Multicarrier Transmission System

Abstract

A converter for use in a cable television system and interconnected between a transmission path and a television receiver for selecting any one of a plurality of television signals carried by the transmission path. The converter includes a broadband section for translating all received television signals to intermediate frequencies higher than the frequencies of the received television signals such that no images appear within the band of input signals. A selected one of the television signals at a predetermined higher intermediate frequency is then translated by a narrow band section to a lower frequency of a predetermined television channel for reception by the television receiver.

Parent Case Text

RELATED APPLICATION

This application is a continuation-in-part of copending application Ser. No. 814,340, filed Apr. 8, 1969.

Description

FIELD OF THE INVENTION

This invention relates to signal transmission systems and more particularly to signal transmission systems in which a plurality of carriers are transmitted over a common path with minimum distortion between transmitted signals.

BACKGROUND OF THE INVENTION

Signal transmission systems are widely employed in which a plurality of carrier signals are transmitted over a common transmission path to a receiving point where individual carrier signals can be selected and the information carried thereon detected. Such systems are used for example in telemetry and cable television systems. The performance of such multicarrier systems is limited however by distortion or interference which occurs between carrier signals, this distortion being usually due to intermodulation between carriers, producing sum and difference beats which interfere with other carriers being transmitted. This interference usually increases with the signal levels and is cumulative along the transmission path, especially where successive amplification is employed.

In cable television systems, for example, a number of television channels are transmitted over a wideband cable having cascaded wideband anplifiers along the length thereof to maintain suitable signal levels. The performance of such cable television systems is dependent in large part on the quality of the repeater amplifiers employed in the transmission system and the number of such amplifiers which can be cascaded along the transmission path. The noise, intermodulation and cross modulation properties of the amplifier seriously limit the number of amplifiers which can be employed in a practical system and still achieve acceptable television reception. The limiting factor is generally the second order effects, that is, sum and difference beats and second harmonics which cause interference between adjacent carrier signals. The deleterious effect of second-order interference can be ignored in systems that carry the standard 12-VHF channels (which occupy the bands 54 to 88 and 174 to 216 MHz.), because the sum and difference of any two carriers within these bands fall outside of the bands. However, any attempt to carry more than 12 channels by expanding the band will cause sum and/or difference frequencies to occur close to original carrier frequencies, with a resultant visible distortion of the received picture. One way to overcome these effects is to use specially designed push-pull amplifiers which by reason of their symmetrical operation suppress the generation of second order beats as well as beats of higher even orders. The design of such amplifiers to minimize second order effects is, however, quite critical and adds considerably to the cost and complexity of the design and manufacture of amplifiers. Selection of particular frequencies to minimize interchannel interference is wasteful of the usable bandwidth and is also dependent upon the sideband structure of particular signals employed and their placement within the band.

SUMMARY OF THE INVENTION

In accordance with the present invention, a multicarrier transmission system is provided which is especially suited to cable television systems and wherein a unique interrelationship of transmitted carriers provides effective elimination of the effects of interchannel interference. The carrier signals transmitted are odd multiples of and can be derived from a selected base or reference frequency to provide system operation insensitive to second order distortion products, which are the major cause of interchannel interference. By use of the present invention in a cable television system, for example, a less severe limitation is imposed on the number of cascaded amplifiers which can be employed along the transmission path since second order effects, which are the limiting factor in conventional systems, are not a limiting factor in the present system. Moreover, the invention permits the transmission of a greater number of channels than by conventional means by reason of the elimination of the effects of interchannel interference. As a major feature of the invention, a portion of the spectrum within each channel is provided in which no carrier or second or third order beats are present and in which other information can be transmitted without interference with or from the television data transmitted.

The invention is not limited to cable television systems, although it is operative to particular advantage in such systems, but is broadly useful in many transmission systems where a plurality of carrier signals are to be propagated over a common path.

