Patent Application
20060280317
The invention relates to a filtering circuit for audio filtering, and in particular relates to an electronic filter array used in conditioning audio signals.
Audio signal conditioning presents a number challenges especially for radio broadcast applications. For example, it is desirable to broadcast audio signals as close to actual live sound as possible, however, various types of distortion and interference due to equipment limitations and environmental factors can have a negative effect on the audio signal.
Accordingly, many types of audio filters have been used with varying levels of success to provide filtering of audio signals. For example, audio notch filters have been used to filter out particular frequencies known to cause problems with the audio signal. A major challenge faced by audio engineers is to provide filtering of specific (often relatively narrow) frequency bands of an audio signal, for example in dealing with sideband distortion, while not attenuating the surrounding frequencies or overall signal. To illustrate the challenge, some tones reside at a frequency that is quite isolated from other tones. Because these tones are isolated, filtering is relatively simple because there are no surrounding tones that will be affected. However, other tones are substantially bunch together, which requires any filtering to be very precise so as not to attenuate the surrounding frequencies and thereby negatively affect sound quality.
Ganged amplification and other devices create additional challenges in presenting a high degree of detailed signal clarity, intelligibility and resonance. Finally, providing many different types of filters that have a substantially constant or steady resistance so as not to negatively affect overall sound quality is also difficult.
Another design criterion that radio broadcast stations must consider are the regulations propagated by the Federal Communications Commission (FCC), which has provided guidelines within, which the station must operate. One such guideline limits the usable audio frequencies of the broadcast station so that the audio signal from one station does not “spill over” into the frequency range allotted to a different station. The need to comply with these regulations has led to the use of filters for attenuating frequencies to comply with FCC criteria, effectively narrowing the total useable frequency band available to the broadcast station. For example, the broadcast station may be allotted a frequency band within which to operate. In order to maintain the audio signals within that particular range, the station may need to limit their audio broadcast signal by means of filters, to a narrower subset of the allotted frequency band to ensure compliance.
Still another regulation the radio station must consider is peak limiting. So-called “multi-function boxes”, while providing peak limiting, have been used, which have the net effect of distorting the signal by deliberate clipping, limiting, unnecessary roll-off of passed frequencies essential to faithful reproduction of source material, and resultant reduction in directionality of a multi-channel signal.
What is needed then is a filter system and method which provides audio signal conditioning on an integrated basis across an extremely wide range of frequencies that substantially eliminates distortion and maximizes resonance per frequency.
It is further desired to provide a filter system and method which provides for audio signal filtering accounting for amplitude considerations but still enhancing frequency quality.
It is still further desired to provide a filter system and method which provides for elimination not only of harmonic distortion, but also of distortions occurring within and across octaves.
It is yet further desired to provide a filter system and method which provides independent of amplitude levels, concomitant articulate musical instrument attacks, natural/open resolution of consonants and vowels in speech and overall presence approaching a free space condition, free of an electronic device.
These and other objectives are achieved in one advantageous embodiment by the provision of an audio filter, and more specifically to an array of audio notch filters in various configurations that provides audio filtering of multiple audio frequency bands. The various selected configurations allow for minimizing and/or substantial elimination of harmonic distortion in selected audio frequency bands in audio signal recording, transmission and broadcasting.
In one advantageous embodiment, a voltage-controlled source active band-reject filter (notch filter) is utilized. A plurality of such filters may be provided in series, providing a cascading chain of filters. In one embodiment, a cascade of up to ten filters may be used for audio filtering in each band. These groups of filters for each band are preferably positioned in series, with successive groups of filters filtering the selected bands. It is, however, possible for the groups of filters to be parallel connected. The notch filters will advantageously be selected/constructed to have a substantially constant or steady resistance relative to other notch filters in the circuit. The notch filters may, however, having varied capacitance values.
The voltage-controlled source active band-reject filter is configured as a combination of resistance and capacitance, cascaded for example, ten times per band and functions as an integrative device. It provides a cut grater than 200 db (−200 db) approaching −300 db per frequency band, over relatively narrow bands within an overall range of approximately 0.25 cycles per second (cps) to as many as 2 petacycles per second (pcps).
A practical advantage of the present filter system is its ability to effectively eliminate the boundaries to obtaining high clarity and resonance due to, for example, ganged amplification and/or other devices where regulatory bandwidth constraints and commercial requirements either cannot be reconciled with or serve to defeat these qualities. In particular, the filter system relieves sideband load and at a minimum, reduces the need for peak limiting. The filter system further eliminates the need for so-called “multi-function boxes.” This is achieved by not only eliminating distortion, but also by maximizing resonance per frequency. Consistent with this, the emphasis is shifted away from mere amplitude considerations in commercial environments to consideration of frequency quality.
Still further, the filter system provides for elimination of distortions occurring within and across octaves such as for example: dominant, tonic, subdominant, mediant and leading tones heterodyning against each other, thereby producing substantially pure resonance per tone, which is accomplished independent of amplitude levels.
