Audio crossover

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Audio crossovers are a class of electronic filters designed specifically for use in audio applications, especially hi-fi. Commonly used loudspeaker drivers are incapable of covering the entire audio spectrum with acceptable loudness and lack of distortion by themselves. Thus, crossovers serve the purpose of splitting the audio signal into separate frequency bands which can be handled by individual loudspeaker drivers optimized for those bands. A combination of multiple drivers each catering to a different frequency band is the design pattern for most hi-fi speaker systems. An audio crossover may also be constructed mechanically and is commonly found in full-range speakers, portions of whose cones/dust caps/whizzer cones are decoupled at progressively higher frequencies.

Another use of crossovers is multiband processing, in which the audio signal is split into bands, which are adjusted (equalized, compressed, echoed, etc) separately. After the adjustments, the individual bands are mixed together again. Some examples are: multiband dynamics (compression, limiting, de-essing), multiband distortion, bass enhancement, high frequency exciters, noise reduction (for example: Dolby A noise reduction).

Contents

[edit] Overview

Comparison of the magnitude response of 2 pole Butterworth and Linkwitz-Riley crossover filters. The summed output of the Butterworth filters has a +3dB peak at the crossover frequency.
Comparison of the magnitude response of 2 pole Butterworth and Linkwitz-Riley crossover filters. The summed output of the Butterworth filters has a +3dB peak at the crossover frequency.

An ideal audio crossover would split the incoming audio signal into separate bands that do not overlap or interact and which, when added together, output the signal unchanged in both frequency, relative levels, and phase response. This behavior cannot be achieved in practice, but can be approximated. Just which of the approximations possible is best, or possible, is a matter of lively debate. Many different crossover types are used, but they generally fall under one of the classifications provided below.

[edit] Crossover classification

[edit] Classification based on the number of filter sections

In loudspeaker specifications, one often sees a speaker classified as an "N-way" speaker. N is a positive whole number greater than 1, and it indicates the number of filter sections. A 2-way crossover consists of a low-pass and a high-pass filter. A 3-way crossover is constructed as a combination of low-pass, band-pass and high-pass filters (LPF, BPF and HPF respectively). The BPF section is in turn a combination of HPF and LPF sections. 4 (or more) way crossovers are not very common in speaker design, primarily due to the complexity involved, which is not generally justified by better acoustic performance.

Recently, a number of manufacturers have developed what is often called "N.5-way" crossover techniques. This usually indicates the addition of an extra bass driver (commonly termed a subwoofer when limited to only low frequencies below all other drivers) with a crossover designed such that it augments the bass response or compensates for cabinet diffraction effects.

Remark: Filter sections mentioned here is not to be confused with the individual 2-pole filter sections that a higher order filter consists of.

[edit] Classification based on components

Crossovers can also be classified based on the design approach (ie, more or less, the type of components used).

[edit] Passive crossovers

A passive crossover is made entirely of passive components, arranged most commonly in a Cauer topology to achieve a Butterworth filter. Passive filters use both non reactive resistors, and reactive components such as capacitors and inductors. Passive crossovers are usually more expensive than active crossovers when intended for high-end systems. The use of non inductive resistors and special capacitor components add up to the most demanding standards, since they all will be subjected to the audio signal path. Polypropylene and mica or oil capacitors are common, as well as air core inductors of high purity copper. Well designed Butterworth filters add no ripple to the pass-band, and behave in a linear manor over it's operating spectrum. On the negative side,

  • These tend to be bulky and always cause power loss.
  • The designs are not only frequency specific, but also impedance specific to the charge or load it will be subjected to. Hence each passive crossover is uniquely designed, and seldom to never will be apropriate for other speakers.
  • They tend to exhibit a roll off that is slighly less steep than the theoretical values.
  • Correct designs are difficult to achieve, even with the help of specialized books. This is especially true on designing crossovers of third and fourth order.
  • For best results on speaker loads, anti-resonant and non-inductive cells must be added, increasing the difficulty of the design.

[edit] Active crossovers

An active crossover contains active components (ie, those with gain) in its filters. In recent years, the most commonly used active device is an op-amp; active crossovers are operated at levels suited to power amplifier inputs in contrast to passive crossovers which operate after the power amplifier's output, at high voltage and current levels. On the other hand, all circuits with gain introduce noise, and in this case, unless carefully designed, the noise will be amplified by the power amplifiers, perhaps unacceptably.