DESCRIPTION OF THE DRAWING

The invention will be more fully understood from the following detailed description, taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a block diagram of a signal transmission system embodying the invention;

FIG. 2 is a block diagram of a cable television system embodying the invention;

FIG. 3 is a block diagram of an alternative embodiment of the system of FIG. 2;

FIG. 4 is a plot of the spectrum of carrier and sideband signals useful in illustrating the performance of the invention;

FIG. 5 is a block diagram of a converter operative in a system according to the invention;

FIG. 6 is a schematic diagram of the up-converter of FIG. 5;

FIG. 7 is a schematic diagram of the down-converter of FIG. 5; and

FIG. 8 is a schematic diagram of the channel selector of FIG. 5.

DETAILED DESCRIPTION OF THE INVENTION

A multicarrier transmission system employing the principles of this invention is illustrated in FIG. 1 and includes means for generating a plurality of uniquely interrelated carrier signals each of which may be individually modulated with information signals, and means for combining the several carrier signals for transmission over a common transmission path such as a cable or radio link. A master oscillator 10 providing a stable signal of predetermined frequency drives a harmonic generator 12 which provides a harmonic spectrum over a frequency band of interest. The output of harmonic generator 12 is applied to a plurality of filters 14a, 146 and 14c, each filter being selectively tuned to provide an output signal of predetermined frequency. Each filter output signal is applied to a respective carrier oscillator 16a through 16c and is operative to control the output frequency of the respective carrier oscillator such that oscillator output signals are provided of predetermined frequency interrelationship. The carrier signals thus generated can be applied to respective modulators 18a and 18c for modulation by respective signals from information sources 20a through 20c. The modulated carrier signals are combined in a combining network 22 which provides a signal output which can be coupled to a transmission path such as a wideband cable or a microwave or other radio link. The carrier frequencies provided by carrier oscillators 16a through 16c are odd multiples of the master oscillator frequency and in accordance with the present invention the sum and difference components between adjacent carrier signals occur in the guardband portions of the spectrum between adjacent channels. The odd-multiple spacing of the carrier frequencies need not be absolutely precise but can vary to a degree depending upon the required bandwidth of the channels in a particular system. The carrier oscillators 16a through 16c need not, therefore, be locked precisely to master oscillator 10; rather, they can be free running oscillators such as crystal controlled oscillators, the precision and stability of which are sufficient to maintain the oscillator frequency sufficiently close to the required odd multiple of the base frequency to effectively prevent second order products from appearing other than in the guard bands between channels.

The invention as embodied in a cable television system is illustrated in FIG. 2. A plurality of television signals are received off the air by individual receivers 30a, 30b and 30c and associated antennas 32a, 32b and 32c. The received television carriers are heterodyned by respective mixers 34a, 34b and 34c to new carrier frequencies which are uniquely interrelated according to the principles of this invention for common transmission over the cable. Each of the television signals transmitted on the cable is different than the standard television broadcast frequencies. Mixers 34d and 34e are provided for transmission of local television or other signals introduced from suitable sources 31a and 31b. As in the embodiment described hereinabove, the interrelated carrier signals are derived and controlled by a master oscillator and harmonic generator and filter system. A master oscillator 36 operating in this embodiment at a frequency of 4.75l MHz. drives a harmonic generator 38, the output of which is applied to a plurality of filters 40a through 40e, the respective outputs of which are applied to respective mixers 34a through 34e. In the case of standard television transmission a base frequency greaterhan 4.5 MHz. must be employed since a standard channel spacing between picture and sound carriers is 4.5 MHz. The carrier signals of adjacent channels should be separated by a factor of two times the base frequency to provide sufficient separation for the sideband components. In the illustrated embodiment, a base frequency of 4.75 MHz . is employed to provide 18 unique transmission channels, as set forth hereinafter, which effectively eliminate the effects of interchannel interference.

The outputs of mixers 34a through 34e are combined in network 42, the output of which drives a wideband cable 44 having amplifiers 46 cascaded at intervals along the length thereof. Subscriber connection is made to the system by a converter 48 being connected to the input of television receiver 50. Converter 48 is operative to select anyone of the channels transmitted over cable 44 and to apply the selected channel to receiver 50. In actual use, receiver 50 is tuned to a channel which matches the output frequency of converter 48 and which channel is selected to be one not used by any local television transmitter.

