GB2180112A - Transparent tone-in band transmitters receivers and systems - Google Patents

Abstract

In transparent tone-in-band communication systems a notch in the frequency band is usually formed and then the resulting spectrum is translated in frequency to an intermediate frequency range as part of the transmission process. In a transmitter of the present invention a more simple arrangement is used in which the notch is formed directly in an intermediate frequency range by using mirror filters 11a and 11b to divide an input signal into two portions and supply respective mixers 12 and 14, each having one output sideband in the intermediate frequency range. The mixers 12 and 14 receive different reference frequencies from oscillators 27 and 28. The mixer outputs are supplied to a summing circuit 16 whose output is passed to a band pass filter to remove the unwanted sidebands. The original frequency spectrum is restored at the receiver (Figure 3) by mixer processes of shifting the two selected sidebands similar to, but the inverse of, those used in the transmitter. In modifications (Figures 5 and 6) one of the portions from 11a and 12a is fed to the summing circuit 16, the other portion from 116 is fed to the summing circuit 16 via two mixers (35, 38 or 57,58) and the other portion from 12b is fed to the summing circuit 16 via two mixers (44,47) or a frequency converter (55'). <IMAGE>

Description

SPECIFICATION Transparent tone-in band transmitters, receivers and systems The present invention relates to transmitters, receivers and systems for transparent tone-in-band operation, particularly for use where transmitter and receiver employ an intermediate frequency.

TTIB systems are described by J.P. McGeehan, A.J.

Bateman and D.F. Burrows in "The Use of 'Transparent' Tone-ln-Band (TTIB) and Feedforward Signal Regeneration (FFSR) In Single Sideband Mobile Communication Systems", IEE Conference on Communications Equipment and Systems 82, pages 121 to 126,1982. In an example of a known TTIB system the spectrum ofa baseband signal,forexamplefrom 300 Hzto 3 kHz, is split into two approximately equal segments, for instance from 300 Hzto 1.7 kHz and 1.7 kHz to 3 kHz. The upperfrequency band is translated upward in frequency by an amount equal to the width of an intervening "notch" and added to the lowerfre quencyband.Ifforexampletherequired "notch" width or band separation is 1.2 the circuit output will comprise a signal extending from 300 kHzto 1.7 kHz and from 2.9 to 4.2kHz. A low level reference tone may then be added at the centre ofthe resulting notch which in this example would be 2.3 kHz and the composite signal is then transmitted using conventional techniques, such as single sideband (SSB), with the pilot tone in the notch acting as the reference for subsequent pilot-based processing.In the receiver, the final stages of audio processing remove the pilot in the usual way (for use in, for example, automatic gain control and automaticfrequencycontrol purposes) and perform a complementary downwards frequency translation of the upper half of the spectrum thereby regenerating the original 300 Hzto 3 kHz baseband signal. Thus TTIB gives a complete transparent channel from the baseband input ofthe transmitter to the baseband receiver output avoiding the disadvantage of removing a section ofthe band in orderto insert the pilottone butobtainingthe advantages of high degree of adjacent channel protection, good correlation between fades on the pilot tone and fades on the audio signal, and a large symmetrical pull-in range for the frequency control to operate.

Other examples of TTI B systems are given in US PatentApplication Serial No.617,733 and UK Patent Applications Nos. 8513649,8421025 and 8430319, all having the same inventors as the present application. These applications and the above mentioned paper are hereby incorporated into this specification by reference.

According to a first aspect of the present invention there is provided a communication system forTTIB operation comprising a transmitter comprising means for dividing a band of interest in the frequency spectrum into upper and lower portions, and fortranslating the upper and lower portions in frequency by differing amounts to provide a notch between the translated upper and lower portions, and means for transmitting the translated portions, and a receiver comprising means for receiving the transmitted portions, and means for dividing the received frequency band into two portions at the notch, and for translating the two portions in frequency by different amounts to provide a continuous output spectrum.

According to a second aspect of the invention there is provided a method oftransmission and reception forTTIB operation comprising dividing a band of interest in the frequency spectrum into upper and lower portions, translating the upper and lower portions in frequency by differing amounts to provide a notch between the translated upper and lower portions, transmitting the translated portions, receiving the transmitted portions, dividing the received frequency band into two portions at the notch, and translating the two portions in frequency bydif- ferentamountsto provide a continuous output spectrum.

The main advantage of the present invention is that the translated upper and lower portions may be in a lowintermediatefrequencyband used bythe transmitter and receiver so relaxing the specification of subsequent stages of frequency translation and filtering used in transmitters and receivers at higher in termediatefrequency bands.

In the communication system according to the first aspect of the invention the means for dividing the band and frequencytranslation may comprise first and second filters preferably formed by mirror filter means as hereinafter defined and first and second mixers connected atthe outputs of the first and second filters and receiving different reference signals, the outputs of the mixers being connected to a summing circuit.

