More particularly, the invention relates to such an arrangement of the kind comprising an input intended to be connected to the subscriber lines, an output intended to be connected to the headend, means for evaluating sig- nals incoming at the input, and blocking means control- lable by said evaluation means and arranged to permit or prevent a transmission of signals corresponding to the incoming signals at the output i. n dependence upon the evaluation of the incoming signals effected by the eva- luation means.

A disadvantage of previously known noi. se reduction arrangements of said kind is that it is not possible in an easy manner to vary the function and manner of opera- tion of said arrangements in view of individual require- ments of different cable TV systems. Nor is it possible, after the installation of said arrangements, to change their function and manner of operation in an easy manner in order to adapt them to new or altered requirements later on presented by individual system operators.

The invention has for its purpose to provide a new and improved noise reduction arrangement of the kind initially specified, wherein the above-mentioned dis- advantages of the known arrangements can be avoided.

The arrangement according to the invention proposed for said purpose is primarily characterized in that said blocking means are formed by a reprogrammable digital sig-

nal processing unit, whi. ch is connected to the input vi. a an analog-to-digital converter in order hereby to receive the incoming signals in a sampled digital form and which is arranged to generate digital output signals corres- ponding to portions of the incoming signals, depending upon the existing programming of the signal processing unit and the result of the evaluation of the incoming signals effected by the evaluation means, and to deliver said digital output signals to a digital-to-analog con- verter connected to the output.

As a consequence of the above construction of the arrangement, it is possible i. n connection with the in- stallation of the arrangement easily to adapt its func- tion and manner of operation to the existing require- ments of the installation location simply by programming the digital signal processing unit in a ccrresponding manner. This means that the arrangement may be produced and delivered in a standard design. Furthermore, if new of altered requirements in respect of the function and manner of operation of the arrangement are presented at a later stage, it is easy to adapt the arrangement to said requirements simply by correspondingly reprogram- ming the signal processing unit. In this connection, it should be mentioned that the arrangement may preferab- ly be arranged to permi. t a remote controlled programming or reprogramming, respectively, of the signal processing unit.

The arrangement according to the invention also offers the specific advantage of making it possible by programming the signal processing unit in an appropriate manner to bring said unit to act as a frequency converter which is capable of causing a change in the carrier fre- quency of an incoming, signal duri. ng the passage of said signal in digital form through said unit.

Furthermore, by appropriately programming the sig-

nal processing unit, it is possible to bring said unit to cause a delay of the incoming signals transmitted i. n digital form thereto before generating any corresponding digital output signals from said unit. Such a design of the arrangement makes it possible to compensate for the time consumed for the evaluation of an incoming signal.

The signal processing unit may suitably contain one or more FIR-filters which for instance may be based on PLD or FPGA solutions offering a high parallel pro- cessing capacity. However, the desired function of the signal processing unit may alternatively be obtained by utilizing a digital signal processor (DSP).

According to a preferred embodiment of the i. nven- ti. on the signal processing unit may contain a FIR-filter which is arranged, in response to each sample X (k) of an incoming signal generated by the analog-to-digital converter, to generate an output signal Y (k) according to the equation Y (k) = aOBX (k) +a1eX (k-1) +a2oX (k-2)... +an_1. X (k-(n-1)) where k designates the order number of the sample in questi. on, aO, a1... an 1 consist of a set of multi. plica- tion coefficients selectable in dependence upon the re- sult of the evaluation of the incoming signal effected by the evaluation means and previously stored in a memory contained in the arrangement.

The evaluation means may ccnsist of analog means which are arranged to receive the incoming signals in their original analog form. However, according to a favourable embodiment of the invention, the evaluation means may instead consist of reprogrammable digital eva- luation means which are arranged to receive the incoming signals in a sampled digital form, preferably from the analog-to-digital converter connected between the input of the arrangement and the signal processing unit. The use of such evaluation means makes it possible, whenever

so desired, to alter the criteria according to whi. ch the evaluation of the incoming signals is to be effected.

In this case, the arrangement may preferably be arranged to permit a remote controlled programming or reprogram- ming, respectively, of the evaluation means.

The evaluation means may suitably be arranged to determine whether an incoming signal satisfies one or more predetermined conditions and to cause the signal processing unit to deliver a digital output signal cor- responding to the incoming signal only when said condition or each of said conditions, respectively, is satisfied.

In this case, the evaluation means may be arranged to determine whether an incoming signal has a frequency falling within a predetermined permitted portion of the carrier frequency band previously mentioned. Moreover, the evaluation means may also be arranged to determine whether an incoming signal has an amplitude amounting to a predetermined lowest permitted value.

Finally, it should also be mentioned that the eva- luation means may be arranged to deliver information, depending upon the result of the evaluation effected by sai. d means, to storage means contained in the arrange- ment and from which sai. d information can be fetched in to the headend.

Below the invention is further described with re- ference to the accompanying drawing which shows a block diagram of a noise reduction arrangement according to an embodiment of the invention, selected by way of example only, said arrangement being connected in a local cable TV network.

