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Digital Dynamic Range Compressor Design— a Tutorial and Analysis

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Digital Dynamic Range Compressor Design— a Tutorial and Analysis PAPERS Digital Dynamic Range Compressor Design— A Tutorial and Analysis DIMITRIOS GIANNOULIS, MICHAEL MASSBERG, AND JOSHUA D. REISS, AES Member ([email protected]) ([email protected]) ([email protected]) Queen Mary University of London, London, UK Dynamic range compression, despite being one of the most widely used audio effects, is still poorly understood, and there is little formal knowledge and analysis of compressor design techniques. In this tutorial we describe several different approaches to digital dynamic range compressor design. Digital implementations of several classic analog approaches are given, as well as designs from recent literature, and new approaches that address possible issues. Several design techniques are analyzed and compared, including RMS and peak–based approaches, feedforward and feedback designs, and linear and log domain level detection. We explain what makes the designs sound different and provide metrics to analyze their quality. Finally, we provide recommendations for high performance compressor design. 0 INTRODUCTION1 we can describe the main parameters of a compressor unit and specify a set of standard stages and building blocks that Dynamic Range Compression (DRC) is the process of are present in almost any compressor design. mapping the dynamic range of an audio signal to a smaller In general, compressors are quite an unexplored field in range [1-2], i.e., reducing the signal level of the higher academia. The authors know of no prior publication that peaks while leaving the quieter parts untreated. DRC is collectively derives and discusses the most common com- used extensively in audio recording, production work, noise pressor design options in depth. A better understanding of reduction, broadcasting, and live performance applications. the different design choices would make compressor design Considering the classic audio effects (equalization, delay, considerably easier and less of a trial-and-error process. It panning, etc.), the dynamic range compressor is perhaps the would also shed light on how the different design choices most complex one. Design choices involve the compressor affect the perceived sonic characteristics of the compressor. topology, the static compression characteristic, placement, The goal of this paper is to provide such analysis, consid- and type of smoothing filters, sidechain filtering, etc. This ering both technical and perceptual aspects. explains why every compressor in common usage behaves The tutorial is structured as follows. Section 1 provides and sounds slightly different and why certain compressor an overview of the most common controls used to operate models have become audio engineers’ favorites for certain a dynamic range compressor. In Section 2, the principles types of signal. The analysis of compressor designs is diffi- of its operation are described. Particular attention is paid cult because they represent nonlinear time-dependent sys- to the level detection and use of attack and release times, tems with memory. The gain reduction is applied smoothly especially their digital implementation based on classic ana- and not instantaneously as would be the case with a simple log designs. Section 3 discusses the different methods and static nonlinearity. Furthermore the large number of design implementation for a complete design. Section 4 compares choices makes it nearly impossible to draw a generic com- the behavior of several different designs using both example pressor block diagram that would be valid for the majority compression curves and formal objective measures. Con- of real world compressors. “No two compressors sound clusions and recommendations for designs are provided in ... alike each one is inaccurate in its own unique way.” [3] Section 5. Some differ in topology, others introduce additional stages, and some simply differ from the precise digital design since these deviations add character to the compressor. However 1 COMPRESSOR CONTROLS 1 Accompanying audio samples and source code (in Mat- A compressor has a set of controls directly linked to lab and as VST plug-ins) for the compressor designs and compressor parameters through which one can set up the analysis are available from www.elec.qmul.ac.uk/digitalmusic/ effect. The most commonly used compressor parameters audioengineering/ may be defined as follows. J. Audio Eng. Soc., Vol. 60, No. 6, 2012 June 399 GIANNOULIS ET AL. PAPERS 0 2 PRINCIPLE OF OPERATION -5 Knee Width The signal entering the compressor is split in two copies. One is sent to a variable-gain amplifier (the gain stage) (dB) -10 G y and the other to the side-chain where the gain computer, a -15 circuit controlled by the level of the input signal, applies -20 the required gain reduction to the