Field of invention

The present invention relates generally to tunable electronic filters, in particular, to a tunable Sallen-Key filter.

Background Art

Filters are an essential component in signal processing where it is desired to remove unwanted frequency components from a signal and to extract a particular frequency component or band of frequency components, and are ubiquitous across fields such as telecommunications, audio-visual processing, healthcare, automotive, and so on. Filters can be implemented in a variety of manners, including analogue and digital forms, in order to achieve a particular frequency response appropriate to a particular application. In many applications, however, it is desirable to be able to control or vary frequency responses in order to enable dynamic filtering, or to adapt to particular incoming signals or filtering requirements. In the case of analogue filters, which are

advantageous for their simplicity, tuning of a frequency response is typically achieved by altering the configuration of an RC network, composed of resistors and capacitors, which enables filter cut-off frequencies as well as filter types (e.g. high-pass, low-pass, band-pass and band-stop) to be specified and controlled. For applications which require dynamic frequency responses, the RC layout as well as their values would need to be altered every time a different frequency response is needed. One conventional way to solve this problem in a multi-channel filter is to implement a number of different filters in parallel, which requires components for all the required frequency responses to be arranged on the same circuit board, which often needs a large number of components, conflicting with space and cost restrictions. In an analogue filter, the RC network is typically used in conjunction with an amplifier, such as an operational amplifier, and so arranging many amplifiers on the same circuit board is particularly disadvantageous due to the particular size, power consumption and cost associated with the amplifier.

Sum m ary of Invention

Embodiments of the present invention aim to provide an efficient architecture or topology in which filter flexibility and scalability can be achieved in a manner which reduces space requirements, and also reduces costs by reducing the number of required components. In addition, embodiments of the present invention aim to reduce power consumption . By reducing the number of components, the present invention also reduces the space occupied on a printed circuit board (PCB), which opens the possibility of new applications, where smaller circuits are required.

According to an aspect of the present invention, there is provided a tunable electronic filter comprising: an input stage coupled to a tuning stage; an amplifier with a non inverting input coupled to an output of the tuning stage, and an inverting input coupled to ground, and an output; and an output stage coupled to the amplifier output; wherein the tuning stage comprises N RC networks and a multiplexer, wherein the N RC networks are connected in parallel between the input stage and respective inputs of the multiplexer; wherein the multiplexer is configured to selectively connect one of the N RC networks to the non-inverting input of the amplifier, such that a second order Sallen-Key filter is formed between the input stage and the output stage, wherein selection of one of the N RC networks tunes the electronic filter to have a respective one of N frequency responses, for N > 2.

Each of the N frequency responses may be associated with a respective cut-off frequency.

The N frequency responses may represent at least one of a low-pass filter and a high- pass filter. A gain may be introduced by connecting two resistors to the inverting input of the amplifier, wherein a first resistor may be connected between the inverting input of the amplifier and ground and a second resistor may be connected between the inverting input of the amplifier and the amplifier output. The multiplexer may be an A: 1 multiplexer comprising N inputs, each connected with a respective one of the N RC networks.

The multiplexer may be an N+1A multiplexer comprising N+ 1 inputs; and wherein the tuning stage may further comprise a non-filter path connected in parallel with the N RC networks between the input stage and a respective input of the A+ l: l multiplexer; wherein the N+ 1: 1 multiplexer may be configured to selectively connect one of the N RC networks or the non-filter path to the non-inverting input of the amplifier, wherein selection of the non-filter path directly couples the input stage of the filter to the non inverting input of the amplifier. The filter apparatus may further comprise two or more tunable electronic filters as described above, arranged in series, in which the output stage of a tunable electronic filter is connected to the input stage of a successive tunable electronic filter.

The filter apparatus may comprise a group of first M tunable electronic filters arranged in series, in which each first tunable electronic filter is of a first type; a group of second M tunable electronic filters arranged in series, in which each second tunable electronic filter is of a second type; wherein either: the first type is a high-pass filter and the second type is a low-pass filter, or the first type is a low-pass filter and the second type is a high-pass filter; for M > 2, wherein the groups of first and second M tunable electronic filters may be connected in series such that the filter apparatus is a Mx 2 order band-pass filter.