Active crossovers always require the use of power amplifiers for each output band. Thus a 2-way active crossover needs two amplifiers — one each for the woofer and tweeter. This means that an active crossover based system will often cost more than a passive crossover based system, although none of the amplifiers needs to provide output as high as for an equivalent sound level full-frequency, power amplifier, which reduces cost. Generally, active crossovers require use of tweeter protection (typically a small capacitor) since the tweeter is now directly connected to an amplifier and may be damaged due to DC in that output, or a short thump produced when an amplifier turns on or off. The cost and complication disadvantages of active crossovers are offset by the following gains:

  • a frequency response independent of the dynamic changes in a driver's electrical characteristics.
  • typically, the possibility of an easy way to vary or fine tune each frequency band to the specific drivers used. Examples would be crossover slope, filter type (eg, Bessel, Butterworth, etc), relative levels, ...
  • isolation of each driver from signals handled by drivers, thus reducing intermodulation distortion and overdriving
  • The power amplifiers are directly connected to the speaker drivers, thereby maximising amplifier damping control of the speaker voice coil, reducing consequences of dynamic changes in driver electrical characteristics, all of which are likely to improve the transient response of the system
  • reduction is power amplifier cost and output requirement. With no energy being lost in passive components, amplifier requirements are reduced considerably (up to 1/2 in some cases), reducing costs, and potentially increasing quality.

[edit] Mechanical crossovers

Main article: Full-range

This crossover type is mechanical and uses the properties of the materials in a driver diaphragm to achieve the necessary filtering. Such crossovers are commonly found in full-range speakers which are designed to cover as much of the audio band as possible. One such is constructed by coupling the diaphragm of the speaker to the voice coil through a compliant section and directly attaching a small light-weight cone called whizzer to the voice coil. The compliant section is intended to ensure that the primary diaphragm responds only to lower frequencies while the whizzer, which is directly coupled to the voice coil, responds only to the rapid movements of the coil at higher frequencies. This combination results in the main diaphragm having an upper cut-off frequency while the characteristics of the whizzer and voice coil set the lower limit to the whizzer's response, thereby implementing a crossover action. The choice/weight of materials used for the diaphragm, whizzer and the speaker's suspension determine the crossover frequency and the effectiveness of the crossover. This sort of crossover is much more complex to design, especially if the highest degree of performance is desired. Extensive trial and error is required. A prominent example of whizzer cone full-range drivers are the Fostex line of drivers.

An alternative is to use the dust cap as a high frequency radiating device, also crossed over by mechanical compliance from the primary diaphragm. High frequency dispersion is somewhat different for this approach than for whizzer cones. A third possibility is to build the primary cone with such curvature, and with such materials, that it effects a mechanical crossover in its own motion; the inner ring radiates most at high freqencies while the entire cone radiates at low freqencies. Currenlty, the E J Jordan range of full-range drivers use the last approach.

Speakers which use these mechanical crossovers have some advantages in sound quality despite the difficulties of designing and manufacturing them, and despite the inevitable output limitations. Full-range drivers have a single acoustic center at all frequencies, and can have reduced phase change across the audio spectrum. For best performance at lower frequencies, these drivers require careful enclosure design. If used conventionally, their small size requires considerable cone excursion at low freqencies, and few of these drivers allow this in the interest of reasonable high frequency performance. But wihtin these constraints, cost and complication are reduced -- no passive or active crossover required.

Due to the way these crossovers are implemented, drivers using them are almost always 2-way. Those who do not prefer the sound of fullrange drivers (eg, a lack of powerful bass and usually strong highs) sometimes argue that the act of making a single diaphragm respond separately to low and high frequencies dooms it to do neither justice. See full-range speaker for construction details.

[edit] Digital Crossovers

Active crossovers can be implemented digitally using a DSP chip or a microprocessor. They either use the digital approximations of traditional analog IIR filters (Bessel, Butterworth, Linkwitz-Riley etc.), having similar character to the analog versions, or they use Finite impulse response (FIR) filters. FIR filters can be constructed easily using DSP chips or microprocessors. They usually have a higher order, but their behaviour is different. They can be designed and built so that they have a linear phase response, which is desirable sound reproduction. As a result, they are often used as crossovers in digital signal processing.

[edit] Classification based on filter order or slope

Just as filters have different orders, so do crossovers, depending on the filter slope they implement. The final acoustic slope may be completely determined by the electrical filter or may be achieved by combining the electrical filter's slope with the natural characteristics of the driver. In the former case, the only requirement is that each driver has a flat response at least to the point where its signal is approximately -10dB down from the passband. In the latter case, the final acoustic slope is usually steeper than of the electrical filters used. A third or fourth order acoustic crossover often has just a 2nd order electrical filter. This requires that speaker drivers be well behaved a considerable way from the nominal crossover frequency, and further that the high freguency driver be able to survive a considerable input in a frequency range below its crossover point. This is difficult in actual practice. In the discussion below, the characteristics of the electrical filter order is discussed, followed by a discussion of crossovers having that acoustic slope and their advantages or disadvantages.

Most audio crossovers use first to fourth order electrical filters. Higher orders are not generally implemented in passive crossovers for loudspeakers, but are sometimes found in electronic equipment under circumstances for which their considerable cost and complexity can be justified.