In the drawing, reference numeral 10 designates a portion of a local cable TV network which may consist of a local area network forming part of a cable TV sys- tem containing a headend 11, located at a substantial distance from the local network and serving as a commu-

nication central.

Network 10 contains a diplex filter 12 which is connected to headend 11 through a port A and to a plura- lity of subscriber lines 13 through a port B and which is arranged to permit a transmission of TV signals, radio signals and data signals in a downstream direction from headend 11 to subscriber lines 13 and subscriber outlets (not shown) connected to said lines within different predetermined carrier frequency bands.

An additional carrier frequency band b1 is utilized for the transmission of data signals in the return direc- tion, i. e. in an upstream direction from lines 13 to headend 11, and for said purpose only. In order to faci- litate a reduction of noise transmitted from network 10 within said carrier frequency band, diplex filter 12 is arranged via a port D to divert signals within carrier frequency band b1 incoming to port B from subscirber lines 13 to an input 14 of a noise reduction arrangement generally designated 15. From said arrangement, those of said signals which are permitted to pass through the arrangement can be fed via an output 16 to a port C of filter 12 in order then to be delivered to headend 11 via port A.

Noise reduction arrangement 15 compri. ses a reprog- rammable digital signal processing unit 17 which is con- nected to input 14 via an analog-to-digital converter 19, controlled by a clock 18, and a parallel data bus 20 i. n order hereby to receive the analog signals incoming to input 14 in a sampled digital form. In order to obtain a sufficient resolution in the digital output signal from converter 19, each sample may suitably ccmprise at least 8 bits.

Furthermore, noise reduction arrangement 15 also comprises an evaluation unit 21 for evaluating the sig- nals incoming to input 14 and delivering control signals,

depending upon the result of said evaluation, to signal processing unit 17.

Unit 21 may consist of an analog evaluation unit which is connected directly to input 14 by means of line 22' (shown dash-dotted) in order to receive the signals incoming to said input in their original analog form.

However, uni. t 21 may alternatively consist of a reprogrammable digital evaluation unit which by means of a data bus 22", shown in full lines, is connected to converter 19 in order hereby to receive the signals incoming to input 14 in a sampled digital form.

Signal processing unit 17 has for its purpose to generate digital output signals depending upon the current programming of said unit and the control signals supplied from evaluation unit 21 and corresponding to portions of the signals incoming to input 14 acceptable for for- warding to headend 11. From unit 17, these output sig- nals are transmitted via a data bus 23 to a digital-to- analog converter 24 which is controlled by clock 18 and in which said signals are converted to corresponding analog signals then delivered from noise reduction ar- rangement 15 via output 16. Hereby, signal processing unit 17 acts as a blocking means which can permit or prevent a transmission at output 16 of output signals corresponding to the signals incoming at input 14 in dependence upon the evaluation of the incoming signals effected by unit 21.

In order to permit a remote controlled program- ming or reprogramming of signal processing unit 17 and also of evaluation unit 21, if the latter consists of a reprogrammable digital unit, there is provided a control unit 25 to which control orders of different kinds can be transmitted from headend 11 via a modem 26 connected to network 10.

The more detailed construction of units 17 and 21

may be varied in many different ways.

Signal processing unit 17 may preferably contain one or more FIR-filters and means for storing coeffi- cient values preprogrammed by means of control unit 25 and means for momentarily loading the coefficient values proper to the control signals from evaluation Unit 21..

Evaluation unit 21 may suitably contain a plurality of band-pass filters, having mutually different pass bands and by mEAns of which frequency band b1 can-be divided into different subbands, and a corresponding ..." . plurality of Selectors connected each to one of said filters. These : :. detectors may be utilized. to determine whether a signal occurring. within a certain subband has an 'mplitude .'iounting to a predetermined lowest per- mitted value and/or whether the front edge of the signal has a slope falling within a predetermined interval. If unit 21 consists of a digital unit, it may easily be re- programmed in order to alter the pass bands of the diffe- rent filters.

In order to facilitate a remote observation of the conditions in the local network, evaluation unit 21' may be arranged to deliver information about the result of the effected evaluation of the incoming signals to storage means contained in control unit 25. This informa- tion may then be fetched in to headend 11 via modem 26 when so desired.

The manner of operation of the noise reduction ar- rangement above described may be varied in many different ways. In the normal case, the arrangement may be program- med to permit signals incoming from the subscriber lines, having a certain lowest permitted amplitude and frequen- cies falling within one or more selected portions of frequency band b1, to pass through said unit. If signal types occur, not permitting the use of a level detection, the incoming signals may. however be permitted to pass

through the arrangement irrespectively of their ampli- tude.

Furthermore, the evaluation unit may be utilized to determine whether a signal incoming from a subscriber line is of a certain permitted modulation type and to permit a delivery of a corresponding signal to the head- end only when this is the case. Since a determination of the modulation type may require a longer time than the longest delay that can be permitted in the noise reduction arrangement, one may consider to permit the signal to pass through the arrangement as scon as it has been ascertained that the carrier frequency falls within a permitted portion of frequency band b1 and then to interrupt the continued transmission of the signal immediately if it later on is found that the signal fails to exhibit a correct modulation.

