GB2145610A - Television transmission systems - Google Patents

Abstract

A wide band frequency modulated DBS signal of the type having time sequential compressed chrominance and luminance (video) components is received by an aerial (1), frequency converted in a down converter (2) and applied to the input (4) of a conversion unit. The conversion unit has a tuner (5), i.f. stage (9, 10) and a frequency demodulator (11) to produce baseband video which is applied after de-emphasis at (12) to a modulator (27) where the video is amplitude modulated onto a vision carrier from an oscillator (28). The modulated carrier is applied through a vestigial sideband filter (29) and an adder (30) to an output (31). Digital data bursts and other additional digital information present in the DBS signal are recovered in a stage (13) and processed in a unit (14) where the data bursts are expanded to occupy the major part of a line period at a bit rate which is an integral sub-multiple of the bit rate for the data bursts in the received DBS signal. The additional digital information is also expanded at the same bit rate and divided into groups, a group being added to the minor part of a line period containing the expanded data burst. The expanded data bursts and the grouped additional information are digitally modulated in a modulator (32) onto a second carrier from an oscillator (33), the modulated second carrier being applied through a bandpass filter (34) and the adder (30) to the output (31). The output (31) is connected to a cable distribution system for which the narrow band signals produced by the conversion unit are suited as such systems cannot cope with high bit-rates. <IMAGE>

Description

SPECIFICATION Television Transmission System The invention provides a television transmission system having a time multiplexed television signal in component form the majority of the lines of a frame period of which each contain a digital data burst component and a vision component whilst at least one other line of said frame period contains the digital sound/data component and additional digital information, the digital data burst component and the additional digital information having a first bit rate, the digital data burst component for each discrete line being expanded to have a second bit rate and to occupy the major part of a discrete period which corresponds to the period of a line, said second bit rate being an integral sub-multiple of said first bit rate, the second bit rate being greater than f1x n/m where f1 is the first bit rate, n is the number of bits in the digital data burst component and mis the number of bits in a line period at the first bit rate. The invention also relates to such a system for use over a media of restricted bandwidth and also to equipment for use with such systems.

Following the decision in March 1982 that direct broadcast by satellite (DBS) of television programmes for the United Kingdom would commence in 1982 an Advisory Panel chaired by Sir Anthony Part was established to report on technical transmission standards. The findings of this Panel published in November 1982 by Her Majesty's Stationery Office as Cmnd 8751 "Direct Broadcasting by Satellite--Report of the Advisory Panel on Technical Transmission Standards" (known as the Part Report) recommended that the Independent Broadcast Authority's Multiplexed Analogue Component (C-MAC) system should be adopted for DBS, which recommendation has subsequently been accepted.

The C-MAC system has been described in the Independent Broadcast Authority's Experimental and Development Report 118/82 "MAMA Television System for High-Quality Satellite Broadcasting" dated August 1982, this report also describing the A-MAC system (the prefix relating to the type of sound and data transmission). Proposed specifications were included in this report for the two systems, that for the C-MAC system having been revised since the adoption of that system for DBS. The changes in the structure of the video waveform include a reduction in transition periods following the sound/data, chrominance and luminance components with a consequent lengthening of the sound/data component.

Figure 1 of the accompanying drawings (which is not to scale) diagrammatically shows one line period of a C-MAC television signal which occupies 64 Ps and each line is divided notionally into a number of bit or sample periods at a clock rate of 20.25 MHz, there being 1296 such samples per line.

Figure 1 is derived from the European Broadcasting Union Draft New Report "Television Standards for 625-line 12 GHz Satellite Broadcasting", SPB 284, dated June 1983, the contents of which is incorporated herein by way of reference. Each line contains the following in the sequence given: a=203 bits-synchronisation, sound/data (data burst) b=4 samples-transition from end of data c=15 samples-main clamp period (zero-level of chrominance reference) SC1 =6 samples-reserved for vision scrambling.

d=354 samples-chrominance (C) g=704 samples-luminance (Y) SC2=6 samples-reserved for vision scrambling.

h=4 samples-transition into data.

The chrominance component is time compressed by 3:1 so that approximately 52 pus of chrominance information is compressed to occupy 17.48 cos (354 samples) with the R-Y colour difference signal being transmitted on alternate lines and the B-Y colour difference signal being transmitted on the intervening lines. The luminance component is time compressed by 3:2 so that approximately 52 cos of luminance information is compressed to occupy 34.76 ups (710 samples). For DBS transmissions the compressed chrominance and luminance components are frequency modulated with a bandwith of 27 MHz whilst the radio frequency carrier is modulated using 2-4 phase shift keying (2--4 PSK) by the digital sound/data component.

The current proposal is that the sound/data component shall be packet multiplexed and occupy 624 data bursts per frame, each packet comprising 751 bits made up from blocks of 195 bits from each of the 624 data bursts (the first 8 bits of each data burst comprising 1 bitfordata-run in followed by 7 bits forming a line sync. word).

Whilst it will be possible for households to directly receive the DBS transmission by means of a dish aerial of appropriate size sighted onto the satellite, with a down-converter at the aerial to bring the frequency of the incoming transmission to just above the broadcast U.H.F. bands, it has also been suggested that many households will prefer to receive such transmission via a cable television distribution system which at the same time can convey other television programmes whilst doing away with the need for individual aerials.Such distribution by way of cable will obviously have advantages where the signal from the satellite is weak e.g. the transmission is not primarily intended for the country in which it is received, where transmissions are being received from a number of satellites located in different geostationary positions thus requiring a complex aerial array, or in areas of high occupancy where the sighting of aerials may be difficult.

