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Equalization Methods with True Response Using Discrete Filters

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Equalization Methods with True Response Using Discrete Filters Audio Engineering Society Convention Paper 6088 Presented at the 116th Convention 2004 May 8–11 Berlin, Germany This convention paper has been reproduced from the author's advance manuscript, without editing, corrections, or consideration by the Review Board. The AES takes no responsibility for the contents. Additional papers may be obtained by sending request and remittance to Audio Engineering Society, 60 East 42nd Street, New York, New York 10165-2520, USA; also see www.aes.org. All rights reserved. Reproduction of this paper, or any portion thereof, is not permitted without direct permission from the Journal of the Audio Engineering Society. Equalization Methods with True Response using Discrete Filters Ray Miller Rane Corporation, Mukilteo, Washington, USA [email protected] ABSTRACT Equalizers with fixed frequency filter bands, although successful, have historically had a combined frequency response that at best only roughly matches the band amplitude settings. This situation is explored in practical terms with regard to equalization methods, filter band interference, and desirable frequency resolution. Fixed band equalizers generally use second-order discrete filters. Equalizer band interference can be better understood by analyzing the complex frequency response of these filters and the characteristics of combining topologies. Response correction methods may avoid additional audio processing by adjusting the existing filter settings in order to optimize the response. A method is described which closely approximates a linear band interaction by varying bandwidth, in order to efficiently correct the response. been considered as important as magnitude response because it was less audible. Still, studies have 1. BACKGROUND confirmed that it is audible in some situations [1,2]. Minimum phase is often chosen for its economy and The audio graphic equalizer has evolved into a set of because it is appropriate for correcting a system with around thirty filters at fixed frequencies, covering the minimum phase characteristics, which may be cancelled audio range. The operator has adjusted the level of each by minimum phase filtering without adding time delay individually, either to correct a magnitude response [3]. Minimum phase filters might also be appropriate variation, or to create one intentionally. This degree of for magnitude correction of system responses with control has remained popular despite the recent unknown phase characteristics. Today, mainly due to advances of technology. Digital Signal Processing DSP, non-minimum phase shift designs have become (DSP) has made it practical to provide many times this more practical, and linear phase with its constant group resolution, to the point where the term arbitrary delay is the most common example. magnitude response is considered applicable. The graphic equalizer with front panel sliders As is well known, equalization has phase shift as a displaying a response curve may be easily mathematical requirement. Phase shift has not always misunderstood as displaying the actual magnitude Miller Equalization Methods with True Response response. Although discrete filters can be designed with filter significantly affects no more than the first few narrow bandwidths, even approaching a filter shape neighboring frequency bands. Figure 1 shows the with a flat top and steep cutoff is expensive, so in combined response for several topologies. The resulting practice each filter has an effect over a wider range of frequency response then doesn’t match the settings (at frequencies than it is intended to affect. The filter band center frequencies). The nature of the combined response curves still have significant magnitude at response depends on the filter combining topology. neighboring filter band frequencies. In practice, each dB 12 Cascaded Band 10 Settings Interpolated Constant-Q 8 Parallel 6 4 2 0 Figure 1: Combined Responses – Settings at +6 dB combined into one filter. The number of bands is Using discrete filters, techniques can be applied to limited only by the size of the filter. An FIR filter may counteract this filter skirt interaction, and produce what be designed to approximate the impulse response of is termed “true response”, that is, the frequency many IIR filters, and provides the ultimate in flexibility, response closely matches the controls. Each band where magnitude and phase may be adjusted more becomes independent, or very nearly so. Otherwise, to specifically and semi-independently. In order to support be effective, an operator must be very accustomed to the low frequencies, many thousands of taps are required. In product’s particular filter and combining behavior. order to economically implement this, complicated methods like multirate processing or the use of FFT for Equalizer filters began as analog second-order fast circular convolution are used. These methods are filters, and have since been implemented as digital IIR outside the scope of this paper with some examples in