Another embodiment of the invention is illustrated in FIG. 3 wherein the television signals to be transmitted over the common transmission path are each produced at the appropriate frequency by respective oscillators rather than from a master oscillator as described above. Referring to FIG. 3, the system is generally similar to that of FIG. 2 except that each mixer 34a through 34e has associated with it a respective oscillator 52a through 52e, which typically is a crystal controlled oscillator operative to provide an output frequency of sufficient stability to achieve intended system performance. Mixers 34a through 34e provide the plurality of television signals of frequencies which are odd multiples of one another in accordance with the present invention, these uniquely interrelated signals being applied as in FIG. 2 to a combining network 42 for transmission over wide band cable 4.

As a typical example, the particular frequencies employed according to the invention for an eighteen channel cable television system are set forth hereinbelow. Each frequency is different than the standard television broadcast frequencies.

Cable channel Frequency (MHz.) __________________________________________________________________________ 1 picture 52.25 2 sound 56.75 2 picture 61.75 sound 66.25 3 picture 71.25 sound 75.75 4 picture 80.75 sound 85.25 5 picture 90.25 sound 94.75 6 picture 99.75 sound 104.25 7 picture 109.25 sound 113.75 8 picture 118.75 sound 123.25 9 picture 128.25 sound 132.75 10 picture 137.75 sound 142.25 11 picture 147.25 sound 151.75 12 picture 156.75 sound 161.26 13 picture 166.25 sound 170.75 14 picture 175.75 sound 180.25 15 picture 185.25 sound 189.75 16 picture 194.75 sound 199.25 17 picture 204.25 sound 208.75 18 picture 213.75 sound 218.25 __________________________________________________________________________

FIG. 4 illustrates the spectrum of a television picture and sound carrier and associated sidebands where the picture carriers of all channels are odd multiples of a base frequency. All second order beats between other data. carriers will occur at the edges of the channel, outside the envelope 60 which indicates the spectrum used for the transmission of the television signal, no matter now many channels are employed in the transmission system. It is a particular feature of the invention that a spectrum 62 within each channel is provided in which no carrier or second or third order beats are present, as illustrated. This spectrum is useful for the transmission of different types of information without interference with or from the television channels and can be used for example for the transmission of voice, signalling, telemetry or otherata.

As discussed hereinabove, converter 48 in the embodiments of FIGS. 2 and 3 is operative to select any one of the channels transmitted over cable 44 and to apply the selected channel to television receiver 50. A converter especially adapted for use in a system according to the invention is illustrated in FIG. 5 and is operative to translate each of the 18 television channels transmitted over cable 44 to a single output channel adapted to be coupled to the antenna terminals of a conventional television receiver.

The converter employs a double conversion technique in which input signals from cable 44 are translated to a higher intermediate frequency from which they are converted to a lower output frequency. The translation upward to the intermediate (IF) frequency substantially eliminates image problems, since the local oscillator, for providing frequency conversion to the IF frequency, is higher in frequency than the input signals and image frequencies are higher than the local oscillator frequency. Thus, no images appear within the band of the input signals. By use of the novel converter, the necessity of bandpass filtering for each channel, required in conventional television converters, is eliminated. In the novel converter, channel selection is accomplished without the use of channel selection filters, as employed in conventional turret tuners. The converter utilizes broadband signal processing to provide a plurality of frequency translated input signals, a selected one of which is at the IF frequency. Signal processing from the IF frequency to the output is accomplished in a narrow band manner at fixed frequencies. No RF switching or tuning is required in operation.