Chapter 7 of the Part Report deals with the interaction between DBS and cable distribution systems and it is reported that the Cable Television Association of Great Britain considered they would be able to provide a cable service even if C-MAC was chosen as the DBS transmission standard. Several examples are given in that chapter and where appropriate to C-MAC the inference is that this type of signal could be directly transmitted over cable systems. Present cable transmission systems use co-axial cable conveying their television programmes in the V.H.F. broadcast bands and whilst there is much debate at present as to whether future installed systems should use optical fibre cables it is quite likely that many of the systems yet to be installed will also be co-axial cable in view of lower installation cost.

It has recently been realised that the transmission of a C-MAC signal over a VHF cable transmission system is not as feasible as originally thought as the 27 MHz bandwidth of such a signal would occupy too much bandwidth thus reducing the number of programmes that such a cable system could carry.

In addition transmission of the sound/data componentatthe high 20.25 Mbit/s rate would pose severe problems on such cable systems because of the short delay reflections produced and that there is a much lower bit rate limit for such cable systems.

With the above in mind some sources have suggested that the only practical way of handling such a signal over a VHF cable transmission system is to convert the C-MAC signal into a PAL type signal prior to its application to a cable system. Such a conversion would lose the advantage of time multiplexed chrominance and luminance and reintroduce the defects of cross-luminance and crosscolour present with colour subcarrier systems but more important where the received DBS signal is scrambled to prevent unauthorised reception e.g.

subscription television services, it would be necessary to descramblethe signal prior to conversion and then rescramble the converted signal.

In our co-pending United Kingdom Patent Application No.8306921 (PHB 32963) we have proposed to overcome the above problem by amplitude modulating the vision component (compressed chrominance and compressed luminance) and by expanding the data burst (the digital sound/data component) such that it has a lower bit rate so that it can then modulate a separate carrier. The data burst from a line then occupies just less than a line period and the whole signal occupies a far narrower bandwidth than the 27 MHz required for satellite broadcasts whilst the reduced bit rate does not cause problems for its transmission over a cable.

Since the filing of the above patent application the system specification has been changed such that provision has been made for line 625 of each frame to carry data during the whole of its period. Line 625 contains a data burst of 203 bits of which 104 bits form the frame synchronisation data, 5 bits provide unified date and time and 94 bits form the static data frame. The data burst is followed by a repeated data frame consisting of five successive identical 94 bit data blocks, the repeated data frame being followed by six further 94 bit data blocks which are so far not allocated which in turn are followed by a 59 bit data block which is at present undefined.Figure 2 of the accompanying drawing (which is not to scaie) shows the proposed overall structure of the line 625 and contains the following in the sequence given: FSD=104 bitsfrnme synchronising data UDT=5 bits-unified date and time SDF=94 bits-static data frame RDF=470 bits-repeated data frame comprising five 94 bit data blocks TDMCTL (1) to (5)-time division multiplex control groups NA=564 bits-six 94 bit data blocks not allocated UDF=59 bits-undefined Figure 3 of the accompanying drawings (also not to scale) shows the proposed structure of the data burst for line 625 and contains the following sequence:: DRI=1 bit-demodulator run-in LSW=7 bits-line synchronisation word CRI=32 bits--clock run-in FSW=64 bits-frame synchronisation word UDT=5 bits-unified date and time CHID 16 bits-satellite channel identification SCR=8 bits-television and sound services configuration reference TDMC=2 bits-time division multiplex configuration NA=54 bits--unailocated ECL=14 bits-error control (function of previous 80 bits) Figure 4 of the accompanying drawings (also not to scale) shows the proposed structure of the repeated data frame each time division multiplex control group containing in the following sequence:: FCNT=7 bits-frame counter UDF=1 bitup-dateflag TDMCID=8 bits-time division multiplex component identification TDMS=42 bits-time division multiplex structure FTS=1 1 bitsfirsttime slot (tdm component subframe) LTS= 11 bits-last time slot (tdm component subframe) FLN=10 bits-first line number (tdm component subframe) LLN=10 bits-last line number (tdm component subframe) SCR=8 bits-services configuration reference UDF=14 bits-undefined ECL=14 bits-error control (function of previous 80 bits).

It will be appreciated that whilst the system proposed can cope with the data burst in each of the lines its does not make provision for the additional digital information in line 625. Therefore it is an object of the present invention to provide a system and apparatus that can readily handle this additional digital information.

The invention provides a television transmission system having a time multiplexed television signal in component form the majority of the lines of a frame period of which contain a digital data burst component and a vision component whilst at least one other line of said frame period contains the digital data burst component and additional digital information, the digital data burst component and the additional digital information having a first bit rate, the digital data burst component for each discrete line being expanded to have a second bit rate and to occupy the major part of a discrete period which corresponds to the period of a line, said second bit rate being an integral sub-multiple of said first bit rate, the second bit rate being greater than f1 x n/m where f1 is the first bit rate, n is the number of bits in the digital data burst component and m is the number of bits in a line period at the first bit rate, characterised in that the additional digital information in said one line is also expanded to have said second bit rate which additional digital information, either prior or subsequent to expansion, is divided into a plurality of groups, an individual group being added at the second bit rate to an individual period containing an expanded digital data burst component so as to be located within the minor part of said period such that the said plurality of groups are contained within a corresponding plurality of periods in a frame period.