[edit] First order crossovers

1st order filters have a 20 dB/decade (or 6 dB/octave) slope. All 1st order filters have a Butterworth filter characteristic. 1st order filters are considered by many audiophiles to be ideal for crossovers. This is because this filter type is 'transient perfect', meaning it passes both amplitude and phase unchanged across the range of interest. It also uses the fewest parts and has the lowest insertion loss (if passive). A 1st order crossover allows signal at unwanted frequencies to get through in the LPF and HPF sections than other configurations. While woofers can easily take this (aside from generating distortion at frequencies above those they can properly handle), smaller high freqnecy drivers (especially tweeters) are more likley to be damaged since they are not capable of handling large power inputs at lower frequcnies than their crossovers.

In practice, speaker systems with true first order acoustic slopes are difficult to design because they require large overlapping driver bandwidth, and the shallow slopes mean that non-coincident drivers interfere over a wide frequency range and cause large response shifts off-axis.

[edit] Second order crossovers

2nd order filters have a 40 dB/decade (or 12 dB/octave) slope. 2nd order filters can have a Bessel, Linkwitz-Riley or Butterworth characteristic depending on design choices and the components used. This order is commonly used in passive crossovers as it offers a reasonable balance between complexity, response, and higher frequency driver protection. When designed with time aligned physical placement, these crossovers have a symmetrical polar response, as do all even order crossovers.

It is commonly thought that there will always be a phase difference of 180° between the outputs of a (second order) low-pass filter and a high-pass filter having the same crossover frequency. And so, in a 2-way system, the high-pass sections output is usually connected to the high frequency driver 'inverted', to correct for this phase problem. For passive systems, the tweeter is wired with opposite polarity to the woofer; for active crossovers the high-pass filter's output is inverted. In 3-way systems the mid-range driver or filter is inverted. However, this is generally only true when the speakers have a wide response overlap and the acoustic centers are physically aligned.

[edit] Third order crossovers

3rd order filters have a 60 dB/decade (or 18 dB/octave) slope. These crossovers usually have Butterworth filter characteristics; phase response is very good, the level sum being flat and in phase quadrature, similar to a first order crossover. The polar response is asymmetric. In the original D'Appolito MTM arrangement, a symmetrical arrangement of drivers is used to create a symmetrical off axis response when using 3rd order crossovers.

Third order acoustic crossovers are often built from first or second order filter circuits.

[edit] Fourth order crossovers

4th order filters have an 80 dB/decade (or 24 dB/octave) slope. These filters are complex to design in passive form. A 4th order crossover with −6 dB crossover point and flat summing is also known as a Linkwitz-Riley crossover (named after its inventors). It can be constructed in active form by cascading two 2nd order Butterworth filter sections. The output signals of this crossover order are in phase, thus avoiding phase inversion in driver connections unless the driver acoustic centers are not aligned.

[edit] Higher order crossovers

Passive crossovers giving acoustic slopes higher than 4th order are not common, because of cost and complexity. They are sometimes used in active crossover modules.

[edit] Mixed order crossovers

Crossovers can also be constructed with mixed order filters. For example, a second order lowpass combined with a third order highpass. These are generally passive and are used for several reasons, often when the component values are found by computer program optimization. A higher order tweeter crossover can sometimes help compensate for the time offset between the woofer and tweeter, caused by non aligned acoustic centers.

[edit] Classification based on circuit topology

Series and parallel crossover topologies. The HPF and LPF sections for the series crossover are interchanged with respect to the parallel crossover since they appear in shunt with the low & high frequency drivers.
Series and parallel crossover topologies. The HPF and LPF sections for the series crossover are interchanged with respect to the parallel crossover since they appear in shunt with the low & high frequency drivers.

[edit] Parallel crossovers

These are by far the most common. Electrically the filters are in parallel and thus the various filter sections do not interact. This makes them easier to design because the sections can be considered separately, and because component tolerance variations will be isolated.

[edit] Series crossovers

Crossovers using this topology are almost always passive because it is easiest to construct in passive form. In this topology, the individual filters are connected in series, with a driver or driver combination connected in parallel to each filter. As can be seen in the image, a low-pass filter in shunt with the tweeter results in a high-pass response for the tweeter, since the lower frequencies are shunted by the LPF via the woofer. Similarly, the HPF in parallel with the woofer shunts away the higher frequencies via the tweeter - a low-pass response for the woofer. One advantage (or disadvantage, depending on how one looks at it) of this crossover, is that the crossover sections interact with each other. Changes in any one component affects both highpass and lowpass sections. To some extent, this makes the crossover somewhat self-balancing - the crossover frequency changes, but the system still sums substantially flat. This characteristic makes them appealing to designers not using using sophisticated measuring equipment or simulation software. These crossovers are also more sensitive to component tolerance variations.

[edit] References

[edit] See also

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