According to a third aspect of the invention there is provided a communication system forTTIB comprising a transmitter including mirrorfilter means as hereinafter defined for dividing a band of interest in the frequency spectrum into upper and lower portions, meansforfrequencytranslating at leastone said portion to derive a frequency spectrum with a frequency notch between the said portions, and means for transmitting the said spectrum, and a receiver including means for receiving the said spectrum, mirrorfilter means as hereinafter defined for dividing the received spectrum into two portions at the notch, and means for translating at least one said portion in frequency to provide a continuous output spectrum.

Mirrorfilter means in this specification means a pair of complementary filters having a predetermined fixed orvariable frequency versus attenuation characteristic between input terminals and output terminals, wherein the characteristic of one filter output isthe complement of that of the other filter output.

The meansforfrequencytranslation may be as mentioned inthefirstaspectoftheinvention or may be a single or double oscillator arrangement as described in the above mentioned paper o r a ny of the above mentioned patent applications.

The present invention also includes a TTIB system using the transmitter and/orthe receiver ofthefirst orthird aspects ofthe invention; andfurtherthe invention includes methods corresponding to the transmitter and/or receiver of the first and second aspects of the invention.

Further aspects of the invention include TTIB communication systems in which mirrorfilter means are used and/or in which the notch is derived bytranslating the input spectrum in frequency wherein one portion is translated by more than another portion so providing the notch. The invention also includes transmitters, receivers for such TTIB systems and methods corresponding to such systems, trans mitters or receivers.

Certain embodiments of the invention will now be described by way of example with reference to the accompanying drawings, in which Figure lisa block diagram of a transmitterfora TTIB system according to the first aspect ofthe invention, Figure2 shows frequency spectra which appear in the circuit of Figure 1, Figure 3 is a block diagram of a receiver for a TTIB system according to the first aspect of the invention, Figure 4 shows frequency spectra which appear in the circuit of Figure 3, and Figure 5and are block diagrams of TTI B systems according to the second aspect of the invention.

The letters "a" to "I" denote the frequency spectra shown in Figures 2 and 4 and the positions atwhich these spectra occur in the circuits of Figures 1 and 3 are indicated by the letters. The spectrum 'a' of Figure 2 is divided into two halves which are shaded in different directions and this shading indicates throughout Figures2 and 4 howsignalsfrom the original spectrum are translated infrequency.

In Figure 1 the baseband input spectrum 'a' is applied to an input terminal 10 where it is divided into lower and upper spectra 'b' and 'c' respectively by a mirror filter 11 shown for convenience in two portions 11 a amd 11 b. The mirror filter may be a digital filter comprising a shift register having a numberof stages each connected by way of a respective multi pliertosumming means. Theoutputofthesumming means provides the spectrum 'b' or 'c' and the summing means is connected to a subtraction circuit which subtracts the input signal at terminal 10 from the output of the summing means to provide the spectrum 'c' or'b', respectively.Mirror filters are described in Johnston J.D. "A Filter Family Designed for Use in Quadrature Mirror Filter Banks" IEEE Conference on Acoustic Systems and Signal Processing, Denver 1982 pages 291-294.

The output ofthe filter portion 11 a is supplied to a balanced mixer circuit 12 which also receives a signal at a frequency f7 from an oscillator 13. The resulting spectrum is shown at 'd'. Similarlythe output from the filter portion 11 b passes to a balanced mixer cir- cuit 14which receives a signal at a frequencyf2from an oscillator 15 and generates the spectrum 'e'. Since balanced mixers are used the spectra 'd' and 'e' do not contain signals atfrequenciesf1 andf2, and hence these signals are shown by dashed lines. Filters 12 and 14 are connected to a summing circuit 16which has an inputterminal 17forinjecting a pilotsignal shown at 18 in 'f'.A bandpass filter 19 removesthe lower side bands 21 and 22 from the spectra 'd' and 'e' to provide the transmitter output spectrum 'f'.

All the information present in the spectrum 'a' is present in the spectrum 'f' but a frequency notch 23 is provided which allows signals, such as control signals,forexamplethe pilot signal 18,to be included in the transmitted signal with the advantages described in the above mentioned paper and patent applications.

Thefrequenciesf1 and f2are chosen to givethe re- quired notch width and to position the spectrum 'f' in the frequency band at a desired position, for example an intermediate frequency ofthetransmitter.