The invention also provides a television transmission system in which a first time multiplexed television signal having a given bandwidth is converted into a second television signal for transmission via a media with a bandwidth which is restricted with respect to that of the first television signal, in which the majority of discrete lines of a frame period of said first television signal each sequentially contain a digital data burst component, a time compressed chrominance component and a time compressed luminance component whilst at least one other line of said frame period contains the digital data burst component and additional digital information, said data burst component and said additional digital information modulating a carrier at a first bit rate whilst said carrier is frequency modulated by said chrominance and luminance components, said second television signal comprising a first carrier amplitude modulated by a video signal discrete lines of which sequentially contain the time compressed chrominance component and the time compressed luminance component at corresponding compression rates and located in corresponding positions as with said first television signal, said second television signal further comprising a second carrier located outside the modulation of said first carrier which second carrier is modulated by the digital data burst component at a second bit rate the major part of each of discrete periods of which, which periods each correspond to the period of a line, contain the data burst component present in the discrete lines of said first television signal but expanded to occupy the major part of each discrete period, said second bit rate being an integral sub-multiple of said first bit rate, the second bit rate being greater than f1xn/m where f1 is the first bit rate, n is the number of bits in the compressed data burst component in said first television signal and m is the number of bits in a line period at said first bit rate, characterised in that the additional digital information in said one other line is also expanded to have said second bit rate which additional digital information, either prior or subsequent to expansion, is divided into a plurality of groups, an individual group being added at the second bit rate to an individual period containing an expanded digital data burst component so as to be located within the minor part of said period such that the said plurality of groups are contained within a corresponding plurality of periods in a frame period.

The groups may each have an equal number of bits which number is less than the number of bit positions in the minor part of a period. The minor part of a period may additionally contain a line number code in multi-bit form at the second bit rate each code being unique to a particular line in a frame period. Alternatively, the groups may each have an equal number of bits which number is equal to the number of bit positions in the minor part of a period.

The invention further provides a conversion unit for use with the above television system comprising means for receiving said first television signal the majority of discrete lines of a frame period of which each contain a digital data burst component and a vision component whilst at least one other line of said period contains the digital data burst component and additional digital information, the digital data burst component and said additional digital information having a first bit rate, means for recovering the digital data burst component from said television signal, means for expanding the recovered data burst component at a second bit rate such that the recovered component from each line of said first television signal occupies a major part of a discrete period which corresponds to the period of a television line, the second bit rate being an integral sub-multiple of the first bit rate which second bit rate is greaterthan f1xn/m whereof1 is the first bit rate, n is the number of bits in the data burst component in said first television signal and m is the number of bits in a line period at the first rate, characterised in that said conversion unit additionally comprises means for recovering said additional digital information from said one other line, means for expanding said recovered additional digital information at said second bit rate, means for dividing said recovered additional digital information, either prior or subsequent to expansion into a plurality of groups, and means for adding an individual group at the second bit rate to an individual period containing an expanded data burst component so as to be located within the minor part of said period, the said plurality of groups being contained within a corresponding plurality of periods in a frame period.

The invention additionally provides a conversion unit for use with the above television system comprising means for receiving said first television signal the majority of discrete lines of a frame period of which each sequentially contain a digital data burst component, a time compressed chrominance component and a time compressed luminance component whilst at least one other line of said frame period contains the digital data burst component and additional digital information, with said digital data burst component and said additional digital information modulating a carrier at a first bit rate whilst the carrier is frequency modulated by said compressed chrominance and luminance components, means for frequency demodulating said modulated carrier to produce said compressed chrominance and luminance components and means for recovering the digital data burst component from said modulated carrier, means for amplitude modulating the demodulated compressed chrominance and luminance components onto a first carrier in such manner that the compression rates and positions of said components correspond to those in said first television signal, means for expanding the recovered data burst component at a second bit rate such that the recovered component from each line of said first television signal occupies a major part of a discrete period which corresponds to the period of a television line, the second bit rate being an integral sub-multiple of the first bit rate which second bit rate is greater than f1x n/m where f1 is the first bit rate, n is the number of bits in the data burst component in said first television signal and m is the number of bits in a line period at the first bit rate, means for modulating the expanded data burst component on a second carrier located outside the modulation of the first carrier, the first and second modulated carriers forming the second television signal, characterised in that said conversion unit additionally comprises means for recovering said additional digital information from said one other line, means for expanding said recovered additional digital information at said second bit rate, means for dividing said recovered additional digital information, either prior or subsequent to expansion into a plurality of groups, and means for adding an individual group at the second bit rate to an individual period containing an expanded data burst component so as to be located within the minor part of said period, the said plurality of groups being contained within a corresponding plurality of periods in a frame period.

Such a unit may additionally comprise means for generating a line number code in multi-bit form at the second bit rate with each code being unique to a particular line in a frame period and means for adding a line number code to the minor part of at least those periods which contain a group from the additional digital information.

The invention also provides a television receiver for use with the above television system comprising selection means connected to a restricted bandwidth transmission media for seiecting one from a number of transmission channels, a first signal processing arrangement connected to said selection means for processing the video signal conveyed by a first carrier, a second processing arrangement having means for recovering the data burst component from the major part of the periods conveyed by a second carrier, and means for recovering a clocking signal corresponding to that of the first bit rate present from the second bit rate, characterised in that said second processing arrangement additionally comprises means for recovering the additional digital information from the groups contained in the minor part of said periods.