The filter apparatus may comprise M groups of tunable electronic filters arranged in series, in which each of the M groups comprises a first and a second tunable electronic filter of respective first and second types; wherein : either: the first type is a high-pass filter and the second type is a low-pass filter, or the first type is a low-pass filter and the second type is a high-pass filter; for M > 2, such that the filter apparatus is a Mx 2 order band-pass filter. According to another aspect of the present invention, there is provided an Mx 2 band- stop filter apparatus comprising an Mx 2 order band-pass filter as described above; and an output amplifier having an inverting and a non-inverting input; wherein the input to the Mx 2 order band-pass filter is coupled to the non-inverting input of the output amplifier, and the output of the Mx 2 order band-pass filter is connected to the inverting input of the amplifier.

The Mx 2 band-stop filter apparatus may further comprise a gain amplifier between the input to the Mx 2 order band-pass filter and the non-inverting input having gain substantially equal to gain provided by the Mx 2 order band-pass filter. According to another aspect of the present invention, there is provided a filter apparatus comprising a plurality of stages, in which at least one of the plurality of stages comprises a first tunable electronic filter apparatus as described above, and at least one of the plurality of stages comprises a second tunable electronic filter apparatus, the second tunable electronic filter apparatus comprising an input stage coupled to a tuning stage; an amplifier with an inverting input coupled to an output of the tuning stage, and an non-inverting input coupled to ground, and an output; and an output stage coupled to the amplifier output; wherein the tuning stage comprises N RC networks and a multiplexer, wherein the N RC networks are connected in parallel between the input stage and respective ones of the N inputs of the multiplexer; wherein the multiplexer is configured to selectively connect one of the N RC networks to the inverting input of the amplifier, such that a second order multiple feedback filter is formed between the input stage and the output stage, wherein selection of one of the N RC networks tunes the electronic filter to have a respective one of N frequency responses, for N > 2.

Brief des criptio n o f drawings

Embodiments of the present invention will be described by way of example only, with reference to the accompanying drawings, in which :

Figure 1 is a system diagram of a tunable electronic filter according to embodiments of the present invention ;

Figure 2 illustrates a fundamental second order Sallen-key (SK) filter;

Figure 3 illustrates a second order tunable SK filter according to an embodiment of the present invention ;

Figure 4 illustrates a second order tunable SK filter with an additional path, according to an embodiment of the present invention ;

Figure 5A shows the frequency response (Bode plot) of a second order tunable low-pass filter (FPF) with the cut-off frequency set at 50Hz according to an embodiment of the present invention ;

Figure 5B shows the frequency response (Bode plot) of a second order tunable high- pass filter (HPF) with the cut-off frequency set at O . lHz, according to an embodiment of the present invention ;

Figure 6A and 6B are block diagrams of tunable band-pass filters (BPFs) with cascaded tunable second order SK filters according to embodiments of the present invention. Figure 7 shows the frequency response (Bode plot) of a fourth order tunable band-pass filter (BPF) with the band pass set at O . lHz - 50Hz, according to an embodiment of the present invention ;

Figure 8 A and 8B are block diagrams of tunable band-stop filters with cascaded tunable second order SK filters according to embodiments of the present invention ;

Figure 9 shows the frequency response (Bode plot) of a tunable band-stop filter according to an embodiment of the present invention ;

Figure lOA and 10B are block diagrams of band-stop filters comprising cascaded tunable second order SK filters according to embodiments of the present invention ; and Figure 11 illustrates a fundamental second order multiple feedback (MFB) filter topology.

Detaile d des criptio n

Figure 1 illustrates a system diagram of a tunable electronic filter 100 according to embodiments of the present invention. The tunable electronic filter 100 comprises an input stage 101 coupled to a tuning stage 102, an amplifier 103 coupled to an output of the tuning stage, and an output of the amplifier is coupled to an output stage 104 of the tunable electronic filter. The input stage 101 of the tunable electronic filter 100 comprises a terminal or circuitry suitable for interfacing with and receiving a signal to be filtered, and the output stage 104 of the tunable electronic filter 100 comprises a terminal or circuity suitable for interfacing with and providing the filtered signal to any downstream system components (not shown).

Tunability is achieved by the tuning stage which comprises a multiplexer (MUX) 106 and N passive RC networks 105, also referred to herein as Filter Forming Circuitries FFC 1-N, according to an embodiment of the present invention, for integer N > 2. The multiplexer 106 in the tuning stage is a multiplexer having N inputs and a single output. The N RC networks 105 share the same input signal from the input stage 101 and the multiplexer 106 operates such that the output of only one out of the N RC networks 105 is available at the output of the multiplexer.

