaling and mean averaging.

Peak Frequency Response

The peak frequency response shows the peak level by frequency reached in any single analysis time period. It is calculated by computing a spectrogram and then calculating the max value on the frequency axis.

Spectrum

The spectrum view is essentially a spectrogram albeit one with some visual differences. See the spectrum view for full details.

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