The second processing arrangement may further comprise means for assembly the said group in an order indicated by the line number codes where such line number codes are present.

The above and other features of the invention will now be described by way of example with reference to the accompanying drawings in which: Figure 5 is a block diagram of a conversion unit for use with the present invention, Figure 6 is a block diagram of part of Figure 5 in greater detail, Figure 7 is a plot of a vision signal for use with the present invention, Figure 8 is a plot of the frequency characteristic of a second television signal which can be used with the present invention, Figure 9 is a block diagram of a television receiver for use with the present invention, and Figure 10 is a block diagram of part of Figure 9 in greater detail.

The diagram of Figure 5 includes a conversion unit for converting a received C-MAC DBS television signal into one suitable for applying a MAC television signal to a cable distribution system. This Figure shows a dish aerial 1 of appropriate dimensions for receiving C MAC/packet DBS television signals which are located in the 12 GHz broadcasting band. Attached to the aerial 1 is a down converter unit 2 in which the frequencies of the incoming signals are shifted such that they are positioned just above the UHF broadcasting bands and lie between 950 and 1750 MHz, so that they can be readily applied via a co-axial cable 3 to an input terminal 4 of the conversion unit.In the conversion unit the signals at terminal 4 are applied to a tuner unit 5 where the required television signal is selected in the usual manner by mixing it with a tuned local oscillator signal to produce an intermediate frequency (i.f.) signal which in this case has a frequency of 134 MHz. The bandwidth of the tuner and the resulting i.f. signal is 27 MHz to match the bandwidth of the DBS signal. Tuning in the tuner unit 5 is by means of a selection voltage applied over a connection 6 from a selector unit (not shown) applied by way of a first input of an adder circuit 7 whose output is connected to the appropriate input of the tuner unit 5. The adder circuit 7 has a second input to which an automatic frequency control (a.f.c.) voltage is applied over a connection 8, this a.f.c. voltage being added to the selection voltage to ensure correct tuning of the tuner unit 5. The i.f. signal from the tuner unit 5 is amplified in an amplifier stage 9 and applied to surface accoustic wave (S.A.W.) filter 10 having a pass band of 27 MHz centered on the i.f. of 134 MHz.

The output of the SAW filter 10 is applied to a limiter and discriminator stage 11 in which the frequency modulated chrominance and luminance vision components of the i.f. signal are demodulated to produce at its output a baseband vision MAC signal which is subjected to de-emphasis in a de-emphasis stage 12. The limiter and discriminator stage 11 also produces the a.f.c. voltage which is applied over connection 8 to adder circuit 7.

The i.f. signal is also applied to a limiter and 2--4 PSK demodulator stage 13 in which the data burst (sound/data component) and the additional digital information all at 20.25 Mbit/s are recovered. These digital signals are applied to a converter 14 which has a number of functions one of which is to expand the digital signals such that they have a much lower bit rate. Certain features of the converter unit 14 are shown in greater detail in Figure 6. In Figure 6 the data burst and the additional digital information at 20.25 Mbit/s is applied to the input A of the converter 14 from where it is applied as a first input to a phase comparator 15 a second input of which receives a signal at the clocking frequency of 20.25 MHz from a voltage controlled crystal oscillator 16.

The comparator 15 produces a voltage at its output depending on the phase relationship between its two inputs which voltage is applied through a low pass filter 17 to the control input of the oscillator thus providing the voltage control for this oscillator.

The output of the oscillator 16 is applied to a frequency divider 18 which divides the output of the oscillator by six to produce a second clocking frequency of 3.375 MHz which is applied to an output C of the converter unit 14. The digital signals at input A are also applied to a sync. word recognition stage 19 together with clock pulses at 20.25 MHz (input C1) from the oscillator 16which stage recognises the sync. words contained at the beginning of each data burst and produces at its outputs H and 2V signals respectively at the line and frame rate which in turn are applied to respective inputs of a timing chain 20 which at an input C1 also receives clock signals at 20.25 MHz from the oscillator 16.The timing chain 20 in response to these inputs produces a sequence of write, read and other signals during each frame period as will be further described. The H output of the sync. word detector 19 is also applied to an output D of the converter unit 14 the purpose of which will be described later. The input A is also coupled to a first digital store 21 and a second digital store 22 both of which may be in the form of shift registers. The data burst appearing during each line of the C-MAC signal is written in the store 21 at the clocking frequency of 20.25 MHz applied to its input C1 under the control of a write signal applied from the timing chain 20 to its input W1.In order to enable this data burst to be successfully conveyed over a cable distribution system this component is expanded as proposed in our co-pending application No. 8306921 such that its bit rate is much lower than that present in the C-MAC signal. As proposed in that application the data burst component is read out during a period which is less than 64 'is at a bit rate which is an integral sub-multiple of the original bit rate of 20.25 Mbit/s (in this case one-sixth of the original bit rate).The data burst component is therefore read out at the second clocking frequency of 3.375 MHz applied to the input C2 under the control of a read signal applied from the timing chain 20 to input R1 during each 64 tis period to occupy 202 bits of the 216 bits (the demodulator run-in bit being ignored) at that rate during such a period and is applied to a first input of a multiplexer 23. The second digital store 22 is arranged so that only the additional digital information appearing after the data burst in line 625 of each frame is written into that store at the clock frequency of 20.25 MHz applied to its input C1 under the control of a write signal applied from the timing chain 20 to its input W2.This additional data information is read out from the store 22 at the second clock frequency of 3.375 MHz applied to its input C2 under the control of write signals produced by the timing chain 20 and applied to the input R2.