N may be freely defined by the user in the design of the architecture shown in Figure 1, but it is advantageous if N = 2 ⁿ , since this facilitates addressing of the RC networks using n binary control lines (not shown) to the multiplexer. For example, eight RC networks can be addressed by three control lines, 32 networks can be addressed by five control lines, 128 networks can be addressed by seven control lines, 512 networks can be addressed by nine control lines, and so on.

The selection of one of the N RC networks 105 by the multiplexer 106 tunes the electronic fdter 100 to have a respective one of N frequency responses, associated with the configuration of the resistors and capacitors in the RC networks 105. The N frequency responses may differ in parameters including any, or combinations of any of cut-off frequency, roll-off and filter type (high-pass or low-pass). An active tunable electronic filter is achieved by coupling, via the multiplexer, the passive RC networks to an amplifier, in order to circumvent any signal attenuation caused by the load impedance of the RC network, and to optimise the frequency response. The amplifier 103 used in the embodiment illustrated in Figure 1 is an operational amplifier (op-amp), having high input impedance, low output impedance, and a power supply (not shown). The output of the multiplexer is connected to the input of the amplifier.

In embodiments of the present invention, the RC networks are advantageously configured such that when connected to the op-amp by the multiplexer, the resultant circuit forms a second order Sallen-Key filter. Sallen-Key filters represent an efficient design , having high performance, such that design of the tunable electronic filter of embodiments of the present invention can be achieved effectively, and replacement, expansion or inclusion of redundancy for individual RC networks for the filter can be performed without difficulty.

Figure 2 illustrates a fundamental second order Sallen-Key (SK) filter 200. A SK filter is a variation on a voltage-controlled voltage-source (VCVS) filter that uses unity-gain amplifier. The circuit comprises an op-amp 20 1, in the arrangement shown in Figure 2 having non-inverting and inverting inputs 208 , 209 and an output. First and second impedances Zl 204 and Z2 205 are connected in series between the input of the filter 202 and the non-inverting input 208 of the amplifier. A third impedance Z3 206 is connected in the feedback loop between a junction between Zl 204 and Z2 205 and the output of the filter. A fourth impedance Z4 207 is connected between the non-inverting input 208 of the amplifier and ground. For a low-pass filter, Zl 204 and Z2 205 are resistors, Z3 206 and Z4 207 are capacitors. For a high-pass filter, Zl 204 and Z2 205 are capacitors, Z3 206 and Z4 207 are resistors. It is conventional to tune both resistors and both capacitors in a SK filter in order to tune the overall filter response, but in embodiments of the present invention, dynamic tuning of an individual RC network is not required.

In the arrangement illustrated in Figure 2, gain is achieved via impedances Z5 210 and Z6 211 are present and the gain is 1 + Z6/ Z5, where Z5 210 and Z6 211 are resistors. If gain is not needed, the inverting input of the amplifier is connected directly to the output of the filter.

Figure 3 shows a configuration of a second order tunable SK filter 300 according to an embodiment of the present invention. Each RC network 301 comprises resistors and capacitors arranged to implement different filter functions based on the principles illustrated with respect to Figure 2, so different RC networks have different frequency responses when connected to the non-inverting input of the amplifier via the multiplexer 305.

In some embodiments, two resistors and two capacitors are used for each RC network. The RC network selected by the MUX determines the bandwidth of the second order tunable SK filter and its Q factor in the selected configuration.

In one embodiment, the amplifier is configured to have unity gain. In alternative embodiments, a gain may be introduced by connecting two extra resistors Z5 306 and Z6 307, wherein Z5 306 is connected between the inverting input of the amplifier 302 and ground and Z6 307 is connected between the inverting input of the amplifier 302 and the output of the amplifier 303. Since the gain is achieved by configuration of the inverting input of the amplifier, and the same amplifier is connectable to each of the N RC networks, the selected gain applies to all the RC networks.