Below follows a more detailed description of an arrangement of the kind shown in the drawing and above described, said arrangement being selected by way of example only.

Evaluation unit 21 is assumed to consist of an analog unit containing five band pass filters which di- vide frequency band b1 into five different pass bands, namely a first band b1A ccvering the frequency range 5- 9 MHz, a second band b1B covering the frequency range 10-16 MHz, a third band b1 covering the frequency range 17-26 MHz, a fourth band b1 covering the fre- quency range 27-42 MHz, and a fifth band b1E covering the frequency range 43-65 MHz. In this case, evalua- tion unit 21 also comprises five detectors which are connected each to one of said filters and which serve each to detect whether a valid message signal occurs within the pass band of the filter in question and to deliver corresponding control signals to signal proces- sing unit 17, which acts as a controllable blocking means.

Signal processing unit 17 is assumed to contain a 15 taps FIR-filter to which the signals within frequency band b1 incoming to input 14 are supplied in a sampled digital form, and more particularly, in the form of an 10 bits parallel data signal consisting of a continuous stream of successive samples X which are generated at a frequency corresponding to the frequency of clock 18.

This frequency may for instance amount to 150 MHz.

For each sample X, the FIR-filter generates a 10 bits output signal Y, formed by a combined multiplication and addition process effected by the FIR-filter. For each sample, this signal may be defined by the equation Y (k) = aOoX (k) +a1*X (k-1) +a2XX (k-2)... +a.. X (k-14) where k designates the order number of the sample in question and aO, a1... 1*'*a14 consist of multiplication co- efficients stored in memories contained in unit 17. In practice, signal Y (k) is delayed some few clock cycles in relation to the time for the generation of sample X (k).

In order to facilitate different operation modes of unit 17 in dependence upon the control signals delivered from unit 21, a plurality of different sets of multiplica- tion coefficients are stored in different memory banks in unit 17. Each set represents a certain predetermined operation mode. Withi. n each set, the coefficient values are symmetrical, i-e-aO = a14 a1 = a13 a2 a12, cetera. Furthermore, each coefficient may suitably con- sist of 9 bits, namely 8 digit bits and 1 sign bit.

Unit 17 may for instance be arranged to operate according to any of the following three operation modes, namely as a high-pass filter permitting only signals within frequency band b1E, i. e. 43-65 MHz, to pass therethrough, as an all-pass filter permitting signals within the entire frequency band b1, i. e. 5-65 MHz, to pass therethrough and as a blocking filter which

blocks the entire frequency band b1.

The firstmentioned one of the three above opera- tion modes may be utilized if the evaluation unit 21 detects the occurrance of a valid message signal only within frequency band b1E. This operation mode can suita- bly be obtained by utilizing the following set of coeffi- cients, namely aO = a.. = 1, a1 13 2 14, 3 a11 23t a4 = a10 = 15 as = a9 =-19 a6 = Bo =-64, a-= 127. These coefficient values have been selected in order to ensure that within frequency band b1E there will not occur any substantial ripple nor any substantial variations in the delay caused by the FIR- filter.

The second one of said operation modes may be uti- lized if unit 21 detects a simultaneous occurrance of valid message signals within more than one of the diffe- rent frequency bands b1A through b1E, for instance within frequency bands b1B, b1C and b1E. In order to achieve said operation mode, the following coefficient values may preferably be utilized, namely aO = a14 = 0, aI = a13=0, a2=a12=0, a3=a11=0, a4=a10=0, a5= a9 = 0, a6 = a8 = 0, a7 = 212. By selecting the coeffi- cient values in this manner, it is possible to ensure that the delay and the amplification in the FIR-filter of a signal within frequency band b1E will not be changed when shifting from the firstmentioned to the second ope- ration mode or inversely.

The third one of the above operation modes may be utilized when no valid message signal at all occurs within frequency band b1. This operation mode may natu- rally be obtained by utilizing a set of coefficients where all coefficients aO through a14 have the value 0.

In the example above described it has been assumed that signal processing unit 17 contains a symmetrical FIR-filter. Said unit may however alternatively contain

an unsymmetrical FIR-filter or an IIR-filter. However, a symmetrical FIR-filter offers the advantage of making it easy to ensure that the delay distorsion and the amp- litude variation of a signal within a pass band common to two or more operation modes will be very low when shifting from one of said operation mcdes to another mode.

Moreover, it has been assumed that evaluation unit 21 ccnsists of an analog unit. Said unit may however in- stead consist of a digital unit containing a number of FIR-filters or IIR-filters acting as band-pass filters and a corresponding number of envelope detectors connec- ted each to one of said filters.

The invention is not restricted to the embodiment above described and shown in the drawing. Instead, many other embodiments are feasible within the scope of the invention as defined in the following claims.