The read signals are arranged such that successive groups of 5 bits are read out during succeeding 64 us periods on a first-in-first-out basis such that each group is read out during the last 5 bit positions of the 216 bit positions in a 64 us (line) period at the 3.375 Mbit/s rate. The groups of bits from digital store 22 are applied to a second input of the multiplexer 23. In addition to producing the various write and read signals W1, W2, R1, R2, the timing chain 20 also produces 9 bit codes which successively indicate the line number in a frame period, these 9 bit codes being arranged to occupy the 9 bits following bit 202 in a 64 us period and preceding the groups of 5 bits at the end of these periods. The 9 bit codes are applied from the timing chain 20 to a third input of the multiplexer 23.

Although there are 625 line (64,us) periods in a frame period a 9 bit binary code can only provide unique codes for 512 of these periods. However, as the 1093 bits of the additional digital information is divided into groups of 5 bits such groups will only need to be placed in 219 of the periods and hence it is not necessary to identify each period. The timing chain 20 will then produce additional sequences of bits to be inserted in the last 5 or 14 bit positions of each line period not occupied by a group derived from the additional digital information or a line number code and a group.The timing chain 20 also produces a control signal for the multiplexer 23 which is applied to its control Input M so that the signals appearing at its first, second and third inputs are correctly assembled such that during a line period the output successively produces at the 3.375 Mbit/s rate the 202 bits from the data burst, the 9 bit line number code followed by the 5 bit group derived from the additional digital information in line 625 (or the last 14 bits being made up using substitute bits as described above). These expanded digital data signals are applied to an output B of the converter unit 14.

Referring again to Figure 5, the vision signal from the de-emphasis stage 12 is applied to a first input of a gated sync, insertion stage 26 whose second input receives a pseudo sync. pulse from a sync. pulse generator 25 which is triggered from the output D of the converter unit 14, this sync. pulse being gated into the vision signal during the period previously occupied by the 203 bits (approx. 10 'is) of the data burst. The combined sync. and vision signal from the gated sync. insertion stage 26 is applied to the modulation input of a modulator 27 in which this signal is amplitude modulated onto a vision carrier received at a second input from a first carrier oscillator 28, the frequency of the carrier being in the frequency bands used for cable distribution systems.The nature of this signal during a line period (64 'its) against its percentage depth of modulation of the carrier (%C) is shown in Figure 7 from which it will be seen that the pseudo sinc.

pulse together with its associated back and front porches (S) occupy the period previously occupied by the data burst whilst the compressed chrominance (C) and luminance (Y) components still occupy the same periods as in the C-MAC signal. In Figure 7 it has been assumed that the vision scrambling periods SC1 and SC2 are included in those periods occupied by the compressed chrominance (C) and luminance (Y) components respectively. It will therefore be appreciated that any scrambling or coding of these components in the received C-MAC signal is unchanged bythe processing of these components in the conversion unit and remains intact. From Figure 7 it will be seen that the pseudo sync. pulse is represented by 100% modulation of the carrier whilst the front and back porches are represented by 70% of carrier modulation.The zero-level of chrominance is represented by 50% of carrier modulations whilst the extremes of the chrominance component and the black and white levels of the luminance component are respectively represented by 80% and 20% of carrier modulation. These levels of modulation are of course by way of example and other suitable levels may be chosen. The output from the modulator 27 is applied to a vestigial side band filter 29 to remove substantially all of the lower sideband of the amplitude modulated carrier from the modulator 27. The output from the filter 29 is applied to the first input of an adder circuit 30 whose output is connected to the output 31 of the conversion unit for application to a cable distribution system.

The outputs B and C respectively conveying the expanded digital signals and the second clocking frequency (3.375 MHz) are applied to a digital modulator 32 which at a further input receives a second carrier from a second carrier oscillator 33 in which this carrier is digitally modulated by the expanded digital signals for example using quadrature phase shift keying. The modulated second carrier output is applied through a bandpass filter 34to a second input of the adding circuit 30 for application to the output terminal 31.

A frequency spectrum for the signal appearing at terminal 31 is shown in Figure 8 where the first (vision) carrier is indicated at VC and the spectrum of the vestigial sideband vision signal extends from approximately 1.25 MHz below the vision carrier to approximately 8.5 MHz above the vision carrier. The second carrier indicated by SC is located 11 MHz above the vision carrier VC and the expanded digital signal modulation extends approximately 3 MHz about the second carrier. However, as proposed in our co-pending United Kingdom patent application No. 8314434 (PHB 32981) the second carrier may be located below the vision carrier.It will be seen from Figure 8 that the bandwidth of the modulated second carrier lies outside the bandwidth of the modulated vision carrierwhilstthe bandwidth occupied by the two modulated components together is approximately 14 MHz which is substantially halfthe 27 MHz bandwidth ofthe DBS C-MAC signal. The same is true for the modified proposal made in our application No. 8314434.