The outputs of the feedback loops introduced by the RC networks are directly connected to the output stage of the filter without changing the transfer function of the system . The system response of the second order tunable SK filter approximates the response of a second order SK topology that uses the selected RC network for impedances Zl 204, Z2 205, Z3 206 and Z4 207 (and Z5 210 and Z6 211 where used) of Figure 2. The second order tunable SK fdter topologies according to embodiments of the present invention are designed with the minimum number of required components, the amount of operating current is reduced, and as a result, the total power dissipation is minimized. A single amplifier is required to create an active filter using any of the N passive RC networks.

Additionally, embodiments of the present invention offer the ability to use a mixture of different types of filter in one device. For example, the user is able to select, via multiplexer control signals, between different types of filtering (low-pass, high-pass), which are input branches connected to the multiplexer. Furthermore, the embodiments of the invention allow for multiple cut-off frequencies to be provided using a single device, either with, or without filter-type variability. In other words, if the application requires only one type of filter (either low-pass or high-pass), the multi-type filter device is easily configured as a single-type device having multiple cut-off frequencies, but the multiple cut-off frequencies can also be used in combination with multiple filter types.

In one embodiment, the multiplexer is configured to be an A: 1 multiplexer.

In an alternative embodiment, the multiplexer is configured to be an A+ l: l multiplexer 401 as shown in Figure 4. An additional path, referred to as a non-filter path 402, is connected in parallel to the N RC networks between the input stage 403 and a respective input 404 of the N+ 1: 1 multiplexer 401 as illustrated in Figure 4. The N+ 1: 1 multiplexer is configured to selectively connect one of the N RC networks or the non filter path to the non-inverting input of the amplifier, such that when the non-filter path is connected, the input stage of the filter is directly coupled to the input of the N+ 1: lmultip lexer . The direct-coupling of the input stage of the filter to the N+ 1: 1 multiplexer bypasses the N RC networks which enables conversion of a signal recording system from AC-coupled to DC-coupled to capture low-frequency (close to DC) signals according to an embodiment of the present invention.

Figures 5A and5B show frequency responses from various types (LPF, HPF) of second order tunable SK filters according to embodiments of present invention. One available RC network/ cut-off frequency is shown in each type of filter. Figures 5A and 5B respectively show the Bode plots of a second order tunable LP SK filter with the cut-off frequency set at 50Hz and a second order tunable HP SK filter with the cut-off frequency set at O . lHz respectively.

It will be appreciated that the use of the MUX and the amplifier does not impose a limit on the number of available frequency responses according to embodiments of the present invention. In other words, a user is free to decide on the number of RC networks/ frequency responses depending on the application requirements (e.g. signals to be filtered, bandwidth, power consumption, area, redundancy to cater for component failure etc.).

In alternative embodiments of the present invention, a filter apparatus comprises two or more instances of the second order tunable SK filters 100 of the type shown in Figure 1 arranged in series, for realisation of filters with different frequency responses. For instance, it may be possible to arrange M stages of tunable SK filters in series, for integer M > 2. If the filter apparatus is made up of M second order tunable SK filters of the same type (either all high-pass or all low-pass), the filter order increases by two for each additional stage (2 xM).

The resultant filter apparatus achieved through such a cascaded system may be low- pass, high-pass, band-pass (BPF) or band-stop (BSF). In some embodiments, to realise a second order BPF, one second order SK high-pass stage of the type shown in Figure 3 (in which all N RC networks represent high-pass filter circuitry) and one second order SK low-pass stage of the type shown in Figure 3 (in which all N RC networks represent low-pass filter circuitry) are connected in series, the two stages connected in either order. In further embodiments, it is possible to“nest” different filter types within each stage, as described below.

Figure 6 A illustrates a cascaded Mx 2 order SK BPF 500 , in which M groups of a BPF (502,503), each group containing a pair of a HPF filter 505, 507 of the type shown in Figure 3 (in which all N RC networks represent high-pass filter circuitry) and a LPF filter 506, 508 of the type shown in Figure 3 (in which all A RC networks represent low- pass filter circuitry) connected in series. M groups of the HPF and LPF pairs 502, 503 are cascaded to realise an Mx 2 order BPF. In such a system, there are 2 M amplifiers and 2 M multiplexers, leading to N ^2M combinations of RC networks in the signal path. According to some embodiments of the present invention, the M groups of the HPF and LPF pairs all have unity gain, such that the Mx 2 order BPF has unity gain. In other embodiments of the present invention, one or more of the M groups of the HPF and LPF pairs introduce a gain, such that the Mx 2 order BPF provides an overall gain G. In some embodiments each of the M groups of the HPF and LPF pairs may have substantially equal gains.