Referring to FIG. 5, it is seen that input signals from cable 44 are applied to a low pass filter 70, the output of which is coupled to a broadband amplifier 72, the output of which is applied to one input of a broadband mixer 74. A voltage tuned oscillator 76 provides a second input signal to mixer 74 such that a predetermined intermediate frequency signal is provided by mixer 74. Mixer 74 is broadband and provides a plurality of output signals each at a frequency which is the sum or the difference between a respective input signal frequency and the frequency of oscillator 76. In the illustrated embodiment, mixer 74 utilizes the difference frequencies. The oscillator 76, as determined by the setting of the channel selector 86, produces an output frequency operative to provide a mixer output for the selected channel which is at the IF frequency. The channel selector 86 includes a voltage divider 88 energized from a suitable voltage source 90 and having a plurality of output taps, each providing a selected different voltage and each of which can be selectively coupled to oscillator 76 by means of a selector switch 92. The IF output from mixer 74 is filtered in IF filter 78 and amplified in IF amplifier 80, and is then applied to one input of a mixer 82, the second input of which is a signal from fixed frequency oscillator 84. The output of mixer 82 is coupled to the antenna input terminals of receiver 50.

The low pass filter 70, amplifier 72, mixer 74, voltage tuned oscillator 76, and part of IF filter 78 comprise an up-converter for translating each of the 18 television channels being transmitted over cable 44 to a predetermined IF frequency. In the illustrated embodiment the IF frequency is approximately 230 MHz. for the input signals which are in the 50- 220 MHz. region. The particular IF frequency is selected to minimize the effect of spurious mixing components on the output channel. Part of the IF filter 78, amplifier 80, mixer 82, and fixed frequency oscillator 84 comprise a down-converter for translating the IF signal to an output frequency adapted for application to television receiver 50. In practice the output frequency from mixer 82 is a standard television channel frequency which is not used by any local television transmitter. The output frequency of mixer 82 can be, for example, either channel 2 or channel 3, selectively determined by suitable adjustment of the output frequency of oscillator 84, since these adjacent television channels are never employed together in any one television reception area to prevent adjacent channel interference. Thus, by providing a converter output frequency which selectively can be either channel 2 or channel 3, one of these output converter frequencies will be suitable in a particular television receiving locale for coupling a television receiver to the novel multicarrier system.

The up-converter is illustrated schematically in FIG. 6. Input signals from cable 44 are applied via an input jack 94 to a low pass filter, including series inductor L1 and shunt capacitors C1 and C2, through an input capacitor C3 to the base of a transistor Ql. Transistor Q1 and its associated components function as a broadband amplifier. Suitable biasing is provided by resistors R1, R2 and R3 connected in the manner illustrated. Resistor R4 and capacitor C5 provide frequency peaking to maintain a relatively flat amplifier response. The collector of transistor Q1 is coupled by means of a tuned circuit to the emitter of a transistor Q2. The tuned circuit consists of inductor L2, connected between the collector of transistor Q1 and ground and having a centertap connection connected to series-connected capacitor C6 and resistor R5. Transistor Q2 functions as a broadband mixer, and the local oscillator signal is applied to the base of transistor Q2 from the output of transistor Q3 which with its associated circuitry operates as a voltage tuned oscillator. Resistors R6, R7 and R8 provide biasing for transistor Q2, as do the resistors R9, R10 and R11 for transistor Q3. Capacitor C9 functions as an RF bypass.

The output frequency of the signal derived from the emitter of transistor Q3 is determined by the capacitance of voltage variable capacitor C12. The capacitance of variable capacitor C12 is determined by the magnitude of the control voltage -V.sub.c applied to terminal 96. This control voltage is derived from a voltage divider, to be described hereinbelow, and is applied to capacitor C12 via an RF feedthrough capacitor C13 and inductor L3. The capacitance of variable capacitor C12 in conjunction with the associated tuned circuit is operative to provide a variable oscillator output frequency for providing proper mixer operation.