The output from Figure 1 is aftertransmission by means of an SSB transmitter and receiverforexample received at a terminal 25 of the receiver shown in Figure 3. The pilottone and any othercontentsof the notch are removed and used, typically in the SSB receiver, before the incoming signal is passed to the terminal 25. The spectrum 'g' is equivalent to the spectrum 'f' except that in general it will be attenuated and distorted. A mirrorfilter 26 having two portions 26a and 26b is used to separatethe lower and upper portions ofthe received signal and gives spectra 'h' and 'i' of Figure 4. These spectra are mixed with frequenciesf3 and f4from oscillators 27 and 28 in balanced mixers 29 and 30 to give spectra 'j' and 'k'.

The frequencies f3 and f4 are so chosen that the lower side bands ofthe outputs of the mixers 29 and 30 when combined in a summing circuit 32 and then filtered in a low passfilter33 givethe original spectrum 'a' shown as'l' in Figure 4. Usually, equalsf3,andf2 equals G.

Typical values forthe frequencies f1 to f4 are f1 =f3=3 KHz and f2=f4=3.6 KHzwith an input base- band of 300 Hzto 3.4KHz.

It will be appreciated that the inventions of US Application 617,733 and UK Applications 8513649 and 8421025 can with advantage be used in the system shown in Figures 1 and 3 ofthe presentspecification.

In Figure 5 the "two oscillator" system of Figure 2 of US Application 617,733 and UKApplication No.

8513649 is modified to employ mirrorfilters. In Figure 5 mirrorfilters 11 and 12 have the samefunctions as in Figures 1 and 3 of the present application; that is inthetransmitterthe baseband is divided into upper and lower portions bythefilter 11 and in the receiver the received band is divided into upper and lower portions by the filter 12. However the output from filter 11 b is passed to a mixer 35 wherethe upper portion of the input spectrum is translated in frequency using asignalfrom an oscillator36.The upper sideband and the resulting signal is removed by a low pass filter 37 leaving the lower sideband which solely comprises signals corresponding to that part ofthe spectrum which is absentfrom the output of filter portion 11 a. However this portion will in general not be in a spectral position to give a required width of frequency notch. For this reason thefil- ter 37 is connected to another mixer 38 which receives a reference signal from an oscillator 39. The outputfromthe mixer 38 is applied to a summing circuit 41 which also receives the output from the fil terportion 1 1a by way of a delay circuit 42 which compensates for the delays experienced by the output from the filter portion 11 bin the mixers 35 and 38 and the filter 37. The resulting spectrum atthe output of the summing circuit 41 includes the upper and lowersidebandsattheoutputfromthemixer38 butthe upper sideband is removed by a bandpass filter 43.

Atthe receiverthe input signal is applied to the mirrorfilter 12 where the outputfrom the portion 12b passes to a mixer 44 receiving a reference signal from an oscillator 45. The lower sideband of theresultant output is selected using a lowpass filter 46 but in general its signal content will not be in its original spectral position in the frequency spectrum and therefore it is applied to a mixer 47 receiving a reference signal from an oscillator 48 before it reaches a summingcircuit49.Signalsfromthefilterportion 12a pass through a delay circuit 51 which com pensates for delays in the circuits 44,46 and 47 before reaching the summing circuit 49. The lowpass filter 52 is connected to the outputofthesumming circuit 49 to remove the upper sideband ofthesignal from the mixer 47 and the output of the filter 52 isthe original baseband restored to its position in the frequencyspectrum.

Anotherexampleofa TTIB system using mirrorfilters is shown in Figure 6 where component circuits which have the same functions as in previous figures havethesamedesignations.

As before incoming signalsto be transmitted are divided into two portions by the mirrorfilters 1 1a and 11 b but the low frequency po rtion is passedtoafre- quency converter 55 which translates this portion by an amount dependenton thefrequency of an oscillator 56. Thus this frequency determines the width of the notch. The upperfrequency portion reaches the summing circuit 41 from the high pass filter 1 la.

Signals entering the frequency converter 55 are passed to separate mixer circuits 57 and 58 but signals received by the mixer circuit 57 are subjectto a phase change of 90 in a circuit 59. The reference frequencyforthis mixer also passes bywayofa90' phase change circuit61. As a resultwhentheside- bands of the outputs ofthe mixer circuits 57 and 58 are summed in a circuit 62 onlythe upper sideband is passed to the summing circuit 41, since the lower sidebands cancel.

On reception the original upperfrequency portion passes directly to the summing circuit 49 by way of the mirrorfilter 12a, butthesidebandfrom thefrequency converter 55 reaches a frequency converter 55' through the mirrorfilter 12b. The converter 55' is the same as the converter 55 exceptthatthe summing circuit 62 is replaced buy a subtraction circuit(not shown). An oscillator 63 ofthe correct frequency to restore the sideband from the converter 55 to its original position in the frequency spectrum is connected in place of the oscillator 56 to apply reference signals to the converter 55'.