In alternative embodiments, a band-pass fdter response can be realised by having a group 512 of first M tunable electronic filters of a first same type 514, 515 (high-pass or low-pass, high-pass in Figure 6B) arranged in series, cascaded with a group 513 of second M tunable electronic filters of a second type 516, 517 (high-pass or low-pass, low-pass in Figure 6B) arranged in series as illustrated in Figure 6B, in which the second type is different from the first type. In other words, to realise an Mx 2 BPF as a stage 1 518 , an Mx 2 order HPF 512 is cascaded with an Mx 2 order LPF 513. Multiple instances of cascaded arrangement (stage 1) 518 may be further cascaded in order to generate yet higher order filters. For example, K instances of a single HPF group with a single LPF group as shown in Figure 6B results in a BPF of order K x M x 2 order BPF is formed by cascading K instances of the stage 1.

As described above with respect to Figure 6A, each of the HPF groups and the LPF groups shown in Figure 6B may have unity gain, such that the Mx 2 order BPF provides an overall unity gain, or one or more of the HPF group and the LPF group may introduce a gain , such that the Mx 2 order BPF provides an overall gain.

Figure 7 shows the Bode plot of a fourth order tunable BP SK filter with the frequency range from 0. l-50Hz according to embodiments of present invention. The filter apparatus associated with the frequency response of Figure 7 is formed either of the type illustrated in Figure 6A, in the case in which M = 2, or the type shown in Figure 6B, for M = 2, as the performance of the two embodiments is the same.

In embodiments of the present invention, a Mx 2 order band-stop or notch filter 600 , 607 can be realised by inverting an Mx 2 order BPF, such as that shown in Figure 6A and Figure 6B, using an output amplifier 602, 608 , which may be an instrumentation amplifier or a differential amplifier. This can be achieved by having the input 601, 606 to the band-pass filter coupled to the non-inverting input 605, 611 of the output amplifier and the output of the band-pass filter connected to the inverting input 604, 610 of the output amplifier as shown respectively in Figures 8 A and 8B. According to embodiments of the present invention, the Mx 2 order band-stop filter may have unity gain.

Figure 9 shows frequency response of a second order tunable band-stop filter formed based on Figure 8 A according to an embodiment of the present invention . The centre frequency of this band-stop filter is 24 Hz, while the low- and high- cut-off frequencies are approximately 11 and 50 Hz, respectively.

Figure lOA and 10B are block diagrams of a band-stop filter apparatuses 700 , 713 comprising cascaded tunable second order SK filters with a gain amplifier 703 and first and second resistors 706, 707 configured to provide a gain to the filter apparatus according to embodiments of the present invention.

The cascaded tunable second order SK filters, for example, can be the Mx 2 order band pass filters illustrated in Figure 6A and 6B. Whereas the x2 order band-pass filters shown in Figure 8 A and 8B may be configured to have unity gain, in the present embodiments, the x2 order band-pass filters may have a gain G, as described above with respect to Figures 6A and 6B. In addition to the output amplifiers, the filter apparatus 700 in Figure 10 A or 10B further comprises a gain amplifier 703 having a second inverting 705 and a second non-inverting input 704 and a second output 711. The second output 711 of the gain amplifier 703 is coupled to the first non-inverting input 709 of the output amplifier 712 in Figure 10 A or 10B. The input 701 to the cascaded tunable SK filters is coupled to the non-inverting input 704 of the gain amplifier. The first resistor 706 is connected between the inverting input 705 of the gain amplifier and ground and the second resistor 707 is connected between the inverting input 705 of the gain amplifier and the output 711 of the gain amplifier. The gain amplifier 703 , together with the first resistor 706 and the second resistor 707, is configured to provide a gain which is substantially the same as the gain G provided by the Mx 2 order band- pass filters. The output of band-pass filter stage is coupled to the inverting input 711 of the output amplifier 712. The band-pass filter stage provides the gain G to the inverting input 711 of the output amplifier 712, and the gain amplifier 703 provides substantially the same gain G to the non-inverting input 709 of the output amplifier 712, such that the filter apparatus 700 , 713 is an Mx 2 order band-stop filter with an overall gain which is substantially equal to the gain G.