Figure 9 is a block diagram of a television receiver capable of receiving television signals from a cable distribution system whether the signals be of the form described above or according to the PAL-I standard. The receiver comprises an input terminal 35 for connection to the cable system which is connected to a tuner unit 36 capable of covering the VHF and UHF frequency bands (40 to 860 MHz), the tuner unit having a received signal bandwidth of approximately 15 MHz. The tuner unit 36 has a local oscillator tuned by means of a control voltage applied over a connection 37 and an adder circuit 38 in a similar manner to the tuner unit 5 in Figure 5, an a.f.c. signal being applied from a connection 39 to a second input of the adder circuit 38.The nature of the i.f. signal from the tuner unit 36 will depend upon the signal being processed though for either type of signal the vision carrier i.f. will be located at the same frequency e.g. 39.5 MHz. For the PAL-I signal the i.f. of the sound or second carrier will then be located at 33.5 MHz whilst that for the MAC signal will be located at 28.5 MHz where the signal transmitted over the cable has the second carrier above the vision carrier. Where the second carrier is below the vision carrier the i.f. for the second carrier will be accordingly positioned above the i.f. for the vision carrier. This output of the tuner unit 36 is amplified by an amplifer stage 40 whose amplified output is applied to a first systems switch 41 in which the incoming signal may be applied either to an upper (PAL) output or a lower (MAC) output.The PAL output is connected to a first (PAL) bandpass filter 42 whilst the MAC output of switch 41 is connected to a second (MAC vision) bandpass filter 43 and to a third (MAC digital components) bandpass filter 44. Suitable bandpass characteristics for the filters 42,43 and 44 are shown in our above mentioned applications Nos. 8306921 and 8314434 and these bandpass filters may conveniently be surface acoustic wave filters. The outputs of the first and second filters 42 and 43 are connected to the input of i.f. amplifier and detector stage 45 which may conveniently incorporate the Philips integrated circuit TDA 3540 or TDA 3541 (the type chosen being dependent on the tuner circuit 36) and for which Development Sample Data has been issued. Besides the amplifying and detection functions of this integrated circuit it also produces the a.f.c. voltage which it applies to the connection 39 and an automatic gain control (a.g.c.) voltage for the tuner unit 36 which it applied thereto over a connection 46, the production of this a.g.c. voltage being one reason why the pseudo sync. pulse is introduced into the vision signal (MAC) at the gated sync.

insertion stage 26 of the conversion unit of Figure 5 (the PAL-I signal already contains a corresponding sync. signal).

The detected output from the stage 45 is applied to a second systems switch 47 being similar to and operated simultaneously with switch 41. The upper (PAL) output of switch 47 is applied to a PAL signal decoder circuit generally depicted by the block 48 where the luminance and chrominance subcarrier components are used (when set for PAL operation) to produce the luminance Y' and the red (R-Y)' and blue (B-Y)' colour difference signals. It is considered that the construction and operation of a PAL signal decoder is so well known as to not warrant a detailed description of its construction and operation.

The PAL output of the switch 47 is also connected to a further bandpass filter 49 having a passband at 6 MHzto select the intercarrierfrequency modulated sound signal when a PAL-I signal is present, this signal being further processed and demodulated in an intercarrier sound stage 50 to produce the demodulated sound signal at its output for application to a loudspeaker system (not shown). A sync. separator stage 51 is also connected to the PAL output of switch 47 which in the presence of a PAL-I signal produces line fH' and field fv' sync.

pulses from this signal.

The output of the third bandpass filter 44 is connected to a stage 52 which with a MAC signal present recovers from the sound carrier i.f. the second clock frequency of 3.375 MHz and provides this at an output 53 and the continuous stream of digital information at 3.375 Mbit/s and provides this at an output 54. These outputs are applied to respective inputs E and F of a unit 55 which produces clocking signals, sync. signals and control signals for the further processing of the MAC signal.

The unit 55 is shown in greater detail in Figure 10 where it will be seen that the 3.375 MHz clocking frequency input E is connected to a first input a phase comparator 56 a second input of which receives a signal at this clocking frequency of 3.375 MHz from a frequency divider circuit 57 which divides a signal applied thereto by 2. The output of the comparator 56 is a voltage dependent upon the phase relationship between its two inputs, this voltage being applied through a low pass filter 58 to the control input of a voltage controlled crystal oscillator 59 of frequency 20.25 MHz. The output of oscillator 59 is successively connected through a frequency divider stage 60 which divides by 3/2 and a further frequency divider stage 61 which divides by 2 to the input of divider stage 57.The divider stages 60, 61 and 57 respectively produces outputs at 13.5MHz, 6.75MHz and 3.375 MHz which together with the output of oscillator 59 (20.25 MHz) are applied to a control stage 62. A further input of the control stage 62 also receives the pseudo sync.