Delay circuits similarto circuits 42 and 51 may be added if required, andthefilters 1 lea, 1 1 b, 12a and 12b need not be mirror filters.

It will be appreciated that the mirrorfilters of Fig ures 5 and 6can also be applied toTTIBtransmitters and receivers which employ a single oscillatorto de rive the notch and restore the frequency spectrum, respectively. Such transmitters and receivers are de scribed in the above mentioned paper, US Application Serial No.617,733 and UKApplications8513649, 8421025and8430319.

It will also be appreciated that the inventions of the above mentioned applications can also be applied with advantage to systems in which mirrorfilters are used to divide the input spectrum and/orthe spectrum received by the receiver as indicated more specifically in Figure 5.

The invention may be put into practice in many otherways: for exa m ple the frequencies fr to f4can be chosen together with filter characteristics to enable other combinations of side bands to be used and selected in circuits corresponding to Figures 1 and 3.

Notches may, of course, be formed directly at other than lFfrequencies.

The filters, mixers, delays, summing circuits and oscillators mentioned above may be implemented in anyofthe arrangements ofthefigures in whole orin part by means of operations and/or methods in one or more programmed computers,forexample microprocessors, signal processing integrated circuits, or gate arrays.

Claims (11)

1. AcommunicationsystemforTTlBoperation comprising a transmitter comprising means for dividing a band of interest in the frequency spectrum into upper and lower portions, and fortranslating the upper and lower portions in frequency bydiffering amountsto provide a notch between the translated upper and lower portions, and means for transmitting the translated portions, and a receiver comprising means for receiving the transmitted portions, and meansfordividingthere- ceived frequency band into two portions at the notch, and for translating the two portions in frequency by different amounts to provide a continuous output spectrum.

2. AtransmitterforTTlB operation comprising means for dividing a band of interest in the frequency spectrum into upper and lower portions, and for translating the upper and lower portions in frequ ency by differimng amounts to provide a notch between the translated upper and lower portions.

3. A receiver for TTIB operation comprising means for dividing a received frequency band containing a notch into two portions at the notch, and for translating the two portions in frequency by different amounts to provide a continuous output spectrum.

4. A method fortransmission and reception for TTIB operation comprising dividing a band of interest in thefrequency spectrum into upper and lower portions, translating the upper and lower portions infrequency by differing amounts to provide a notch between the translated upper and lower portions, transmitting the translated portions, receiving the transmitted portions, dividing the received frequency band into two portions at the notch, and translating the two portions in frequency by different amounts to provide a continuous output spectrum.

5. A communication system forTTIB comprising a transmitter including mirrorfilter means as hereinbefore defined for dividing a band of interest in the frequency spectrum into upper and lower portions, means forfrequencytranslating at least one said portion to derive a frequency spectrum with a frequency notch between the said portions, and means for transmitting the said spectrum, and a receiver including means for receiving the said spectrum, mirror filter means as hereinbefore def ined for dividing the received spectrum into two portions atthe notch, and meansfortranslating at least one said portion in frequency to provide a continuous output spectrum.

6. AtransmitterforTTIB operation comprising mirrorfilter means as herein before defined for dividing a band of interest in the frequency spectrum into upper and lower portions, and meansforfrequency translating at least one said portion to derive a frequency spectrum with a frequency notch betweeen the said portions.

7. A receiverforTTIB operation comprising mirrorfilter means as hereinbefore defined for dividing a received frequency band containing a notch intotwo portions atthe notch, and meansfortranslating at least one said portion infrequencytoprovide a continuous outputspectrum.

8. Acommunicationsystem,transmitterorre- ceiver according to any of Claims 1 to 3 and 5 to 7, wherein at least one of the means for dividing and frequencytranslating, or the means for frequency translating, includes a frequency converter in which the following operations are carried out: an incoming signal is mixed with a reference signal having a frequency equal to the width of the required notch, the phase ofthe incoming signal and that ofthe reference signal are changed by 90 and the resulting signals are mixed together, and the signals obtained after mixing are combined in a way in which one of the sidebands from one mixing operation is cancelled by the corresponding sideband from the other mixing operation and a required sideband only remains.

9. A communication system, transmitter or receiver according to any of Claims 5 to 7 wherein at least one of the meansforfrequencytranslating comprises first and second mixers arranged to receive the outputs ofthe complementary filters, respectively, of the mirror filter means, means for supplying different reference signals to the first and second mixers, and summing meansforsumming the outputs of the first and second mixers.

10. A communication system, transmitter or receiver according to any of Claims 1 to 3 and 5to 9 wherein at least some of the said means are formed byaprogrammedcomputerorsignal processing integrated circuit.

11. Acommunicationsystem,transmitterorre- ceiver substantially as hereinbefore described with reference to Figures 1,3,5 or6 ofthe accompanying drawings.