Audio crossover
Audio crossovers are a type of electronic filter circuitry that splits an audio signal into two or more frequency ranges, so that the signals can be sent to loudspeaker drivers that are designed to operate within different frequency ranges. The crossover filters can be either active or passive. They are often described as two-way or three-way, which indicate, respectively, that the crossover splits a given signal into two frequency ranges or three frequency ranges. Crossovers are used in loudspeaker cabinets, power amplifiers in consumer electronics and pro audio and musical instrument amplifier products. For the latter two markets, crossovers are used in bass amplifiers, keyboard amplifiers, bass and keyboard speaker enclosures and sound reinforcement system equipment.
Crossovers are used because most individual loudspeaker drivers are incapable of covering the entire audio spectrum from low frequencies to high frequencies with acceptable relative volume and absence of distortion. Most hi-fi speaker systems and sound reinforcement system speaker cabinets use a combination of multiple loudspeaker drivers, each catering to a different frequency band. A standard simple example is in hi-fi and PA system cabinets that contain a woofer for low and mid frequencies and a tweeter for high frequencies. Since a sound signal source, be it recorded music from a CD player or a live band's mix from an audio console, has all of the low, mid and high frequencies combined, a crossover circuit is used to split the audio signal into separate frequency bands that can be separately routed to loudspeakers, tweeters or horns optimized for those frequency bands.
Passive crossovers are probably the most common type of audio crossover. They use a network of passive electrical components to split up an amplified signal coming from one power amplifier so that it can be sent to two or more loudspeaker drivers.
Active crossovers are distinguished from passive crossovers in that they split up an audio signal prior to the power amplification stage so that it can be sent to two or more power amplifiers, each of which is connected to a separate loudspeaker driver. Home cinema 5.1 surround sound audio systems use a crossover that separates out the very-low frequency signal, so that it can be sent to a subwoofer, and then sending the remaining low-, mid- and high-range frequencies to five speakers which are placed around the listener. In a typical application, the signals sent to the surround speaker cabinets are further split up using a passive crossover into a low/mid-range woofer and a high-range tweeter. Active crossovers come in both digital and analog varieties.
Digital active crossovers often include additional signal processing, such as limiting, delay, and equalization. Signal crossovers allow the audio signal to be split into bands that are processed separately before they are mixed together again. Some examples are multiband compression, limiting, de-essing, multiband distortion, bass enhancement, high frequency exciters, and noise reduction such as Dolby A noise reduction.
Overview
The definition of an ideal audio crossover changes relative to the task and audio application at hand. If the separate bands are to be mixed back together again, then the ideal audio crossover would split the incoming audio signal into separate bands that do not overlap or interact and which result in an output signal unchanged in frequency, relative levels, and phase response. This ideal performance can only be approximated. How to implement the best approximation is a matter of lively debate. On the other hand, if the audio crossover separates the audio bands in a loudspeaker, there is no requirement for mathematically ideal characteristics within the crossover itself, as the frequency and phase response of the loudspeaker drivers within their mountings will eclipse the results. Satisfactory output of the complete system comprising the audio crossover and the loudspeaker drivers in their enclosure is the design goal. Such a goal is often achieved using non-ideal, asymmetric crossover filter characteristics.Many different crossover types are used in audio, but they generally belong to one of the following classes.
Classification
Classification based on the number of filter sections
Loudspeakers are often classified as "N-way", where N is the number of drivers in the system. For instance, a loudspeaker with a woofer and a tweeter is a 2-way loudspeaker system. An N-way loudspeaker usually has an N-way crossover to divide the signal among the drivers. 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. The BPF section is in turn a combination of HPF and LPF sections. 4 way crossovers are not very common in speaker design, primarily due to the complexity involved, which is not generally justified by better acoustic performance.An extra HPF section may be present in an "N-way" loudspeaker crossover to protect the lowest-frequency driver from frequencies lower than it can safely handle. Such a crossover would then have a bandpass filter for the lowest-frequency driver. Similarly, the highest-frequency driver may have a protective LPF section to prevent high-frequency damage, though this is far less common.