pulse present at an input G. This pseudo sync. pulse present in the MAC vision signal is separated from the signal present at the MAC output of switch 47 by a pseudo sync. detector 63 whose output is connected to the input G (Figure 9). The digital information signals present at terminal F are applied to a sync. word recognition stage 64 to which clock pulses at 3.375 MHz from divider stage 57 are applied to an input C and which produces outputs at the line (H) and frame (2V) frequencies which are applied as inputs to a timing chain 65 which also receives at an input C the 3.375 MHz clock pulses. A de-multiplexer circuit 66 also receives the digital information signals present at terminal F, the demultiplexer 66 also receives a control signal at its control input M from the timing chain 65.The de-multiplexer 66 is controlled such that during each line (64,us) period the first 202 bits (forming the data burst) is present at its lower output which in turn is applied to an output terminal L of the unit 55, the last 5 bits (forming the group of 5 bits) are fed from its middle output into a digital store 67 whilst the 9 bit line code is applied from the upper output to a further input of the timing chain 65. Successive groups of 5 bits from successive line periods (or at least for those 219 line periods containing relevant information) are assembled in the digital store 67 under the control of write signals received at a terminal Wfrom the timing chain 65 to form the expanded repeated data frame.The timing chain 65 provides read signals which are applied to a read input R of the digital store 67 to allow the repeated data frame (up to 1093 bits) to be read out in streams or as required for application to the control stage 62 which also receives the data bursts from the lower output of the de-multiplexer 66. The control stage 62 produces timing signals necessary for the processing of the MAC vision signal and which appear at outputs H and I of unit 55 for application to multi-lead connections 68 and 69 and include the appropriate clocking frequencies, this stage also producing the necessary control signals in response to information contained in the repeated data frame for, amongst other things, changing the displayed picture's aspect ratio.The H and 2V outputs of the sync. word recognition stage 64 are also applied to a sync. generator 70 to which the pseudo sync. pulse from terminal G is also applied and from these inputs produce line (fH) and field (fv) sync. signals which are applied to respective output terminals J and K of the unit 55.

Reverting again to Figure 9, the MAC output of the switch 47 is connected to an analogue-to-digital converter 71 which also receives control and 20.25 MHz clock signals from unit 55 over the connection 68. The time sequential vision signal is converted to digital form in the converter 71 the parallel bits of which are conveyed over multi-lead connection 72 to a MAC vision processor 73 where the chrominance and luminance components are separately stored and expanded in known manner under the control of the control signals and clock signals present in connection 69. The processor 73 has three outputs which are connected as shown to respective digital-to-analogue converters 74,75 and 76 and low pass filters 77, 78 and 79 to produce respective luminance (Y) and red (R-Y) and blue (B-Y) colour difference signals.

The Y, R-Y and B-Y signals from a processed MAC signal and theY', (R-Y)' and (B-Y)' signals from a processed PAL-I signal are applied to respective inputs of a further multi-pole systems switch 80 to which the sync. signals fH, fv and fH, fv are also applied, the switch 80 being operated simultaneously with switches 40 and 47. Switch 80 applies the appropriate signals, depending on the type of signal received, to a display unit 81 where these signals are further processed in known manner to produce a television display.

The clock frequency output 53 of stage 52 and the data bursts from output L of unit 55 are applied to a sound/data decoder 82 which separates and demultiplexes the sound and data signals. The decoder 82 is not shown in greater detail as its construction depends on the composition of the sound/data components in the data burst. The sound signals from the decoder will be applied to the loudspeaker system (not shown) through a further systems switch (also not shown) which operates simultaneously with switches 41,47 and 80.

From Figure 9 it will be seen that the dual standard receiver utilises a number of common stages no matter which of the signals is being received. Whilst reference has been made to one of the signals being of the PAL-I standard it could be any form of frequency multiplexed colour television signal.

In the above description the invention has been described in relation to a system and apparatus suitable for producing a component form time multiplexed television signal for distribution over a cable network. However, the converter unit 14 of Figure 6 may be used in a home receiver or adaptor unit for direct reception of satellite broadcasts to reduce the data rate to a more manageable rate of 3.375 M bit/s rather than the high rate of 20.25 M bit/s at which it is transmitted. In addition, the additional digital information in line 625 need not be divided into blocks of 5 bits for line-by-line insertion into the expanded data but may be divided in a way considered to be more suited to the information it contains. The number of lines in which this information is inserted may also be varied, or may be repeated say twice in a frame period. As an alternative the additional digital information may be divided into blocks or groups of 14 bits and the line number code dispensed with.

Claims (13)

1. A television transmission system having a time multiplexed television signal in component form the majority of the lines of a frame period of which contain a digital data burst component and a vision component whilst at least one other line of said frame period contains the digital data burst component and additional digital information, the digital data burst component and the additional digital information having a first bit rate, the digital data burst component for each discrete line being expanded to have a second bit rate and to occupy the major part of a discrete period which corresponds to the period of a line, said second bit rate being an integral sub-multiple of said first bit rate, the second bit rate being greater than f1x n/m where f1 is the first bit rate, n is the number of bits in the digital data burst component and m is the number of bits in a line period at the first bit rate, characterised in that the additional digital information in said one line is also expanded to have said second bit rate which additional digital information, either prior or subsequent to expansion, is divided into a plurality of groups, an individual group being added at the second bit rate to an individual period containing an expanded digital data burst component so as to be located within the minor part of said period such that the said plurality of groups are contained within a corresponding plurality of periods in a frame period.

2. A television transmission system in which a first time multiplexed television signal having a given bandwidth is converted into a second television signal for transmission via a media with a bandwidth which is restricted with respect to that of the first television signal, in which the majority of discrete lines of a frame period of said first television signal each sequentially contain a digital data burst component, a time compressed chrominance component and a time compressed luminance component whilst at least one other line of said frame period contains the digital data burst component and additional digital information, said data burst component and said additional digital information modulating a carrier at a first bit rate whilst said carrier is frequency modulated by said chrominance and luminance components, said second television signal comprising a first carrier amplitude modulated by a video signal discrete lines of which sequentially contain the time compressed chrominance component and the time compressed luminance component at corresponding compression rates and located in corresponding positions as with said first television signal, said second television signal further comprising a second carrier located outside the modulation of said first carrier which second carrier is modulated by the digital data burst component at a second bit rate the major part of each of discrete periods of which, which periods each correspond to the period of a line, contain the data burst component present in the discrete lines of said first television signal but expanded to occupy the major part of each discrete period, said second bit rate being an integral sub-multiple of said first bit rate, the second bit rate being greater than f, x n/m where f, is the first bit rate, n is the number of bits in the compressed data burst component in said first television signal and m is the number of bits in a line period at said first bit rate, characterised in that the additional digital information in said one other line is also expanded to have said second bit rate which additional digital information, either prior or subsequent to expansion, is divided into a plurality of groups, an individual group being added at the second bit rate to an individual period containing an expanded digital data burst component so as to be located within the minor part of said period such that the said plurality of groups are contained within a corresponding plurality of periods in a frame period.