A further aspect of the invention is the identification and selection of numerous (e.g. 130 or more) relatively narrow frequency bands to be filtered within the overall frequency range of from approximately 20 cycles per second (cps) to approximately 43,000 cps to provide the best acoustical output from a system using a cascading audio notch filter. In this manner, the system may clearly reproduce material down to amplitudes as low as 3 decibels on playback.
It should be noted that, the terms “coupled”, “coupled to”, and “coupled with” as used herein each mean a relationship between or among two or more devices, apparatus, files, programs, media, components, networks, systems, subsystems, and/or means, constituting any one or more of (a) a connection, whether direct or through one or more other devices, apparatus, files, programs, media, components, networks, systems, subsystems, or means, (b) a communications relationship, whether direct or through one or more other devices, apparatus, files, programs, media, components, networks, systems, subsystems, or means, and/or (c) a functional relationship in which the operation of any one or more devices, apparatus, files, programs, media, components, networks, systems, subsystems, or means depends, in whole or in part, on the operation of any one or more others thereof.
In one advantageous embodiment a filter system for filtering an audio signal is provided comprising an audio input signal and a plurality of notch filters. Each notch filter has an input terminal, with each input terminal electrically connected to each other, and each notch filter has an output terminal, with each output terminal electrically connected to each other. The system is provided such that the plurality of notch filters has selected resistance and capacitance values corresponding to a frequency to be attenuated. The system is further provided such that each of the parallel connected notch filters has the same selected resistance and capacitance values so as to attenuate the same frequency.
In another advantageous embodiment a filter system for filtering an audio signal including a plurality of parallel connected notch filters each of said plurality of filters having an input terminal is provided and comprises a capacitor (C1) electrically connected to the input terminal and a capacitor (C2) electrically connected to capacitor (C1), the capacitor (C2) being electrically connected to a non-inverting input of an operational amplifier. The system further comprises a resistor (R1) electrically connected to the input terminal and a resistor (R2) electrically connected to resistor (R1), the resistor (R2) electrically connected to the non-inverting input of the operational amplifier. The system still further comprises a capacitor (C3) electrically connected to a point between resistor (R1) and resistor (R2) and a ground connection and a resistor (R3) electrically connected to a point between capacitor (C1) and capacitor (C2) and the inverting input of the operational amplifier.
In still another advantageous embodiment a filter system for filtering an audio signal is provided comprising a plurality of notch filters each of the plurality of filters having an input terminal and an output terminal. The system is provided such that the input terminals of the plurality of notch filters electrically are connected to each other and the output terminal of the plurality of notch filters are electrically connected to each other. The system is further provided such that the plurality of notch filters each have selected resistance and capacitance values selected from Table A corresponding to a frequency to be attenuated, and each of the parallel connected notch filters have the same selected resistance and capacitance values so as to attenuate the same frequency.
In yet another advantageous embodiment a filter system for filtering an audio signal is provided comprising a plurality of parallel connected notch filters having selected resistance and capacitance values corresponding to a frequency to be attenuated. The system is provided such that each of the parallel connected notch filters has the same selected resistance and capacitance values so as to attenuate the same frequency.
In still another advantageous embodiment a method for filtering an audio signal is provided comprising the steps of identifying a bandwidth of audio frequencies and identifying a plurality of frequency bands within the bandwidth of audio frequencies. The method further comprises the steps of electrically connecting a plurality of sets of audio filters in series and inputting the audio signal into the plurality of sets of audio filters. The method still further comprises the steps of attenuating all the frequencies in the bandwidth of audio frequencies except for the identified plurality of frequency bands and outputting the filtered audio signal from the plurality of sets of audio filters.
Other objects of the invention and its particular features and advantages will become more apparent from consideration of the following drawings and accompanying detailed description.
Referring now to the drawings, wherein like reference numerals designate corresponding structure throughout the views.
Filter system 100 is illustrated in
FDn+1=Dn((Dn+1−Dn)/Dn) Formula 1
where, FDn+1 is the final density value, Dn is the initial density value (density in the first medium), and Dn+1 is the next density value (density in the next medium and on an iterative basis, successive media).
An important general relationship reflected in formula 1 is the difference between two groups of frequencies; 1) those frequencies whose tones reflect very high “atomistic” densities, comparatively high spectral Q values and distinct, isolated, highly concentrated/cohesive “elemental” domains, and 2) those whose densities and Q values necessarily and variably depend on molecular fusions, structures and geometries of diffuse matter endemic to their part of the energy bandwidth and the medium/dielectric through which they pass. Frequencies in the second group coalesce as the net result of absorptive and reflective processes and are found primarily within the approximately middle fifty percent of each octave of sound, light and other forms of energy.
For purposes of further illustration, using values provided in Table A, Formula 2 should be applied for computations above 43,000 cps, while Formula 3 should be applied for computations below 20 cps:
2x(tef−Tef−1)=BWrej Formula 2
0.5x(Tef−Tef−1)=BWrej Formula 3
Where, x is the octave raised (or lowered below 20 cps), Tef is the center frequency (cf) first considered, Tef−1 is the next lower frequency below cf, and BWrej is the rejected bandwidth.