Recently, a number of manufacturers have begun using what is often called "N.5-way" crossover techniques for stereo loudspeaker crossovers. This usually indicates the addition of a second woofer that plays the same bass range as the main woofer but rolls off far before the main woofer does.
''Remark: Filter sections mentioned here is not to be confused with the individual 2-pole filter sections that a higher-order filter consists of.''
Classification based on components
Crossovers can also be classified based on the type of components used.Passive
A passive crossover splits up an audio signal after it is amplified by a single power amplifier, so that the amplified signal can be sent to two or more driver types, each of which cover different frequency ranges. These crossover are made entirely of passive components and circuitry; the term "passive" means that no additional power source is needed for the circuitry. A passive crossover just needs to be connected by wiring to the power amplifier signal. Passive crossovers are usually arranged in a Cauer topology to achieve a Butterworth filter effect. Passive filters use resistors combined with reactive components such as capacitors and inductors. Very high-performance passive crossovers are likely to be more expensive than active crossovers since individual components capable of good performance at the high currents and voltages at which speaker systems are driven are hard to make.Inexpensive consumer electronics products, such as budget-priced Home theater in a box packages and low-cost boom boxes, may use lower quality passive crossovers, often utilizing lower-order filter networks with fewer components. Expensive hi-fi speaker systems and receivers may use higher quality passive crossovers, to obtain improved sound quality and lower distortion. The same price/quality approach is often used with sound reinforcement system equipment and musical instrument amplifiers and speaker cabinets; a low-priced stage monitor, PA speaker or bass amplifier speaker cabinet will typically use lower quality, lower priced passive crossovers, whereas high-priced, high-quality cabinets typically will use better quality crossovers. Passive crossovers may use polypropylene, metalized polyester foil, or electrolytic capacitors. Inductors may have air cores, powdered metal cores, ferrite cores, or laminated silicon steel cores, and most are wound with enameled copper wire.
Some passive networks include devices such as fuses, PTC devices, bulbs or circuit breakers to protect the loudspeaker drivers from accidental overpowering. Modern passive crossovers increasingly incorporate equalization networks that compensate for the changes in impedance with frequency inherent in virtually all loudspeakers. The issue is complex, as part of the change in impedance is due to acoustic loading changes across a driver's passband.
Two disadvantages of passive networks are that they may be bulky and cause power loss. They are not only frequency specific, but also impedance specific. This prevents their interchangeability with speaker systems of different impedances. Ideal crossover filters, including impedance compensation and equalization networks, can be very difficult to design, as the components interact in complex ways. Crossover design expert Siegfried Linkwitz said of them that "the only excuse for passive crossovers is their low cost. Their behavior changes with the signal level-dependent dynamics of the drivers. They block the power amplifier from taking maximum control over the voice coil motion. They are a waste of time, if accuracy of reproduction is the goal." Alternatively, passive components can be utilized to construct filter circuits before the amplifier. This implementation is called a passive line-level crossover.
Active
An active crossover contains active components in its filters, such as transistors and operational amplifiers. In recent years, the most commonly used active device is an operational amplifier. In contrast to passive crossovers, which operate after the power amplifier's output at high current and in some cases high voltage, active crossovers are operated at levels that are suited to power amplifier inputs. On the other hand, all circuits with gain introduce noise, and such noise has a deleterious effect when introduced prior to the signal being amplified by the power amplifiers.Active crossovers always require the use of power amplifiers for each output band. Thus a 2-way active crossover needs two amplifiers—one for the woofer and one for the tweeter. This means that a loudspeaker system that is based on active crossovers will often cost more than a passive-crossover-based system. Despite the cost and complication disadvantages, active crossovers provide the following advantages over passive ones:
- 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, relative levels, etc.
- better isolation of each driver from the signals being handled by other drivers, thus reducing intermodulation distortion and overdriving
- the power amplifiers are directly connected to the speaker drivers, thereby maximizing 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 in power amplifier output requirement. With no energy being lost in passive components, amplifier requirements are reduced considerably, reducing costs, and potentially increasing quality.