filters. Equalization curves with complex shapes can [5,6,7,8]. also be accomplished with single large FIR filters, allowing multiple frequency band settings to be AES 116th Convention, Berlin, Germany, 2004 May 8–11 Page 2 of 17 Miller Equalization Methods with True Response Banks of IIR filters have some advantages over presence of a complex musical signal, critical bands single FIR filters, namely simplicity and speed of limit our ability to hear narrow response variations, design, and better efficiency for a limited number of because of masking caused by closely spaced frequency bands that include low design frequencies [4]. Analog components. Signals with fewer spectral components filters are designed once, while a digital filter may need enable perception of much narrower response to be redesigned when any parameter changes. Small variations. IIR filters can be adjusted (redesigned) quickly, compared to large FIR filters. Even assuming unlimited perceptual abilities, there are other practical limitations. A typical room is not Equalizers are usually adjusted manually, but in anechoic, and has more than one listener position, so it theory an automated analysis of the sound system may has a response that can’t be perfectly flattened. The be used. Care then must be taken with correcting frequency and phase response vary with listener nonminimum phase responses, where matching the position because of room reflections and off-axis phase requires added delay. Also, equalization of deep speaker response variations. The most that can be done notches in the response should not be attempted because is to flatten the speaker/crossover and make some of potential adverse effects in other areas of a room, as compromise for the room reflections [17,18]. Of course well as the possibility of amplifier overload. These a good sound system won’t have too many severe techniques are also beyond the scope of this paper. narrow variations. It appears that one-sixth octave resolution matches 2. DISCRETE FILTERS AND REQUIRED the minimum size of the critical bands and should work FREQUENCY RESOLUTION very well, particularly if the center frequencies could be adjusted more finely. One-third octave matches low Because of the sheer quantity of filters, using frequency critical bands, and should be sufficient at discrete filters generally limits the practical frequency higher frequencies, particularly if a parametric equalizer resolution to one-third octave. How much of a is available to tackle areas requiring better resolution. limitation this is is debatable, and the one-third octave The ideal equalizer might have 25 times the resolution equalizer is widely recognized as effective, familiar, and to match our psychophysical resolution, but at greater easy to use. expense and with marginal improvement for a good sound system. A discrete filter equalizer is the most practical choice for analog designs, given the difficulty of making 3. FILTER RESPONSE AND BAND transversal (FIR) analog filters. Second order discrete INTERACTIONS filters have been universally used, and their digital versions are designed using equations that can be fairly simple (or sophisticated) [9-14]. These equalizers have The discrete equalizer filters are probably always proven to be cost-effective, popular, and effective. Note based on second-order bandpass filters. Higher order that thirty bands at one-third octave spacing have an filters have been shown to have so much phase shift that effect over roughly ten octaves, covering the widely they are impractical for use in equalizers [19]. An accepted 20 Hz to 20 kHz range. examination of the analog second-order bandpass filter helps in understanding the equalizer characteristics, The ideal equalizer would have unlimited frequency whether it is implemented as an analog circuit or in resolution, but what is really useful? Human hearing DSP. This filter has a magnitude response that can be has limited resolution, with a physical component quite sharp at its peak, but which asymptotically defined by the critical bands, and a psychophysical pitch approaches a 6.02 dB per octave slope farther away, as resolution that’s around 25 times better [15,16]. The shown in Figure 2, along with the phase response. This critical bands are around one-sixth octave wide above 1 gives it the potential for significant effect over a wide kHz, increasing beyond one-third octave below this. frequency range. Its phase response is zero degrees at Their exact characteristics may be a matter of debate. center, and approaches 90 degrees at low frequencies They are not at fixed frequencies but are a physical and –90 degrees at high frequencies. Its Nyquist plot ability to resolve a combination of frequencies. In the traces a circular path in the complex plane as shown in AES 116th Convention, Berlin, Germany, 2004 May 8–11 Page 3 of 17 Miller Equalization Methods with True Response 0 10 dB 20 30 100 Degrees 0 -100 Figure 2: Bandpass Magnitude with the 6 dB/octave Asymptotes and Phase Below Figure 3, with some analysis of this in Appendix I. The magnitude and phase are given by the length and angle of the vector from the origin to a point on the path.
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