Electrical audio signal processing systems and devices – Including frequency control – Having crossover filter
Reexamination Certificate
1999-02-23
2002-04-30
Isen, Forester W. (Department: 2644)
Electrical audio signal processing systems and devices
Including frequency control
Having crossover filter
Reexamination Certificate
active
06381334
ABSTRACT:
BACKGROUND OF THE INVENTION
1. The Field of the Invention
This invention relates generally to electro-acoustic or audio loudspeaker systems. More particularly, the invention relates to a partitioning by frequency of the electrical audio signal from the output of an audio amplifier, into a plurality of frequency bands for presentation to the electro-acoustic transducers within a loudspeaker system.
2. Present State of the Art
Audio systems present as an audible signal simultaneous divergent audio frequencies, for example, music or speech for appreciation by a user. The divergent frequency content of audio may generally be considered to consist of differing frequencies. While an audio system may reinforce or reproduce the electrical audio frequency spectrum in a single pair of wires or inputs to a speaker, specific physical implementations of speaker components are optimized for responding to a compatible band of frequencies. For example, low frequencies tend to be better replicated by physically larger drivers commonly known as woofers. Mid-range frequencies, likewise, are more favorably reproduced by a mid-range sized driver. Additionally, higher frequencies are better reproduced by physically smaller drivers commonly known as tweeters.
While an amplifier may electrically deliver the entire audio frequency spectrum to a speaker over a single pair of wires, it is impractical to expect that the high, middle and low frequencies autonomously seek out the corresponding tweeter drivers, mid-range drivers and woofer drivers within a speaker. In fact, connecting high-power, low-frequency signals to a tweeter driver, will cause audible distortion and will typically cause fatigue and destruction of the tweeter driver.
Therefore, modern higher-fidelity audio system speakers incorporate a crossover electrical network that divides the electrical audio frequency spectrum received in a single pair of wires into distinct frequency bands or ranges and ensures that only the proper frequencies are routed to the appropriate driver. That is to say, a crossover is an electric circuit or network that splits the audio frequencies into different bands for application to individual drivers. Therefore, a crossover is a key element in multiple-driver speaker system design.
Crossovers may be individually designed for a specific or custom system, or may be commercially purchased as commercial-off-the-shelf crossover networks for both two and three-way speaker systems. In a two-way speaker system, high frequencies are partitioned and routed to the tweeter driver with low frequencies being routed to the woofer driver. A two-way crossover, which uses inductors and capacitors, accomplishes this partitioning when implemented as an electrical filter. Crossover networks have heretofore incorporated at least one or more capacitors, and usually one or more inductors, and may also include one or more resistors, which are configured together to form an electrical filter for partitioning the particular audio frequencies into bands for presentation to the appropriate and compatible drivers.
FIG. 1
depicts a typical two-way crossover network within a speaker system. The crossover network of
FIG. 1
may be further defined as a first-order crossover network since the resultant response of each branch of the network attenuates the signal at 6 dB per octave. The graph of
FIG. 1
depicts the responses of a woofer driver and a tweeter driver resulting in a first-order crossover in a two-way speaker system. An amplifier provides a signal into input pair
10
comprised of a positive input
12
and a negative input
14
. In the upper branch
16
of crossover network
8
, the high frequencies are filtered and allowed to pass to high frequency driver
18
. Filtering is performed by capacitor
20
which inhibits the passing of lower frequencies and allows the passing of higher frequencies to high frequency driver
18
. Such a portion of the crossover network is commonly referred to as a “high pass” filter.
Lower frequencies are filtered through branch
22
of crossover network
8
to low frequency driver
24
through the use of a filtering element shown as inductor
26
. This portion of the crossover network is commonly referred to as a “low pass” filter. It should be pointed out that crossover networks typically implement the partitioning of the frequencies into bands through the use of network branches which are parallelly configured across positive input
12
and negative input
14
of input pair
10
. The graph of
FIG. 1
illustrates the frequency responses of woofer and tweeter drivers resulting from the two-way crossover network
8
. Crossover network
8
is depicted as a first order crossover in a two-way speaker system. The low frequency or woofer response
28
begins rolling off at approximately 200 Hertz. As depicted in
FIG. 1
, at approximately 825 Hertz, the woofer response
28
is attenuated to a negative 3 dB from the reference response of 0 dB. Tweeter response
30
is increasing in magnitude at a rate of 6 dB per octave and at 825 Hertz is also a negative 3 dB from the reference response of 0 dB. However, after 825 Hertz, tweeter response
30
increases to 0 dB while woofer response
28
continues to roll off at a rate of 6 dB per octave. The intersection of the curves depicting the woofer and tweeter response defines the “crossover frequency.” Frequencies above the crossover frequency presented at input pair
10
increasingly follow the lower impedance path of branch
16
terminating at the high frequency or tweeter driver
18
rather than the higher impedance path, through branch
22
which leads to the low frequency or woofer driver
24
. An implementation for selection of the crossover frequency must be carefully evaluated and selected by weighing certain characteristics to avoid further difficulties or less than ideal matching of the crossover network to the drivers of the speaker system.
FIG. 1
depicts a first-order crossover network which has a characteristic rate of attenuation of 6 dB per octave.
FIG. 2
depicts a second-order crossover network which has a characteristic rate of attenuation of 12 dB per octave.
FIG. 3
depicts a third-order crossover network which has a characteristic rate of attenuation of 18 dB per octave.
FIG. 4
depicts a fourth-order crossover network which has a characteristic rate of attenuation of 24 dB per octave. This demonstrates that to obtain higher rates of attenuation, the number of elements in the network increases in each parallel branch of the crossover network.
Higher order crossover networks are sharper filtering devices. For example, a first order crossover network attenuates at the rate of −6 dB per octave while a second order crossover network attenuates at the rate of −12 dB per octave. Therefore, if a sufficiently low crossover frequency was selected and a first order crossover network employed, a substantial amount of lower frequencies will still be presented to the tweeter. What this means is that such an effect causes undesirable audible distortion, limits power handling, and can easily result in tweeter damage that could be avoided by using a higher order crossover network filter.
While
FIGS. 1-4
have depicted crossover networks, such examples depict that crossover networks are generally implemented as a parallel set of individual filters.
Parallel configured crossover networks have been plagued by phase shifts in the input signal which occurs due to the parallel filter stages resulting in interfering signals when more than one branch or stage of the crossover conducts a portion of the input signal to the respective speakers. Therefore, sharp filters have been employed resulting in distinct and pronounced crossover points.
Furthermore, crossover networks have heretofore required the inclusion of at least one capacitive component such as capacitor
20
for providing the requisite filtering or partitioning of the electrical audio spectrum into frequency bands. Those familiar with high-fidelity appreciate that capacitors are less than ideal
Isen Forester W.
Pendleton Brian Tyrone
Workman & Nydegger & Seeley
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