gain stage. The copy of -25 the signal entering the sidechain has its bipolar amplitude Make-up No compression converted into a unipolar representation of level. When -30 Gain 5:1 compression, hard knee 5:1 compression, soft knee the level of the signal is determined by its absolute value -35 (instantaneous signal level), this is known as peak-sensing. An alternative is to use RMS-sensing and determine the -40 Threshold signal level by its root-mean-square (RMS) value. -45 -45 -40 -35 -30 -25 -20 -15 -10 -5 0 2.1 The Gain Stage xG (dB) The gain stage is responsible for attenuating the input Fig. 1. Static compression characteristic with make-up gain and signal by a varied amount of decibels (dB) over time. The hard or soft knee. heart of every compressor is the element that applies this gain reduction: a voltage controlled amplifier (VCA) that attenuates the input signal according to an external control Threshold defines the level above which compression voltage (hereafter denoted c) coming from the side chain. starts. Any signal overshooting the threshold will be re- Building a VCA with analog components is non-trivial and duced in level. thus many approaches have been applied. Optical com- Ratio controls the input/output ratio for signals over- pressors like the Teletronix LA-2A use a light-dependent shooting the threshold level. It determines the amount of resistor (LDR) as the bottom leg of a voltage divider lo- 2 compression applied. cated within the signal path. The control voltage is used Attack and release times provide a degree of control over to drive a light source which will then—with increasing how quickly a compressor acts. They are also known as brightness—lower the resistance of the LDR and therefore, time constants, although the latter is usually referenced to apply the necessary gain reduction. A similar approach is dB, denoting the gain decrease in dB that the compressor taken by the Universal Audio 1176 compressor, but instead will apply for the given attack time and the opposite for of using an LDR, they used the drain-to-source resistance 3 the release time. Instantaneous compressor response, as de- of a field effect transistor (FET compressor). The con- scribed in [4], is not sought because it introduces distortion trol voltage could then be applied to the gate terminal in on the signal. order to lower the FET’s resistance [5]. An even earlier ap- The attack time defines the time it takes the compressor proach used in so-called variable-mu compressors like the 4 to decrease the gain to the level determined by the ratio Fairchild 670 exploited the fact that altering the grid-to- once the signal overshoots the threshold. The release time cathode voltage would change the gain of a tube amplifier. defines the time it takes to bring the gain back up to the More modern compressor designs make use of specialized normal level once the signal has fallen below the threshold. integrated VCA circuits. These are much more predictable A Make-Up Gain control is usually provided at the com- than the earlier approaches and offer improved specifica- pressor output. The compressor reduces the level (gain) of tions (such as less harmonic distortion and a higher usable the signal; therefore, applying a make-up gain to the signal dynamic range). allows for matching the input and output loudness level. In a solely digital design, one can model an ideal VCA The Knee Width option controls whether the bend in the as multiplying the input signal by a control voltage coming compression characteristic (see Fig. 1) has a sharp angle or from the sidechain. If x[n] denotes the input signal, y[n] has a rounded edge. The Knee is the threshold-determined the output signal and c[n] the control voltage then y[n] = point where the input-output ratio changes from unity to c[n]·x[n]. In addition, make-up gain is often used to add a a set ratio. A sharp transition is called a Hard Knee and constant gain back to the signal in order to match output provides a more noticeable compression. A softer transition and input levels. In a digital compressor we can easily where the ratio gradually grows from 1:1 to a set value in implement a make-up gain by multiplying the compressor’s a transition region on both sides of the threshold is called a output by a constant factor corresponding to the desired Soft Knee. It makes the compression effect less perceptible. make-up gain value. So, representing the signals in decibels, Depending on the signal one can use hard or soft knee, where M is the make-up gain, with the latter being preferred when we want less obvious ydB[n] = xdB[n] + cdB[n] + M. (1) (transparent) compression. A compressor has a set of additional controls most of 2 www.recproaudio.com/diy pro audio/diy files/la2a/la2a which are found in most modern compressor designs. These 1968.jpg include a Hold parameter, Side-Chain filtering, Look- 3 www.gyraf.dk/gy_pd/1176/1176sch.gif Ahead, and many more.
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