According to alternative embodiments of the present invention, additional types of filter may be employed in the tunable filter, for example, second order tunable multiple feedback (MFB) filters. Filter apparatuses according to embodiments of the present invention which combine such MFB filtering with SK filters are referred to herein as “hybrid filter topologies”.

Figure 11 shows a fundamental second order MFB filter 800. A MFB filter 800 uses an operational amplifier 803 with inverting 809 and non-inverting 8 10 inputs and an output 8 11 as an integrator as shown in Figure 11, therefore the dependence of its transfer function on the amplifier is greater than the SK configuration . First Zl 804 and fourth Z4 807 impedances are connected in series between the input 801 of the filter and the inverting input 809 of the amplifier. Second impedance Z2 805 is connected in a first feedback loop between a junction between Zl 804 and Z4 807 and the output 8 11 of the amplifier. Third impedance Z3 806 is connected in a second feedback loop between a junction between Zl 804 and Z4 807 and ground. Fifth impedance Z5 808 is connected between the inverting input 809 of the amplifier and the output 8 11 of the amplifier. The non-inverting input 8 10 of the amplifier is grounded.

For a low-pass filter, Zl 804 , Z2 805 and Z4 807 are resistors, Z3 806 and Z5 808 are capacitors. For a high-pass filter, Zl 804, Z2 805 and Z4 807 are capacitors, Z3 806 and Z5 808 are resistors. The 2nd order MFB filter 800 is capable of forming a bandpass filter without requiring a cascaded structure. For the band-pass configuration, Zl 804, Z3 806 and Z5 808 are resistors, Z2 805 and Z4 807 are capacitors.

A second order tunable MFB filter comprises an input stage coupled to a tuning stage, an amplifier with an inverting input coupled to an output of the tuning stage, and a non-inverting input and an output, and an output stage coupled to the amplifier output. The tuning stage comprises N RC networks and a multiplexer, wherein the N RC networks are connected in parallel between the input stage and respective inputs to the multiplexer. The multiplexer is configured to selectively connect one of the N RC networks to the inverting input of the amplifier, such that a second order multiple feedback filter is formed between the input stage and the output stage. The selection of one of the N RC networks tunes the electronic filter to have a respective one of N frequency responses.

It will be appreciated that, depending upon a particular application, the present invention can be embodied in a number of different ways, based on the principle of selectively connecting multiple RC networks to an amplifier, via a multiplexer.

Depending on the required application, a single device or printed circuit board may contain RC networks enabling high-pass filters and/ or low-pass filters and/ or band pass filters and/ or band-stop filters and/ or notch filters to be selectively employed, with tunability being available for both filter types, and cut-off frequencies, filter order, and roll-off for a given type, using minimal components.

As a comparative example, consider a three-channel configuration having a low-pass channel, a high-pass channel and a band-pass channel on a single hardware device, based on embodiments and cascades of embodiments as described above. This configuration will use eight higher-cost components, namely one multiplexer and one amplifier for each of the low-pass and high-pass channels, and two multiplexers and two amplifiers for the cascaded band-pass channels. This number of components will apply, regardless of the number of cut-off frequencies to be used for each channel type, whereas a conventional filter would need to scale up. Consequently, it can be seen that the area and cost savings increase as the number of channels increases. For the case where there are four cut-off frequencies per channel, the resultant device occupies around 44% of the space of a conventional three-channel filter, which will require 16 amplifiers in total, four for each of the high-pass and low-pass channels, and eight for the band-pass filters. Furthermore, the efficiency of the arrangement of the present invention is particularly highlighted by the fact that in the case of the band-pass filter channels, the example arrangement of the present invention will achieve 16 (or more, depending on scaling of the HPF and LPF stages) band-pass frequency bands using only two amplifiers (4 LPF x 4HPF), whereas conventionally, eight amplifiers only enable four frequency bands. The architecture described above may find particular applicability in multi-channel signal processing systems, such as data acquisition apparatuses. One particular example is in brain monitoring, where signals having a very large number of frequency channels may require rapid and dynamic fdtering, which would not be possible if the hardware required changing in order to switch between each channel. The

embodiments of the present invention would add bandwidth, gain and fdter-type flexibility in the process of recording potential signals.

The present invention is not limited to the embodiments described above, and it will be appreciated that combinations of compatible features of different embodiments may be permissible within the scope of the invention as defined by the appended claims.