The output signal from the mixer is derived from the collector of transistor Q2 and is applied to a triple tuned bandpass filter, two sections of which are depicted in FIG. 6 and one section of which, depicted in FIG. 7, is part of the down-converter. The IF filter sections illustrated in FIG. 6 include resistor R13, inductor L5 and capacitor C14 connected in shunt between the collector of Q2 and ground, and coupled, via an inductor L6, to an LC shunt section including inductor L7 and capacitor C15. Point 98 of the IF filter is common to point 98 in FIG. 7 which serves as the input of the third stage of the IF filter. The third section of the IF filter includes series inductor L8, and inductor L9, capacitor C16 and resistor R14 connected in shunt. The output of the IF filter is coupled viz capacitors C17 and C18 to the base of a transistor Q4 which functions as tuned IF amplifier. Capacitor C19 provides RF bypassing while resistors R15, R16 and R17 provide biasing for transistor Q4. A parallel tuned circuit consisting of inductor L10 and resistor R18 is connected between the collector of transistor Q4 and ground. The output of transistor Q4 is coupled by means of capacitor C20 to the base of a transistor Q5 which functions as narrow band mixer. The output of the mixer includes a tuned filter composed of capacitors C22, C23 and L12 connected as shown, tuned to the output channel which is to be coupled to the television receiver. The local oscillator signal for the mixer is provided by transistor Q6 functioning as a fixed frequency oscillator to provide a local oscillator signal to transistor Q5 for providing a mixer output frequency of predetermined value.

The channel selector is illustrated more particularly in FIG. 8 and includes a resistive voltage divider and selector switch for providing a control voltage -V.sub.c of a value corresponding to the switch contact selected. The selector is operative to select one of the 18 television channels being transmitted over cable 44. It will be appreciated that the present converter requires no RF switching and channel selection is accomplished by direct current switching of a control voltage. Typically the control voltage varies between 0 and -20 volts to provide the intended channel selection.

The voltage divider includes four series connected resistors R30 connected at one end to a source of negative supply valtage -B, and connected at the other end to a variable resistor R30 which in turn is connected to ground. Four variable resistors R32 are connected in parallel to respective resistors R30, the variable tap of each resistor R32 being connected to a respective output switch contact 4-17. Similarly a series string of resistors R33 is connected to the variable tap of adjustable resistor R34, which has one end connected to ground and the other end connected via resistor R35 to the negative supply terminal -B. An output connection from the junction between each adjacent pair of resistors R33 and from the free end of the uppermost resistor R33 to respective output switch contacts 1-13 is provided. The selector switch 92 includes a wiper arm 100 selectively connectable to each of the 18 switch contacts. Wiper arm 100 is coupled via resistor R36 to the voltage source -B B and is also coupled directly to the output terminal 102 from which the control voltage -V.sub.c is derived. It will be noted that the 18th switch position is not coupled in circuit and when wiper arm 100 is in this 18th position, the control voltage is simply the full value of the energizing voltage -B. The selector switch typically is a rotary multiposition switch. Adjustable trimming resistors R37 are provided as shown in parallel with resistors R33 to provide precise adjustment of the control voltage for each channel.

Thus, according to the invention a greater number of channels than conventionally transmitted can be propagated along a transmission path without the deleterious effects of second order interference and with the additional provision of band space which can be employed for other types of signals, as this spectrum space is by virtue of the invention clear of any interference from the television channels. The total system bandwidth is not limited by any specific ratio between highest and lowest frequencies as in other systems, and the system bandwidth can be extended according to particular system requirements. By use of the invention visible distortion which may be received by the television receiver is eliminated since the products of interference are in such portions of the spectrum that they are not received at all by the receiver at the subscribers station.

The invention is not to be limited by what has been particularly shown and described as various modifications and alternative implementations will occur to those versed in the art.

Claims

What is claimed is:

1. A converter adapted to be interconnected between a transmission path and a television receiver for selecting any one of a plurality of television signals carried by said transmission path and for converting the selected signal to a predetermined channel for application to said receiver, said converter comprising:

broadband means adapted for coupling to said transmission path and operative to simultaneously translate said plurality of television signals to intermediate frequencies higher than the frequencies of said television signals;

means for selecting one of said television signals at a predetermined one of said higher intermediate frequencies; and

narrow band means for translating said selected television signal at said predetermined intermediate frequency to the lower frequency of a predetermined television channel.

2. A converter according to claim 1 wherein said broadband means has a bandwidth sufficient to process said plurality of television signals.

