SELECTION AND OPERATION AUDIO SIGNAL PROCESSORS By Gino Sigismondi A Shure Educational Publication Selection and Operation of Table of Contents AUDIO SIGNAL Processors Introduction . 4 What Are Audio Signal Processors? . 4 What Types of Problems Can Benefit from Audio Processing? . 5 Feedback . 6 Chapter 1 Types of Audio Processors . 7 1.1 Volume (Gain) Control . 7 1.2 Filters and Equalization . 7 1.3 Dynamics Processors . 12 1.4 Delay . 16 1.5 Adaptive Audio Processors . 17 Chapter 2 Practical Applications For Audio Signal Processors . 23 2.1 Maximizing Gain-Before-Feedback . 23 2.2 Improving Speech Intelligibility . 24 2.3 Sound System Gain Structure . 26 2.4 Digital Signal Processing . 28 Reference Information . 30 Appendix A: Sound Waves . 30 Appendix B: Potential Acoustic Gain (PAG) and Needed Acoustic Gain (NAG) . 32 Glossary . 34 Shure Product Selection Charts . 36 Bibliography . 38 Acknowledgements . 38 About the Author . 39 Audio Signal Processors 3 Selection and Operation of AUDIO SIGNAL Processors Introduction For any sound system, the primary goal is good if the three primary measures are not satisfied, any sound. What, however, constitutes "good" sound? The subjective terms take on even less importance. Speech three primary measures of good sound are audibility, that is "warm" but unintelligible does the listener little good. intelligibility, and fidelity. Many factors contribute to the Audio signal processors offer a variety of tools to assist quality of the sound, including the quality of the sound in optimizing a sound system for audibility, intelligibility, sources, the sound system, and the room acoustics. and fidelity. While not usually essential for a sound system The audibility of speech or music at the furthest to operate (i.e., provide high-level sound reinforcement listener must be sufficient to achieve the desired effect: of low-level sources), audio signal processors can be usually a comfortable listening level for speech, and more invaluable tools in sound system design. A basic sound powerful levels for certain kinds of music. These levels system consists of four components: should be attainable without distortion or feedback. • Input devices (microphones, CD players, etc) Intelligibility is determined by the signal-to-noise ratio and • Mixers (to combine inputs, control levels, direct-to- reverberant ratio at the listener’s ear. The "signal" and provide preamplification, if necessary) is the desired sound source (speech, musical instruments, • Amplifiers etc.), while the "noise" is ambient sound in the room as • Output devices (loudspeakers) well as electrical noise produced by the sound system. Audio signal processors are typically employed Maximum speech intelligibility requires a speech level of within or just after the mixer stage, but before amplification. at least 20 dB above the noise floor at the listener’s ear. (See Figure 1-1.) A processor can be used at the input The direct-to-reverberant ratio is determined by the stage, but since most processors are designed to operate directivity of the loudspeakers and the reverberation with line level sources this is rare. Signal processors can characteristics of the room. High levels of reverberation be analog or digital, single- or multi-function, stand-alone can severely degrade intelligibility by making it difficult to devices or integrated with other components in the sound distinguish the end of one word and the start of the next. system. Most signal processors originated as stand alone Finally, fidelity of sound is primarily defined by the overall devices designed for a specific purpose. Over time, frequency response of the sound arriving at the listener’s integration of similar processors into one device became ear. The frequency range must be sufficiently wide and popular (e.g. compressor/limiters). The development of relatively uniform in order to provide realistic and accurate audio processors that operate in the digital domain allowed reinforcement of speech and music. Every component in for further integration, leading to multi-function digital signal the signal chain contributes to this, and a limitation at any processors (DSP) that combine seemingly disparate point will affect the fidelity of the entire system. functions into a single unit. Perhaps more importantly, DSP Other more subjective terms may be applied to good devices offer these functions at a cost that is a fraction of sound ("warmth", "punch", etc.), but these colloquialisms the purchase price of several individual processors. are not measurable in any meaningful way. Additionally, Introduction 4 Selection and Operation of AUDIO SIGNAL Processors INSTRUMENT MICROPHONE AMPLIFIER LOUDSPEAKER VOCAL MICROPHONE BOUNDARY MIXER PROCESSOR MICROPHONE Figure 1-1: basic