Referring again to
The configuration of filter system 100 therefore, typically comprises a number of parallel connected filters 10′, 10″, 10′″ . . . 10n, which forms a filter set, each individual filter selected to filter out the same frequency or band of frequencies. Any number of filter sets may preferably be connected in series, however, it is contemplated that filter sets may also be connected in parallel.
Accordingly, filter system 100 includes a signal input 108 where an audio signal may be introduced to the filter system. In the advantageous embodiment illustrated in
Filter (F1) 10′, 10″, 10′″ . . . 10n are all designed substantially identically, so as to provide superior filtering of the frequency or band of frequencies to be removed from the audio signal. In this manner, as steeper notch is attainable, this will help to minimize side band distortion. The sets of filters may then be connected to each other (whether in series as shown, or in parallel) with the total number of filter sets used corresponding to the frequencies or band of frequencies to be attenuated.
Finally, the filtered audio signal exits filter system 100 as signal output 110, which will be a filtered signal that still contains very high signal quality.
Referring now to
The filter (10), in this advantageous embodiment, is constructed as follows. Input 12 is electrically connected to both capacitor (C1) 16, and resistor (R1) 18. Capacitor (C1) is further electrically connected to capacitor (C2) 20, which in turn is electrically connected to the non-inverting input 22 of operational amplifier 24. Resistor (R1) 18 is electrically connected to resistor (R2) 26, which is also electrically connected to the non-inverting input 22 of operational amplifier 24. In this manner, resistors (R1) and (R2) are electrically connected in parallel with capacitors (C1) and (C2) from input 12 to the non-inverting input 22 of operational amplifier 24.
Also shown in
Still further, as seen in
The filtered signal is then output from operational amplifier 24 at output 38. It should be noted that, each output 38 of each filter 10 in a given filter set will be electrically connected to each other such that each filter in a filter set are parallel connected.
The values of resistors (R1) (R2) (R3) and of capacitors (C1) (C2) (C3), will determine the frequency or band of frequencies the filter 10 will notch or reject.
As the selection of these values will determine the accuracy of the filter 10, a Table A is provided, which provides various values for the resistors and the capacitors and shows a corresponding frequency or band of frequencies that will accordingly be rejected. The Table A is provided to be read across.
For example, in referring to Table A 1-1 through 1-3, the right hand column illustrates a desired notch frequency range to be filtered in bold between two horizontal lines (for example, in the first entry in Table A on page 1-1, the filtered frequency range is 20.83659 to 20.96074 cps). Additional information if provided including the bandwidth corresponding to 0.12415 cps, with the notch center frequency (CF) being 20.898665 cps. Various values are provided as read across the table including, for example, the system sharpness or quality (Q), conductance, and frequency normalization value (u), which is calculated as 6.28×(CF). Also provided is, for example, impedance scaling value (ISF), which is calculated as (CF)/62.8.
Also shown are the values for capacitors and resistors in Table A on page 1-2. In this example, capacitor (C1) is selected to be approximately 0.022896174 F, capacitor (C2) is selected to be approximately 0.022896174 F, and capacitor (C3) is selected to be approximately 0.045792347 F. Additionally, resistor (R1) is selected to be approximately 0.000988455Ω, resistor (R2) is selected to be approximately 112.0368054Ω, and resistor (R3) is selected to be approximately 0.000988447Ω. Further circuit information is provided in Table A from pages 1-2 through 1-3.
It should be noted that, while various functions and methods have been described and presented in a sequence of steps, the sequence has been provided merely as an illustration of one advantageous embodiment, and that it is not necessary to perform these functions in the specific order illustrated. It is further contemplated that any of these steps may be moved and/or combined relative to any of the other steps. In addition, it is still further contemplated that it may be advantageous, depending upon the application, to utilize all or any portion of the functions described herein.
This was merely provided as an example of one configuration of filter 10 according to Table A. As seen in Table A, a relatively large number of notch frequencies are listed having corresponding values provided for the various capacitors and resistors in addition to providing various overall circuit information. While Table A is relatively large, it is not meant to be comprehensive and should not be taken as limiting in any regard, but merely provides circuit design information for particularly troublesome frequencies that typically are required to be filtered out of an audio frequency. It is contemplated that fine-tuning of each system may need to be accomplished, however, the formulas provided herein along with Table A will provide a guidance to one seeking to notch out a particular frequency or band of frequencies not listed in Table A. Accordingly, while a great deal of information is listed in Table A, it is contemplated that additional frequency bands may further need to be removed from the audio signal and such can be done without deviating from the invention.
It is further contemplated that the filter system 100 may be provided in, for example, a hermetically sealed protective housing to protect the relatively sensitive electronic equipment from harsh environmental conditions. The housing may be provided has a sturdy structure comprising, for example, a hard plastic, metal, an alloy or combinations thereof.
Although the invention has been described with reference to a particular arrangement of parts, features and the like, these are not intended to exhaust all possible arrangements or features, and indeed many other modifications and variations will be ascertainable to those of skill in the art.
This application claims the benefit of the filing date of U.S. Provisional Patent Application Ser. No. 60/684,390 filed May 25, 2005.
| Number | Date | Country | |
|---|---|---|---|
| 60684390 | May 2005 | US |