3. A converter according to claim 1 wherein said broadband means includes a broadband mixer and a voltage tuned oscillator and said selection means includes voltage selection means for applying a selected control voltage to said oscillator to cause said mixer to produce a selected television signal at said intermediate frequency.

4. A converter according to claim 3 wherein said broadband means includes a broadband amplifier and said voltage selection means includes a voltage divider and switching means for applying a selected control voltage to said oscillator.

5. A converter according to claim 3 herein said voltage selection means includes a voltage divider, a source of reference voltage for said divider and a multiposition switch manually operable to apply a selected control voltage to said oscillator.

6. A converter according to claim 5 wherein said narrow band means includes

an IF filter,

an IF amplifier,

a narrow band mixer, and

a fixed frequency oscillator for applying a local oscillator signal of higher frequency than said television signals to said narrow band mixer to cause said narrow band mixer to produce said selected television signal at said predetermined television channel frequency.

7. A converter according to claim 4 including a low pass filter coupled to said broadband amplifier and adapted to receive said plurality of television signals from said transmission path.

8. For use in a cable television wherein a plurality of television signals are transmitted on a common transmission path, a converter adapted for interconnection between said transmission path and a television receiver comprising:

broadband means for simultaneously translating said television signals to intermediate frequencies higher than the frequencies of said television signals and including

a broadband mixer, and

a voltage tuned oscillator;

voltage selection means for applying a selected control voltage to said oscillator to cause said mixer to produce a selected television signal at a predetermined one of said higher intermediate frequencies; and

narrow band means for translating said selected television signal at said predetermined intermediate frequency to the lower frequency of a predetermined television channel, said narrow band means including

an IF filter,

an IF amplifier,

a narrow band mixer, and

a fixed frequency oscillator for applying a local oscillator signal of higher frequency than said television signals to said narrow band mixer to cause said narrow band mixer to produce said selected television signal at said predetermined television channel frequency.

9. A converter according to claim 8 wherein said transmission path is a wideband cable.

10. A converter according to claim 9 wherein said television signals are each different from standard television broadcast frequencies.

11. A system for transmitting a plurality of television signals a common transmission path comprising:

means operative to simultaneously receive a plurality of television signals;

means operative in response to the received television signals to provide a plurality of television signals, the frequency of each being a different odd multiple of a reference frequency;

a plurality of mixers each associated with a respective receiver;

a master oscillator for providing a signal of a reference frequency;

a harmonic generator operative in response to said reference frequency signal to provide a predetermined harmonic spectrum;

a plurality of filters coupled to said harmonic generator and each associated with a respective mixer and each being selectively tuned to provide an output signal to a respective mixer to cause said mixer to provide a television signal of a frequency which is a different odd multiple of said reference frequency;

a network for combining said television signals for application to a common transmission path; and

means for converting said television signals to the frequency of a predetermined television channel, said conversion means including:

broadband means for translating said plurality of television signals to intermediate frequencies higher than the frequencies of said television signals;

voltage switching means for selecting one of said television signals at a predetermined one of said higher intermediate frequencies; and

narrow band means for translating said selected television signal to the lower frequency of a predetermined television channel.

12. A system according to claim 11 wherein said conversion means comprises:

broadband means for translating said television signals to a higher intermediate frequency and including broadband mixer and a voltage tuned oscillator and said voltage switching means includes means for applying a selected control voltage to said oscillator to cause said mixer to produce a selected television signal at said intermediate frequency.

13. A system according to claim 12 wherein said voltage switching means includes a voltage divider and a multiposition switch for applying a selected control voltage to said oscillator.

14. A system according to claim 13 wherein said narrow band means includes

an IF filter,

an If amplifier,

a narrow band mixer, and

a fixed frequency oscillator for applying a local oscillator signal of higher frequency than said television signals to said narrow band mixer to cause said narrow band mixer to produce said selected television signal at said predetermined television channel frequency.

15. A system according to claim 11 wherein said plurality of television signals are of different frequencies than standard television broadcast frequencies.