3. A television transmission system as claimed in Claim 1 or 2, characterised in that said groups each have an equal number of bits which number is less than the number of bit positions in the minor part of a period.

4. A television transmission system as claimed in Claim 1,2 or 3, characterised in that said minor part of said period additionally contains a line number code in multi-bit form at the second bit rate each code being unique to a particular line in a frame period.

5. A television transmission system as claimed in Claim 1 or 2, characterised in that said groups each have an equal number of bits which number is equal to the number of bit positions in the minor part of a period.

6. A television transmission system substantially as herein described with reference to the accompanying drawings.

7. A conversion unit for use with the television system as claimed in any of the preceding claims comprising means for receiving said first television signal the majority of discrete lines of a frame period of which each contain a digital data burst component and a vision component whilst at least one other line of said period contains the digital data burst component and additional digital information, the digital data burst component and said additional digital information having a first bit rate, means for recovering the digital data burst component from said television signal, means for expanding the recovered data burst component at a second bit rate such that the recovered component from each line of said first television signal occupies a major part of a discrete period which corresponds to the period of a television line, the second bit rate being an integral sub-multiple of the first bit rate which second bit rate is greater than f1x n/m where f1 is the first bit rate, n is the number of bits in the data burst component in said first television signal and m is the number of bits in a line period at the first bit rate, characterised in that said conversion unit additionally comprises means for recovering said additional digital information from said one other line, means for expanding said recovered additional digital information at said second bit rate, means for dividing said recovered additional digital information, either prior or subsequent to expansion, into a plurality of groups, and means for adding an individual group at the second bit rate to an individual period containing an expanded data burst component so as to be located within the minor part of said period, the said plurality of groups being contained within a corresponding plurality of periods in a frame period.

8. A conversion unit for use with the television system as claimed in Claim 2 or Claims 3,4 or 5 when dependent upon Claim 2 comprising means for receiving said first television signal the majority of discrete lines of a frame period of which each sequentially contain a digital data burst component, a time compressed chrominance component and a time compressed luminance component whilst at least one other line of said frame period contains the digital data burst component and additional digital information, with said digital data burst component and said additional digital information modulating a carrier at a first bit rate whilst the carrier is frequency modulated by said compressed chrominance and luminance components, means for frequency demodulating said modulated carrier to produce said compressed chrominance and luminance components and means for recovering the digital data burst component from said modulated carrier, means for amplitude modulating the demodulated compressed chrominance and luminance components onto a first carrier in such manner that the compression rates and positions of said components correspond to those in said first television signal, means for expanding the recovered data burst component at a second bit rate such that the recovered component from each line of said first television signal occupies a major part of a discrete period which corresponds to the period of a television line, the second bit rate being an integral sub-multiple of the first bit rate which second bit rate is greater than f1 x n/m where f1 is the first bit rate, n is the number of bits in the data burst component in said first television signal and m is the number of bits in a line period at the first bit rate, means for modulating the expanded data burst component on a second carrier located outside the modulation of the first carrier, the first and second modulated carriers forming the second television signal, characterised in that said conversion unit additionally comprises means for recovering said additional digital information from said one other line, means for expanding said recovered additional digital information at said second bit rate, means for dividing said recovered additional digital information, either prior or subsequent to expansion, into a plurality of groups, and means for adding an individual group at the second bit rate to an individual period containing an expanded data burst component so as to be located within the minor part of said period, the said plurality of groups being contained within a corresponding plurality of periods in a frame period.

9. A conversion unit as claimed in Claim 7 or 8, characterised in that said unit additionally comprises means for generating a line number code in multi-bit form at the second bit rate with each code being unique to a particular line in a frame period and means for adding a line number code to the minor part of at least those periods which contain a group from the additional digital information.

10. A conversion unit substantially as herein described with reference to Figures 1.2, 3, 4, 5, 6, 7 or 8 of the accompanying drawings.

11. A television receiver for use with the television system as claimed in Claims 1 to 6 comprising selection means connected to a restricted bandwidth transmission media for selecting one from a number of transmission channels, a first signal processing arrangement connected to said selection means for processing the video signal conveyed by a first carrier, a second processing arrangement having means for recovering the data burst component from the major part of the periods conveyed by a second carrier, and means for recovering a clocking signal corresponding to that of the first bit rate present from the second bit rate, characterised in that said second processing arrangement additionally comprises means for recovering the additional digital information from the groups contained in the minor part of said periods.

12. A receiver as claimed in Claim 11 for use with a television receiver as claimed in Claim 4, characterised in that said second processing arrangement further comprises means for assembling the said groups in an order indicated by the line number codes.

13. A television receiver substantially as herein described with reference to Figures 1,2,3,4,5,6,9 or 10 of the accompanying drawings.