sound system What Types of Problems Can Benefit from effect on the sound. Reverberation can be reduced only by Audio Processing? absorptive acoustic treatment or structural modification; To understand the purpose of audio signal processing, electronics cannot remove it. If additional acoustic it is necessary to examine the problems encountered in treatment is not an option, directional loudspeakers allow a typical sound system. Note that an audio processor the sound to be "aimed" toward the listener and away from cannot solve all the potential problems in a sound reflective surfaces. Simply raising the level of the sound reinforcement system. The most common problems are system will only aggravate the problem by raising the listed to the on the next page: reverberation level as well. Long reverberation times The importance of good room acoustics cannot be severely reduce intelligibility. In audio teleconferencing underestimated. In any room where sound reinforcement systems, this results in a hollow, or "bottom-of-the-barrel" will be used, excess reverberation times introduce a sound received by the remote site. myriad of problems that cannot be solved by any audio processors. Reverberation time is the length of time that a sound persists in a room after the sound source has Want to know more about proper microphone usage? stopped. All attempts should be made to keep unwanted Shure offers the following educational guides free sounds from entering the microphone in the first place. of charge: The level of desired sound at the microphone should be • Microphone Techniques for Studio Recording at least 30 dB above any ambient sound picked up by the microphone. Proper microphone placement (a full • Microphone Techniques for Live Sound discussion of which is beyond the scope of this publication) Reinforcement is also crucial. A good rule of thumb: always keep Visit shure.com or contact your local Shure microphones as close as possible to the sound source. office (see back cover) to request your Once sound energy is introduced into the acoustic complimentary copies. space by the loudspeaker, processing no longer has any 5 Selection and Operation of AUDIO SIGNAL Processors Feedback Problems: Remedies: Feedback is characterized by a sustained, Feedback Parametric Equalizer/ ringing tone, which can vary from a low rumble to a Automatic Mixer/ piercing screech. Echoes and reverberation caused Feedback Reducer by room acoustics, as well as ground buzz and other Poor tone quality (subjective) Graphic equalizer extraneous noises, are not the same thing as feed- Sound source too loud Compressor/Limiter/AGC back, and cannot be cured in the same manner. Sound source too quiet AGC Feedback occurs whenever the sound entering a microphone is reproduced by a loudspeaker, picked Varying signal levels Compressor/Limiter/AGC up by the microphone, and re-amplified again and from multiple sound sources again. The familiar howl of feedback is an oscillation Unwanted noise Noisegate/Downward expander that is triggered by sound entering the microphone. Unexpected transients Compressor/Limiter/No overshot The easiest way to (intentionally) create feedback is to ("Look-ahead") Peak Limiter point a microphone directly into a loudspeaker. Comb filtering Automatic Microphone Mixer Placing the microphone too close to the loudspeaker, too far from the sound source, or simply turning the due to open microphones microphone up too loud exacerbates feedback Frequency response Delay problems. Other contributing factors are too many anomalies due to open microphones, poor room acoustics, and uneven misaligned loudspeakers frequency response in either the microphones or Poor intelligibility Parametric Equalizer/ loudspeakers. Automatic Microphone Mixer The single easiest way to reduce feedback is to move the microphone closer to the desired sound Acoustic echoes Acoustic Echo Canceller source. Additionally, using a directional microphone (in teleconferencing systems) (cardioid, supercardioid, etc.) will slightly increase the Distortion Compressor/Limiter amount of gain-before-feedback. Reducing the (due to wide dynamic range) number of open microphones with an automatic mixer will also improve the situation. Try to keep Problems that cannot be solved by audio processing: microphones and lou speakers as far away from each other as possible. Lastly, acoustically treat the room • Echoes because of poor room acoustics to cover hard, reflective surfaces such as glass, • Poor sound due to excess room reverberation times marble, and wood. Realize, though, that in certain • Feedback caused by operating beyond the limits of PAG rooms long reverberation times may be desirable, (see Appendix 2) such as a house of worship used for acoustic music performance. • Noise